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Propagation of unducted whistlers from their source lightning: A case study
Abstract
1] We analyze nightside measurements of the DEMETER spacecraft related to lightning activity. At the 707 km altitude of DEMETER, we observe 3-D electric and magnetic field waveforms of fractional-hop whistlers. At the same time, the corresponding atmospherics are recorded by a very low frequency (VLF) ground-based station located in Nançay (France). The source lightning strokes are identified by the METEORAGE lightning detection network. We perform multidimensional analysis of the DEMETER measurements and obtain detailed information on wave polarization characteristics and propagation directions. This allows us for the first time to combine these measurements with ray-tracing simulation in order to directly characterize how the radiation penetrates upward through the ionosphere. We find that penetration into the ionosphere occurs at nearly vertical wave vector angles (as was expected from coupling conditions) at distances of 100–900 km from the source lightning. The same distance is traveled by the simultaneously observed atmospherics to the VLF ground station. The measured dispersion of fractional-hop whistlers, combined with the ionosonde measurements at the Ebro observatory in Spain, allows us to derive the density profile in the topside ionosphere.
... Although only one wave normal component is represented in the data theoretical predictions and satellite observations confirm that the wave vector is near vertical within the ionosphere e.g. [51], at least for 0+ whistlers, and so we expect the measured component to capture most of the field intensity. In any case in concert with the IGRF or the satellite ephemeris we know the angle between the wave vector and the local geomagnetic field and so could reconstruct the full intensities from the polarization ratios. ...
... Rocket and satellite observations show that sferics can couple to the whistler mode over distances of at least 1000 km from the flash [32,33,51]. In the most complete study of its kind Fiser et al. [54] analyzed ~30000 lightning whistler pairs and found that the average amplitude of a whistler is largest when the magnetic footprint of the detecting satellite is approximately 1 degree north of the causative lightning location. ...
... Simple geometry tells us that from an altitude of 100 km the line of sight viewing is approximately cos -1 (6380/6480) = 10° or about 1000 km. This is very similar to the maximum distance for sferic to whistler conversion estimated from in situ rocket and satellite observations [32,33,51]. The main drawback to this approach is that there is no frequency dependent attenuation. ...
... The wave propagation parameters such as the polarization, ellipticity, the wave normal angle, azimuthal angle, planarity, etc., were computed by the singular value decomposition (SVD) method [25], which has been widely applied to study wave propagation feature [6,10,11,26,27]. The SVD algorithm was introduced in detail by Santolík et al. [25]; our works [28,29] further introduced how to build the Magnetic Field Aligned Coordinate (MFAC) coordinate system for CSES-01. ...
... It is widely accepted that these fractional-hop whistlers propagate through the ionosphere without being ducted until they potentially enter ducts at higher altitudes ranging from 1000 to 2000 km. The fractional-hop whistlers' wave vector directions are very close to vertical just after they penetrate upward into the ionosphere [10]. ...
Driven by the scientific objective of geophysical field detection and natural hazard monitoring from space, China launched an electromagnetic satellite, which is known as the China Seismo-Electromagnetic Satellite (CSES-01), on 2 February 2018, into a circular sun-synchronous orbit with an altitude of about 507 km in the ionosphere. The CSES-01 has been in orbit for over 6 years, successfully exceeding its designed 5-year lifespan, and will continually operate as long as possible. A second identical successor (CSES-02) will be launched in December 2024 in the same orbit space. The ionosphere is a highly dynamic and complicated system, and it is necessary to comprehensively understand the electromagnetic environment and the physical effects caused by various disturbance sources. The motivation of this report is to introduce the typical electromagnetic waves, mainly in the ELF/VLF band (i.e., ~100 Hz to 25 kHz), recorded by the CSES-01 in order to call the international community for deep research on EM wave activities and geophysical sphere coupling mechanisms. The wave spectral properties and the wave propagation parameters of those typical EM wave activities in the upper ionosphere are demonstrated in this study based on wave vector analysis using the singular value decomposition (SVD) method. The analysis shows that those typical and common natural EM waves in the upper ionosphere mainly include the ionospheric hiss and proton whistlers in the ELF band (below 1 kHz), the quasiperiodic (QP) emissions, magnetospheric line radiations (MLR), the falling-tone lightning whistlers, and V-shaped streaks in the ELF/VLF band (below 20 kHz). The typical artificial EM waves in the ELF/VLF band, such as power line harmonic radiation (PLHR) and radio waves in the VLF band, are also well recorded in the ionosphere.
... Zahlava et al. [29] found through observation with the DEMETER spacecraft and Van Allen Probes that the degree of attenuation of the dispersion characteristic of lightning whistlers is lower at night, and concluded that the density of the spatial ionosphere is lower at night. Santolik et al. [30] pointed out that the form of lightning recorded by ground VLF observation stations is very similar to that observed by satellites, and both forms have dispersion characteristics. Putri and Kasahara et al. [31] proposed two correction functions to modify the empirical electron density model, and theoretically calculated the dispersion of lightning whistlers observed by the Arase satellite using ray tracing. ...
... The time-frequency spectrum exhibits an obvious dispersion pattern, and the frequency gradually decreases with time. Santolik et al. [30] pointed out that the form of lightning observed by the satellite is very similar to that recorded by the ground VLF observation stations, and both have dispersion characteristics. Therefore, the signal can be judged to be a lightning whistler. ...
The data assimilation algorithm is a common algorithm in space weather research. In this paper, the time-frequency information in the dispersion spectrum of lightning whistlers received by the ZH-1 satellite is used as the observed value, and the international reference ionospheric model serves as the background model to construct the calculation model of the propagation time of lightning whistlers in the ionosphere. Kalman filtering is adopted to assimilate the electron density distribution along the propagation path of lightning whistlers. The results show that the situation where the electron density of the background model deviates greatly from the true value is significantly improved through data assimilation. The electron density after assimilation is in good agreement with the true value, which effectively helps realize the process of using observed values to correct the background value. On this basis, the influence of the frequency difference on the assimilation inversion effect is studied, and the results show that the assimilation effect is worse when the frequency difference between frequency points is less than 1 kHz.
... The lightning activities from the atmosphere also serve as an embryonic source for strong ELF/VLF emissions in the upper ionosphere (Santolík et al., 2009;Shklyar et al., 2012;Zhima et al., 2017). The azimuthal angles of wave vector of lightning induced ELF/VLF emissions usually predominate around 0° in the above defined FAC coordinate system, which means that this kind of wave propagation direction points away from the Earth direction to outer space (in the increasing L shell direction). ...
... The azimuthal angles of wave vector of lightning induced ELF/VLF emissions usually predominate around 0° in the above defined FAC coordinate system, which means that this kind of wave propagation direction points away from the Earth direction to outer space (in the increasing L shell direction). The lightning induced wave also presents either right or left handed polarization (Santolík et al., 2009), but most importantly, the lightning induced ELF/VLF emissions usually appear as a series of intensive burst spectra with vertical lines or whistlermode falling/rising tones along the whole frequency range from a few hertz up to over 3 kHz or even 10 kHz . In this study, the strong ELF emissions over the Sumatra epicenter zone appeared in a much lower frequency range (below 1,100 Hz), mainly at 300-800 Hz. ...
The abnormal electromagnetic emissions recorded by DEMETER (the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite associated with the April 6, 2010 Mw 7.8 northern Sumatra earthquake are examined in this study. The variations of wave intensities recorded through revisiting orbits from August 2009 to May 2010 indicate that some abnormal enhancements at Extremely Low Frequency range of 300–800 Hz occurred from 10 to 3 days before the main shock, while they remained a relatively smooth trend during the quiet seismic activity times. The perturbation amplitudes relative to the background map which were built by using the same-time seasonal window (February 1 to April 30) data from 2008 to 2010 further suggest strong enhancements of wave intensities during the period prior to the earthquake. We further computed the wave propagation parameters for the electromagnetic field waveform data by using the Singular Value Decomposition method, and results show that there are certain portions of the Extremely Low Frequency emissions obliquely propagating upward from the Earth toward outer space direction at 10 and 6 days before the main shock. The potential energy variation of acoustic-gravity wave suggests the possible existence of acoustic-gravity wave stability with wavelengths roughly varying from 5.5 to 9.5 km in the atmosphere at the time of the main shock. In this study, we comprehensively investigated the link between the electromagnetic emissions and the earthquake activity through a convincing observational analysis, and preliminarily explored the seismic-ionospheric disturbance coupling mechanism, which is still not fully understood at present by the scientific community.
... The sferic signal radiated by lightning can be reflected and propagated in the form of tweek in the Earth-ionosphere waveguide. Part of the sferic signal can escape from the Earth-ionosphere waveguide and leak into the ionosphere, propagating along the geomagnetic field lines to another hemisphere (Santolík et al., 2008(Santolík et al., , 2009Santolík & Parrot, 1998). Because this part of the electromagnetic wave has a whistle-like sound, it is named lightning whistlers (LW) (Carpenter, 1988;Crouchley & Duff, 1962). ...
Based on the magnetic field data recorded by the ZH‐1 electromagnetic satellite, we cerat a training set of 1,300 spectrograms containing the dispersion spectrum of lightning whistlers (LW). The Segment Anything Model (SAM) in the field of image segmentation is trained through the training set to obtain a fine‐tuned SAM model that can be used to detect and segment the dispersion spectrum of LW at pixel level. All track regions of LW are effectively separated from other non‐lightning whistlers regions in the spectrograms after being segmented by the model. The segmentation effect is excellent and detection accuracy is 96.89%, which is better than the previous segmentation model for LW based on ground station data. Then we apply the traditional image processing methods to extract the dispersion spectrum of LW one by one, and develop an algorithm to automatically extract the physical parameters of each LW. The root mean square error between the automatically extracted dispersion parameter and the manually extracted dispersion parameter is only 0.1654 s1/2. The model and algorithm studied in this paper are employed to analyze the dispersion of LW received by the ZH‐1 satellite over China. It is found that the whistlers dispersion received by satellites during summer in the northern hemisphere and summer in the southern hemisphere shows opposite trends with receiving latitude. Both trends can be explained by the relationship between the dispersion and the length of propagation paths of LW.
... Since the Swarm data do not provide whistler wave vectors, we cannot compute backward propagation. Instead, the algorithm iteratively searches the suitable entry point of the ray at the base of the ionosphere and performs forward propagation, assuming a vertical initial propagation vector k → (Helliwell, 1965;Santolík et al., 2009). With this assumption, the condition on ψ guaranteeing Equation 5 becomes a requirement on the local magnetic inclination I: |I| > 1°. ...
Plain Language Summary
A lightning strike generates an electromagnetic impulse that propagates within Earth's atmosphere and eventually leaks out into the ionosphere. As it propagates through the ionosphere toward low‐Earth orbiting (LEO) satellites, it gets converted into a so‐called whistler, with high frequencies arriving earlier than low frequencies. This frequency dispersion depends on the state of the ionosphere. Here, we analyse such whistler waves detected by magnetometers onboard the European Space Agency Swarm satellites to recover information about the state of the ionosphere below the satellites. We first introduce a new metric, the Total Root Electron Content (TREC), which quantifies the cumulative value of the square root of electron density along the path of the whistler. We next propose a method to recover the TREC from the analysis of the whistler dispersion. We finally validate this method by using independently derived ionospheric electron density profiles to infer expected TREC values. Our results show that whistlers detected by LEO satellites can be used to locally improve the widely used empirical International Reference Ionosphere model. Such whistler inferred TREC values could be used to sound the ionosphere above places difficult to sample with conventional measuring techniques, and help better model and understand the highly dynamic ionosphere.
... Lightning strikes in the Earth's atmosphere can produce broadband electromagnetic emissions, which are mostly confined in the waveguide between the Earth and the ionosphere, within a horizontal distance of up to ∼1,000 km from the corresponding lightning sources (Fiser et al., 2010). Part of this lightning-associated radiation can leak through the ionosphere within a narrow transmission cone near the vertical direction (Santolík et al., 2009) and propagate into the inner magnetosphere as lightning-generated whistler (LGW) waves (Carpenter, 1968;Clilverd et al., 2008;Helliwell, 1965Helliwell, , 1969Inan & Bell, 1977;Storey, 1953;Tao et al., 2010). LGW waves are coherent emissions with a frequency range from ∼100 Hz to >30 kHz (Marshall et al., 2021), and most LGW waves are confined inside the plasmasphere (Bortnik et al., 2003a;Oike et al., 2014). ...
Plain Language Summary
A fraction of broadband electromagnetic emissions produced by lightning strikes in the Earth's atmosphere can propagate through the ionosphere into the magnetosphere as lightning‐generated whistlers (LGWs). The LGW plays an important role in the electron scattering of the radiation belts, the efficiency of which highly depends on their wave normal angles and thus the propagation modes of the LGWs. In this study, we develop an algorithm to identify the burst mode waveform data from Van Allen Probes' observations that capture the LGW signals. Using these identified waveforms, we statistically study the wave normal distribution for LGWs in the inner magnetosphere. The results indicate that most LGWs are observed in the low altitude region (<2RE, RE is the Earth's radius) with an oblique, radially outward wave normal. In the high altitude region (∼3RE), parallel wave normals dominate, and in the very low altitude region (<0.2RE), the oblique inward wave normal exists. A ray tracing simulation is performed to confirm the dominant oblique wave normal of LGWs and the existence of inward wave normals caused by the great plasma density gradient in the very low altitude region. The results lead to our systematic understanding of LGWs and advance the predictive capability of radiation belts.
... Where conditions are conducive to penetration through the EIWGUB and continuation to an ionospheric detector, EMPs in the EIWG may convert to F mode plasma waves at the EIWGUB and travel along nearly vertical paths (Jacobson et al., 2011;Santolik et al., 2009) as indicated by the three upward directed blue arrows extending from the EIWGUB to the Van Allen probe altitude at three locations along its orbit in Figure 3. Because conditions are not always conducive, not every EMP produces F waves observable by satellites in the ionosphere. ...
Plain Language Summary
Using satellite data, just as the acoustic noise from distant lightning is heard to rumble, sometimes a considerable time later, for a much longer duration than the visible flashes, an even more greatly delayed and spread‐out series of plasma‐sound waves are found in the earth's ionosphere after every lightning bolt. The noise of these plasma‐sound waves comes in two forms. One set of plasma‐sound waves is slow and is found to be always present over the entire globe. Properly accounting for this noise in satellite electromagnetic field measurements could improve the quality of measurements of the earth's magnetic field from space, and lead to a better understanding of our earth's magnetic field and its ionosphere. A second set of plasma‐sound waves is fast, and more sporadic in appearance. These fast plasma‐sound waves are associated with plasma bubbles that can interfere with radio wave communications around the globe. Better understanding these fast waves and bubbles could possibly allow for better radio communication processes.
... concrete kind of whistler mode waves, quasiperiodic emissions (Carson et al., 1965;Sato & Kokubun, 1980), and a detailed analysis of available multipoint observations. Recent examples of case studies focused on a comparison of whistler mode wave measurements performed simultaneously by a ground-based station and a spacecraft were presented, for example, by Santolík et al. (2009), Jaynes et al. (2015), Demekhov et al. (2017Demekhov et al. ( , 2020, and . These works are often motivated by searching for a possible source region of the waves and their exact propagation toward the ground. ...
Events characterized by substantial intensity enhancements in the frequency range between about 1.5 and 4 kHz in the measurements of the ground‐based Kannuslehto station, Finland are analyzed. Altogether, as many as 465 events are identified in the Kannuslehto data measured during the campaigns between December 2012 and October 2019. It is shown that the events usually last for several hours and they occur preferentially on the dawn side during geomagnetically active periods. Simultaneous measurements performed by the Van Allen Probes spacecraft are used to reveal the L‐shells and magnetic local times where a corresponding intensity increase occurs in space. A backward ray tracing analysis is further employed to investigate the wave propagation between the tentative source region and the ground. Wave normal angles of waves eventually detectable at Kannuslehto are determined and compared with those obtained from a detailed wave analysis. Either a wave ducting, propagation in the Earth‐ionosphere waveguide, or their combination seem to be needed for the waves to reach Kannuslehto.
... In order to better suppress the noise and highlight the natural signals, it is necessary to choose the optimal filtering methods. From the VLF burstmode observations of CSES, there are plenty of typical electromagnetic (EM) waves recorded, such as the ionospheric hiss [12,13], chorus [14,15], quasiperiodic (QP) waves [17], or the lightning-induced strong whistler waves in the whole ELF/VLF frequency band [16]. These typical EM waves can be used to examine the stability of the electromagnetic field detection payloads for their noticeable wave spectral property both in the electric and magnetic components. ...
The China-Seismo-Electromagnetic Satellite (CSES), which was launched in February 2018, carries the search coil magnetometer (SCM) and the electric field detector (EFD) to realize the high-resolution electromagnetic field and wave detection in the upper ionosphere. Due to the complexity and variability of the ionospheric environment, the stability of such a high sampling rate and high-precision electromagnetic field detection systems is always an essential link in data processing and the scientific application of CSES. This work evaluates the stability of the very-low-frequency (VLF) band detection by validating the systemic sampling-time differences between SCM and EFD in the VLF burst-mode observations. The optimal waveform data preprocessing method is put forward according to the noise levels of the VLF burst-mode observation and the inherent design characteristics of EFD. The VLF waveform data of EFD is rebuilt by filling the data gaps among the sampling sub-periods, making it with a similar sample length to SCM. Then by precisely intercepting the maximum and minimum values of the burst-mode waveforms, the variation of the sampling-time difference between EFD and SCM is statistically evaluated. Results show that during the three years’ operation, the sampling-time difference between EFD and SCM predominately keeps below 0.5 s, indicating good stability of EFD and SCM on orbit. Then we developed an automatic synchronization tool based on the similarity function and STA/LTA (short time average over long time average) characteristic function. This tool can effectively realize the precise synchronization between SCM and EFD in the VLF burst-mode observation. This work is helpful to upgrade the data quality of CSES and provides technical support for electromagnetic wave propagation studies.
... To simulate the propagation of whistler-mode waves in a multicomponent cold plasma, we numerically solve 3D ray tracing equations with the 4th/5th order Dormand-Prince Runge-Kutta method from the SciPy Python library. The overall implementation is similar to the one found in Santolík et al. (2009), Section 2.3; the code and a quick guide can be downloaded from the link provided in the data availability statement. We assume that the traversal of the plasmapause happens very early during the wave propagation so that the plasmapause can be excluded from our ray simulations. ...
Equatorial noise is an electromagnetic emission with line spectral structure, predominantly located in the vicinity of the geomagnetic equatorial plane at radial distances ranging from 2 to 8 Earth's radii. Here we focus on the rare events of equatorial noise occurring at ionospheric altitudes during periods of strongly increased geomagnetic activity. We use multicomponent electromagnetic measurements from the entire 2004–2010 DEMETER spacecraft mission and present a statistical analysis of wave propagation properties. We show that, close to the Earth, these emissions experience a larger spread in latitudes than they would at large radial distances and that their wave normals can significantly deviate from the direction perpendicular to local magnetic field lines. These results are compared to ray tracing simulations, in which whistler mode rays with initially nearly perpendicular wave vectors propagate down to the low altitudes with wave properties corresponding to the observations. We perform nonlinear fitting of the simulated latitudinal distribution of incident rays to the observed occurrence and estimate the distribution of wave normal angles in the source. The assumed Gaussian distribution provides the best fit with a standard deviation of 2° from the perpendicular direction. Ray tracing analysis further shows that small initial deviations from the meridional plane can rapidly increase during the propagation and result in deflection of the emissions before they can reach the altitudes of DEMETER.
... Therefore, the simple whistler dispersion approximation for frequencies much lower than the local group velocity maximum at f ce /4 becomes invalid, and a more complex approach is necessary. We performed a backward ray-tracing simulation with an adaptive integration step (Cerisier 1970;Santolik et al. 2009), beginning from the position of Van Allen Probe B, and using a diffusive equilibrium model of the plasma density distribution. We obtained the group delays between the spacecraft position and the ionosphere at five frequencies between 2 and 10 kHz, using an initial wave vector inclined 20°inward from the local field line, antiparallel to the direction obtained from the Van Allen Probe B measurements. ...
This paper presents the highlights of joint observations of the inner magnetosphere by the Arase spacecraft, the Van Allen Probes spacecraft, and ground-based experiments integrated into spacecraft programs. The concurrent operation of the two missions in 2017–2019 facilitated the separation of the spatial and temporal structures of dynamic phenomena occurring in the inner magnetosphere. Because the orbital inclination angle of Arase is larger than that of Van Allen Probes, Arase collected observations at higher L L -shells up to L ∼ 10 . After March 2017, similar variations in plasma and waves were detected by Van Allen Probes and Arase. We describe plasma wave observations at longitudinally separated locations in space and geomagnetically-conjugate locations in space and on the ground. The results of instrument intercalibrations between the two missions are also presented. Arase continued its normal operation after the scientific operation of Van Allen Probes completed in October 2019. The combined Van Allen Probes (2012-2019) and Arase (2017-present) observations will cover a full solar cycle. This will be the first comprehensive long-term observation of the inner magnetosphere and radiation belts.
... We use the same ray-tracing code as employed formerly by Santolík et al. (2006) 92% of oxygen ions and 8% of hydrogen ions). The waves are started at the bottom of the ionosphere (110 km) with wave vectors oriented vertically upward, as stems from the Snell's law (e.g., Santolík, Parrot, et al., 2009). Three different wave rays are started, at geomagnetic latitudes of 40°, 45°, and 50°, roughly corresponding to the geomagnetic latitudes of the Alpha transmitters. ...
Plain Language Summary
Powerful military very low frequency transmitters operating at frequencies of a few tens of kHz are important manmade sources of electromagnetic waves propagating in the Earth’s inner magnetosphere. They can, in particular, interact with energetic electrons trapped in the radiation belts and result in their precipitation. The efficiency of such interactions depends, among other parameters, crucially on the wave normal angles (WNAs) of propagating transmitter signals. Unfortunately, these are not well known and are expected to depend significantly on the mode of propagation. If the wave propagation is governed by density ducts following Earth’s magnetic field lines, the respective WNAs are very low. In the absence of such density ducts, the WNAs are expected to be comparatively large. We use electromagnetic wave measurements performed by the Van Allen Probes spacecraft to determine and systematically analyze WNAs of propagating transmitter signals. This allows us to experimentally distinguish the two propagation types and to evaluate their relative significance.
... [27][28][29]); the lightning-induced whistlermode electromagnetic waves in all ELF/VLF frequency band (e.g., refs. [24,30,31]). ...
The China Seismo-Electromagnetic Satellite (CSES) deploys three payloads to detect the electromagnetic environment in the ionosphere. The tri-axial fluxgate magnetometers (FGM), as part of the high precision magnetometer (HPM), measures the Earth magnetic vector field in a frequency range from direct current (DC) to 15 Hz. The tri-axial search coil magnetometer (SCM) detects the alternating current (AC) related magnetic field in a frequency range from several Hz to 20 kHz, and the electric field detector (EFD) measures the spatial electric field in a broad frequency band from DC to 3.5 MHz. This work mainly cross-calibrates the consistency of these three payloads in their overlapped detection frequency range and firstly evaluates CSES’s timing system and the sampling time differences between EFD and SCM. A sampling time synchronization method for EFD and SCM waveform data is put forward. The consistency between FGM and SCM in the ultra-low-frequency (ULF) range is validated by using the magnetic torque (MT) signal as a reference. A natural quasiperiodic electromagnetic wave event verifies SCM and EFD’s consistency in extremely low-frequency and very low-frequency (ELF/VLF) bands. This cross-calibration work is helpful to upgrade the data quality of CSES and brings valuable insights to similar electromagnetic detection solutions by low earth orbit satellites.
... The equatorward propagation of the middle-latitude hiss waves suggests that most of the inner belt hiss waves come from high latitudes (|MLAT| > 22.4°). Previous ray tracings indicate that outer plasmaspheric hiss Zhima et al., 2017) and lightning generated whistlers (LGWs) (Santolík et al., 2009) can propagate along magnetic field lines to high latitudes and then are refracted inwardly or outwardly. Therefore, outer plasmaspheric hiss and LGWs are two potential sources of the inner belt hiss waves but their contributions are perhaps different in different-L regions. ...
Using the Van Allen Probes A and B observations from 01 January 2013 to 28 February 2018, we surveyed statistically the occurrence rate, intensity, and propagation properties of hiss waves in the inner radiation belt (1.1 < L ≤ 2). Like the outer plasmaspheric hiss (L > 2), the occurrence rate and amplitude of lower‐band hiss (<600 Hz) are higher in the dayside high‐L region (L > 1.3 and magnetic local time [MLT] = 6–20 hr) than the nightside (MLT ∼ 20–6 hr) and increase with enhanced substorm activities (AE increases). Furthermore, their peak power spectral densities are located nearly in the same band (∼200–500 Hz). The equatorward propagation of middle‐latitude hiss suggests that the lower‐band hiss waves in the inner radiation belt mostly originate from the outer plasmaspheric hiss at high latitudes. Although the outer plasmaspheric hiss is also a likely source of weak upper‐band hiss (≥600 Hz) in the dayside high‐L region (L > 1.3 and MLT = 6–20 hr), the intense upper‐band hiss waves mostly appear in the dayside low‐L region (L < 1.3) and most nightside regions. In the low‐L region, the amplitude of the upper‐band hiss has no obvious substorm dependence, and the average of its wave normal angles is comparable to that of lightning‐generated whistlers reported in the past.
... The accurate global model of VLF transmitter signals, including their frequency spectra, wave amplitudes, propagation properties (from the Earth-ionosphere waveguide into the geospace), geomagnetic activity dependence, is crucial to quantify the exact effects of terrestrial VLF transmitters on radiation belt electrons (e.g., Abel and Thorne, 1998a;Inan et al., 1984Inan et al., , 2003Claudepierre et al., 2020;Hua et al., 2020Hua et al., , 2021, and reference therein). Very early studies dating back to 1958 (e.g., Helliwell and Gehrels, 1958;Helliwell and Morgan, 1959;Helliwell et al., 1964;Diesendorf, 1972) provided fundamental insights into the VLF transmitter wave distributions, which have been further deepened both globally and theoretically by following researches based on either observations (e.g., Clilverd et al., 2008;Parrot et al., 2009;Cohen and Inan, 2012;Ma et al., 2017;Meredith et al., 2019;Xiang et al., 2021) or simulations (e.g., Koons et al., 1981;Starks et al., 2008;Santolík et al., 2009;Tao et al., 2010;Zhang et al., 2018;Xia et al., 2020). ...
Wave-particle interactions play a fundamental role in the dynamic variability of Earth’s donut-shaped radiation belts that are highly populated by magnetically trapped energetic particles and characteristically separated by the slot devoid of high energetic electrons. Owing to the continuous accumulation of high-quality wave and particle measurements from multiple satellites in geospace, the important contribution of ground-based very-low-frequency (VLF) transmitter waves to the electron dynamics in the near-Earth space has been unprecedently advanced, in addition to those established findings of the significant effects of a variety of naturally occurring magnetospheric waves. This paper focuses on the artificial modification of Earth’s inner radiation belt and slot by artificial VLF transmitter emissions. We review the global distributions of VLF transmitter waves in geospace, their scattering effects on radiation belt electrons in terms of both theoretical and observational analyses, and diffusion simulation results of wave-particle interactions along with data-model comparisons. We start with a brief review of the radiation belt electron dynamics and an introduction of anthropogenic VLF transmitter waves. Subsequently, we review the global morphology of in situ VLF transmitter waves corresponding to different transmitter locations, including their day-night asymmetry, geographic distributions, seasonal and geomagnetic activity dependence, and wave propagation features. Existed theoretical and observational analyses of electron scattering effects by VLF transmitter waves are then reviewed to approach the underlying physics that can modulate the spatio-temporal variations of the electron radiation belts. Further Fokker-Planck electron diffusion simulations and their comparisons with realistic satellite observations clearly indicate that VLF transmitter emissions can effectively remove energetic electrons to produce a radially bifurcated electron belt, thereby quantitatively confirming the direct link between operations of VLF transmitters at ground and changes of the energetic electron environment in space. We finally discuss the unsolved problems and possible future research in this area, which has important implications for potential mitigation of the natural particle radiation environment with active means.
... We have therefore used a backward ray-tracing simulation procedure based on an earlier technique of Cerisier (1970) with a dipole magnetic field model and a diffusive equilibrium model of the plasma density distribution. The procedure includes an additional adaptive integration step algorithm with verification of the Wentzel-Krammers-Brillouin (WKB) approximation of the geometric optics (Santolík et al., 2006(Santolík et al., , 2009. ...
Measurements of electromagnetic waves in space plasmas are an important tool for our understanding of physical processes in this environment. Inter‐calibration of data from different spacecraft missions is necessary for combining their measurements in empirical models or case studies. We show results collected during a close conjunction of the Van Allen Probes and Arase spacecraft. The inter‐calibration is based on a fortuitous case of common observations of strong whistlers at frequencies between a few hundred hertz and 10 kHz, which are generated by the same lightning strokes and which propagate along very similar paths to the two spacecraft. Measured amplitudes of the magnetic field fluctuations are the same within ∼14% precision of our analysis, corresponding to 1.2 dB. Currently, archived electric field measurements show twice larger amplitudes on Arase compared to Van Allen Probes but they start to match within ∼33% precision (2.5 dB) once the newest results on the interface of the antennas to the surrounding plasma are included in the calibration procedures. Ray tracing simulations help us to build a consistent scenario of wave propagation to both spacecraft reflected by a successful inter‐calibration of the polarization and propagation parameters obtained from multicomponent measurements. We succeed in linking the spacecraft observations to localizations of lightning return strokes by two different ground‐based networks which independently verify the correctness of the Universal Time tags of waveform measurements by both spacecraft missions, with an uncertainty better than 10 ms.
... LGWs (e.g., Colman & Starks, 2013;Meredith et al., 2007;Němec et al., 2010;Ripoll et al., 2020). However, LGW wave power is often observed to extend below 2 kHz, down to a few hundred Hz (e.g., Santolík et al., 2009;Záhlava et al., 2018Záhlava et al., , 2019. Therefore, to achieve a complete understanding of LGW spectral properties, it is critical to include wave power over a broad frequency range (~100 Hz to a few tens of kHz). ...
Plain Language Summary
After initial lightning strikes, a type of plasma wave is generated, typically referred to as a lightning generated whistler (LGW), a portion of which can propagate into near‐Earth space. These waves can interact with the trapped energetic electron population in the Earth's radiation belts, causing pitch angle scattering, and thus play an important role in energetic electron loss into the Earth's upper atmosphere. Using high‐resolution wave burst data from the twin Van Allen Probes over the entire Van Allen Probes era (2012–2019), we evaluate the typical properties and global distributions of LGWs. The newly constructed LGW models are used to quantify their global effects on energetic electron loss in the near‐Earth space and indicate that LGWs play an important role in scattering electrons over a broad energy range (tens of keV to several MeV) in the inner radiation belt and beyond.
... To do so, we will use ray tracing to trace the waves from (1) the source region to each satellite and (2) the source to the station of IST. Here we use the ray tracing model described in detail in Santolík et al. (2006), Santolık et al. (2009), andSantolík et al. (2016). We use a simple 3-D ray tracing model that takes into account the diffusive equilibrium model for density and IGRF to approximate the location of the source region of the QP emission observed during the main event (Martinez-Calderon, 2016;Schunk & Nagy, 2009). ...
Recent availability of a considerable amount of satellite and ground‐based data has allowed us to analyze rare conjugated events where extremely low and very low frequency waves from the same source region are observed in different locations. Here, we report a quasiperiodic (QP) emission, showing one‐to‐one correspondence, observed by three satellites in space (Arase and the Van Allen Probes) and a ground station. The main event was on 29 November 2018 from 12:06 to 13:08 UT during geomagnetically quiet times. Using the position of the satellites we estimated the spatial extent of the area where the one‐to‐one correspondence is observed. We found this to be up to 1.21 Earth's radii by 2.26 hr MLT, in radial and longitudinal directions, respectively. Using simple ray tracing calculations, we discuss the probable source location of these waves. At ∼12:20 UT, changes in the frequency sweep rate of the QP elements are observed at all locations associated with magnetic disturbances. We also discuss temporal changes of the spectral shape of QP observed simultaneously in space and on the ground, suggesting the changes are related to properties of the source mechanisms of the waves. This could be linked to two separate sources or a larger source region with different source intensities (i.e., electron flux). At frequencies below the low hybrid resonance, waves can experience attenuation and/or reflection in the magnetosphere. This could explain the sudden end of the observations at the spacecraft, which are moving away from the area where waves can propagate.
... These timescales fit well within the range of observed durations of lightning generated radio pulses on the low-Earth orbit. The whistlers at frequencies <1.25 kHz exhibit dispersion over a few tens of milliseconds above the nightside ionosphere(Santolík et al., 2009) and longer on the dayside(Santolík et al., 2008), Waves data in the high-and low-band frequency channels of the low-frequency receiver (LFR-Hi and LFR-Lo) combine to give new constraints on the highest frequencies and shortest durations associated with lightning at frequencies below 150 kHz. Snapshots were obtained with 1-s cadence over the period from 11:58:00 to 12:00:30 on 6 April 2019 in LFR-Hi (a) and LFR-Lo (d). ...
Jupiter lightning discharges produce various kinds of phenomena including radio wave pulses at different frequencies. On 6 April 2019, the Juno Waves instrument captured an extraordinary series of radio pulses at frequencies below 150 kHz on timescales of submilliseconds. Quasi‐simultaneous multi‐instrument data show that the locations of their magnetic footprints are very close to the locations of ultrahigh frequency (UHF) sferics recorded by the Juno MWR instrument. Hubble Space Telescope images show that the signature of active convection includes cloud‐free clearings, in addition to the convective towers and deep water clouds that were also recognized in previous spacecraft observations of lightning source regions. Furthermore, the detections of 17 very low frequency/low‐frequency (VLF/LF) radio pulses suggest a minimum duration of lightning processes on the order of submilliseconds. These observations provide new constraints on the physical properties of Jupiter lightning.
... In recent years, the number of studies of whistlers originated from lightning has increased gradually from both ground-station and satellite observations. Santolik et al. [11] analyzed data measured by the DEMETER satellite in the night-side region, which are closely related to the lightning activity detected by the METEORAGE lightning detection network. Fiser et al. [12] developed software for the automatic detection of fractional-hop whistlers in the very-low-frequency (VLF) spectrograms recorded by the ICE (Instrument Champ Electrique; Electric Field Instrument) experiment onboard the DEMETER satellite and compared their result to the EUCLID (European Cooperation for Lightning Detection, www.euclid.org) ...
The automatic detection of shapes or patterns represented by signals captured from spacecraft data is essential to revealing interesting phenomena. A signal processing approach is generally used to extract useful information from observation data. In this paper, we propose an image analysis approach to process image datasets produced via plasma wave observations by the Arase satellite. The dataset consists of 31,380 PNG files generated from the dynamic power spectra of magnetic wave field data gathered from a one-year observation period from March 2017 to March 2018. We implemented an automatic detection system using image analysis to classify the various types of lightning whistlers according to the Arase whistler map. We successfully detected a large number of whistler traces induced by lightning strikes and recorded their corresponding times and frequencies. The various shapes of the lightning whistlers indicate different very-low-frequency propagations and provide important clues concerning the geospace electron density profile.
... They can propagate in a duct formed by a plasma density enhancement, which is usually nearly aligned with the ambient magnetic field, ducting waves at fre-Geophysical Research Letters 10.1029/2019GL083918 quencies below one half of the local electron cyclotron frequency (Helliwell, 1965). Alternatively, the waves can propagate unducted along the trajectory governed by the gradients of refractive index (Bortnik et al., 2003;Hughes, 1981;Maxworth & Gołkowski, 2017;Santolík et al., 2009;Smith & Angerami, 1968). The lightning power leaking from the Earth-ionosphere waveguide into the magnetosphere is strongly influenced by ionospheric attenuation and significantly depends on the direction of the local magnetic field and therefore on the geomagnetic latitude. ...
Plain Language Summary
We analyze contribution of thunderstorms to the intensity of electromagnetic radiation at audible frequencies observed at altitudes between 600 and 32,000 km, where these waves can influence the Van Allen radiation belts. We use the World Wide Lightning Location Network to obtain information about lightning locations and times. Based on that, a lightning activity level is assigned to individual electromagnetic wave measurements of two spacecraft missions: the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions and the Van Allen Probes. Subsequently, we compare median wave intensities obtained at the times of high and low lightning activity. Their ratio reveals that the radio waves originating in strong lightning storms can significantly overpower all other natural waves in a wide range of frequencies and L‐shells. The strength of this effect substantially depends on the local time. Specifically, it is the best pronounced in the afternoon/evening/night sector and nearly absent in the morning/noon sector. This agrees with the local time dependence of both, lightning occurrence and the wave attenuation in the ionosphere. The observed lightning contribution mainly occurs at frequencies over 500 Hz and with a bandwidth decreasing from 12 to 4 kHz for L between 1.5 and 5.
... The distance l at which the electromagnetic energy induced by a lightning discharge can enter into the magnetosphere, and thus, the area S ∼ π l 2 has been estimated by various authors, using different methods. Chum et al. (2006) gave the estimation l ∼ 1,500 km (see also Bourriez et al., 2016;Hayakawa et al., 1992 andSantolík et al., 2009). Vavilov and Shklyar (2014) gave the estimation of lightning-illuminated region in the Earth-ionosphere waveguide to be of the order of 4,000 km Journal of Geophysical Research: Space Physics in linear dimension, thus l ∼ 2,000 km. ...
For the first time an evaluation of the whistler rate around the Earth is performed using results from the neural network aboard the microsatellite DEMETER. It is shown that the rate of whistlers with low dispersion calculated all around the Earth as a function of longitude vary between 1 and 6 s⁻¹ during nighttime (22.30 LT) and between 0.5 and 0.7 s⁻¹ during daytime (10.30 LT). The whistler rate is anticorrelated with the F10.7‐cm solar flux. A decrease by 25% of the solar flux corresponds to an increase of 62% (26%) of the averaged whistler rate calculated for the entire Earth during nighttime (daytime). Using this averaged whistler rate, the global lightning rate is estimated to be of the order of 123 s⁻¹ (27 s⁻¹) during nighttime (daytime). The main conclusion concerns the precipitation of the electrons in the radiation belt by interaction with the whistlers. It is shown that the decrease of the lightning activity at solar minimum (shown with the help of the Schumann resonances) is largely counterbalanced by the increase of the whistler rates in the upper part of the ionosphere due to the decrease of the ionospheric absorption.
... The same assumption and justification are used in Colman & Starks (2013). Observed unducted propagation can however be computed by ray tracing techniques (e.g., Bortnik et al., 2006;Santolik et al., 2009). ...
Plain Language Summary
Lightning flashes strongly emit electromagnetic lightning‐generated waves (LGW). This radiation propagates inside the Earth‐Ionosphere waveguide and escapes into the magnetosphere. Trapped electrons originating from solar eruptions bounce and drift around Earth magnetic field lines. They can be scattered into the atmosphere by any resonant interactions with surrounding LGW that causes diffusion of the electron velocity. Computing accurately the electron diffusion is critical for space weather. This diffusion is proportional to the LGW drift intensity, that is, the LGW intensity affecting the electron along its drift around the Earth, which is not directly measurable since many satellites would be required. Here, we propose a new method that uses the ground‐based World‐Wide Lightning Location Network (WWLLN) to estimate both the local and the drift lightning power density at the Van Allen Probes (RBSP) footprints. Their ratio defines a time‐resolved WWLLN‐based LGW power density ratio, RWWLLN, which multiplied by the local LGW intensity measured by RBSP allows direct computation of LGW diffusion required in space weather codes. Statistical maps of power density and ratios are discussed. We conclude that the large geographical and seasonal variability is important to keep in assessing effects in space and that RWWLLN > 1 suggests significant LGW effects in the inner belt.
... They can be generated by various mechanisms that include electron temperature anisotropies (e.g., Gary & Madland 1985) and current or beam-driven instabilities (e.g., Zhang et al. 1999;Fujimoto 2014). Whistler waves propagating in the magnetosphere ( Santolik et al. 2009;Breuillard et al. 2012Breuillard et al. , 2014 can be guided by density enhancements or depletions (e.g., Smith 1961;Moullard et al. 2002;Streltsov et al. 2006;Li et al. 2011) and modulated by ULF waves (e.g., Xia et al. 2016). The occurrence of whistler waves is generally well correlated with the existence of magnetic structures with magnetic field strength minimum (Smith & Tsurutani 1976;Dubinin et al. 2007) or dipolarization fronts (Le Contel et al. 2009;Deng et al. 2010;Huang et al. 2012a;Li et al. 2015). ...
A new type of electron-scale coherent structure, referred to as electron vortex magnetic holes, was identified recently in the Earth's magnetosheath turbulent plasma. These electron-scale magnetic holes are characterized by magnetic field strength depression, electron density enhancement, temperature and temperature anisotropy increase (a significant increase in perpendicular temperature and a decrease in parallel temperature), and an electron vortex formed by the trapped electrons. The strong increase of electron temperature indicates that these magnetic holes have a strong connection with the energization of electrons. Here, using high time resolution in situ measurements from the MMS mission, it is further shown that electron-scale whistler waves coexist with electron-scale magnetic holes. These whistler waves were found not propagating from remote regions, but generated locally due to electron temperature anisotropy (T eperp;/T e∥) inside the magnetic holes. This study provides new insights into the electron-scale plasma dynamics in turbulent plasmas. © 2018. The American Astronomical Society. All rights reserved.
... Each lightning stroke produces a short pulse of electromagnetic emissions over a wide frequency range called spheric (sometimes spelled as sferic). A part of these emissions can escape from the Earth-ionosphere waveguide and propagate through the plasma environment to higher altitudes (Fišer et al., 2010;Santolík & Parrot, 1996Santolík et al., 2009;Walker, 1976). Considering that plasma is a dispersive medium, the group velocity of waves is a function of their frequency. ...
We investigate the influence of lightning-generated whistlers on the overall intensity of electromagnetic waves measured by the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions spacecraft (2004-2010, quasi Sun-synchronous polar orbit with an altitude of about 700 km) at frequencies below 18 kHz. Whistler occurrence rate evaluated using an onboard neural network designed for automated whistler detection is used to distinguish periods of high and low whistler occurrence rates. It is shown that especially during the night and particularly in the frequency-geomagnetic latitude intervals with a low average wave intensity, contribution of lightning-generated whistlers to the overall wave intensity is significant. At frequencies below 1 kHz, where all six electromagnetic wave components were measured during specific intervals, the study is accompanied by analysis of wave propagation directions. When we limit the analysis only to fractional-hop whistlers, which propagate away from the Earth, we find a reasonable agreement with results obtained from the whole data set. This also confirms the validity of the whistler occurrence rate analysis at higher frequencies.
... In this test, we have acquired magnetic field signal (VLF band) from the analog backend in which a continuous 8 kHz communication channel is visible (portion A of figure 7). Also, we have got some wide band (portion B and C of figure 7) spikes (sferics) and by from initial visual observations we found that the received signal is very similar to GC lightning sferics [18] [19]. ...
Lightning location information plays a significant role in proper and safe working of Power utility staff, Air traffic managers, etc. To capture the location of lightning strokes, Lightning Detection Networks (LDN) are widely used. LDN comprises of several such sensor nodes distributed in a certain geographical area which captures lightning signals. The single sensor node system consists of the antenna, analog circuit front end, and digital back end. These sensor nodes then transmit lightning data in real time to central lightning location system (CLLS) which calculates the exact location of the lightning stroke. We have designed and implement cost effective digital back-end system for a sensor node which digitizes analog signals and performs some signal processing in the digital domain before sending the data to CLLS. Analog front-end receives and amplifies lightning very low frequency signal (VLF) signals, then presented digital system samples the analog signals at a rate of 1 Mega samples per second. This is followed by digital filtering and digital down conversion; after which the data samples are time-stamped with the corresponding real time (UTC time) then this data is sent to CLLS by UART link. This digital design has been simulated and implemented on the Xilinx Spartan 6 series lx9csg324 FPGA board. Presented digital system has been tested on LabVIEW GUI and Xilinx ChipScope pro analyzer and received signals spectrogram has been calculated. The Proposed work of sensor node back-end is carried out for the development of the North-East Indian LDN system with resource efficiency and has real-time features.
link : http://ieeexplore.ieee.org/document/8203933/
... The generation mechanism of ionospheric hiss is a challenging issue due to the complicated atmosphere-ionosphere-magnetosphere system. Except for the downward magnetospheric emission (e.g., Bortnik et al., 2008, Chen et al., 2017, and reference therein), the lightning activities from the atmosphere also serve as another embryonic source (Green et al., 2005;Meredith, Horne, Clilverd, et al., 2006;Santolík et al., 2009). In Figures 2a and 2b, the intense emissions at 14:50 to 14:54 UT show a series of burst spectra as vertical lines over the whole frequency range are induced by lightning activities. ...
We present a conjugate observation on whistler mode electromagnetic hiss from the low Earth orbit satellite Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) and the high-altitude elliptical orbit spacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS). The conjugate observation was performed at 14:51:10 to 15:12:00 UT on 15 June 2010, when DEMETER was flying across the L shell region from ~1.39 to 2.80 at an altitude of ~660 km; meanwhile, THEMIS probes were passing through the L shell region from ~1.64 to 1.91 at altitudes from ~1.6 to 2.0 RE. The conjugated observations demonstrate similar time-frequency structures between the ionospheric hiss (~350 to 800 Hz) captured by DEMETER and the plasmaspheric hiss (~350 to 900 Hz) recorded by THEMIS probes, including similar peak frequencies (~500 to 600 Hz), similar lower cutoff frequencies (~350 to 400 Hz), and upper cutoff frequencies (~730 to 800 Hz). The wave vector analyses show that the ionospheric hiss propagates obliquely downward to the Earth and slightly equatorward with right-handed polarization, suggesting that its source comes from higher altitudes. Ray tracing simulations with the constraint of observations verify that the connection between ionospheric and plasmaspheric hiss is physically possible through wave propagation. This study provides direct observational evidence to support the mechanism that high-altitude plasmaspheric hiss is responsible for the generation of low-altitude ionospheric hiss.
... DEMETER then detects these reflected waves at lower geomagnetic latitudes. To show the possibility of such propagation, we employed a ray-tracing simulation using a 3-D ray-tracing code with continuous verification of the Wentzel-Kramers-Brillouin (WKB) approximation of geometric optics, described by Santolík et al. [2009]. Our objective is to initiate the ray-tracing procedure with the observed wave vectors of earthward propagating waves at the spacecraft position and then compare the theoretical kB values of the ionospherically reflected waves with the observed values in the appropriate latitudinal range. ...
Quasiperiodic (QP) electromagnetic emissions are whistler mode waves at typical frequencies of a few kHz characterized by a periodic time modulation of their intensity. The DEMETER spacecraft observed events where the QP emissions exhibit a sudden change in the wave vector and Poynting vector directions. The change happens in a short interval of latitudes. We explain this behavior by ionospheric reflection and present a ray tracing simulation which matches resulting wave vector directions. We also attempt to locate the source region of these emissions and conclude that they are most probably generated at the inner boundary of the plasmapause which also acts as a guide during the propagation of the QP emissions.
... For our ray tracing simulation we assume that the trajectory of the wave is invariant to time reversal. In order for the wave to penetrate the lower layers of the ionosphere, the back tracing starting point above ATH should be such that the wave vector ⃗ k is approximately perpendicular to the ground [Santolik et al., 2009]. We also assume that the source of the QP emission is punctual and located at the geomagnetic equatorial plane; thus, the ending point of the back trace is the intersection of the ray and the equatorial plane. ...
We report simultaneous observation of ELF/VLF emissions, showing similar spectral and frequency features, between a VLF receiver at Athabasca (ATH), Canada, (L = 4.3) and Van Allen Probes A (Radiation Belt Storm Probes (RBSP) A). Using a statistical database from 1 November 2012 to 31 October 2013, we compared a total of 347 emissions observed on the ground with observations made by RBSP in the magnetosphere. On 25 February 2013, from 12:46 to 13:39 UT in the dawn sector (04–06 magnetic local time (MLT)), we observed a quasiperiodic (QP) emission centered at 4 kHz, and an accompanying short pulse lasting less than a second at 4.8 kHz in the dawn sector (04–06 MLT). RBSP A wave data showed both emissions as right-hand polarized with their Poynting vector earthward to the Northern Hemisphere. Using cross-correlation analysis, we did, for the first time, time delay analysis of a conjugate ELF/VLF event between ground and space, finding +2 to +4 s (ATH first) for the QP and −3 s (RBSP A first) for the pulse. Using backward tracing from ATH to the geomagnetic equator and forward tracing from the equator to RBSP A, based on plasmaspheric density observed by the spacecraft, we validate a possible propagation path for the QP emission which is consistent with the observed time delay.
... One can see at 05:58:22 UT (indicated by a white arrow) a very intense whistler without noticeable dispersion (see also the second spectrogram), which is very different from the usual dispersion of 0 ? whistlers observed by DEMETER [Santolik et al. 2008[Santolik et al. , 2009]. The origin of this whistler is due to a powerful positive cloud-to-ground (?CG) lightning stroke with a very large peak current of 180 kA which occurred above the Pacific ocean, approximately 80 km Other very intense lightning strokes occurred before the arrival of the satellite in the same area. ...
DEMETER was a low Earth orbiting microsatellite in operation between July 2004 and December 2010. Its scientific objective was the study of ionospheric perturbations in relation to seismic activity and man-made activities. Its payload was designed to measure electromagnetic waves over a large frequency range as well as ionospheric plasma parameters (electron and ion densities, fluxes of energetic charged particles). This paper will show both expected and unusual events recorded by the satellite when it was in operation. These latter events have been selected from the DEMETER database because they are rare or even have never been observed before, because they have a very high intensity, or because they are related to abnormalities of the experiments under particular plasma conditions. Some events are related to man-made radio waves emitted by VLF ground-based transmitters or power line harmonic radiation. Natural waves, such as atypical quasi-periodic emissions or uncommon whistlers, are also shown.
... Previous studies show that hiss is usually observed at frequencies below 3 kHz [Thorne et al., 1979;Santolík et al., 2006a;Bortnik et al., 2008b;Chen et al., 2012] and is rarely seen above 3 kHz . At the DEMETER orbit, many whistlers are observed and some of them are associated with lightning activities , Santolík et al., 2009. Whistler mode chorus which usually propagates outside the plasmapsphere Li et al., 2009a;Fu et al., 2012b] is also observed by DEMETER [Zhima et al., 2013]. ...
In this paper, using the data of sunsynchronous satellite DEMETER, we investigated the storm-time variations of ELF/VLF waves during the intense CME-driven storms from 2005 to 2009. The results show that there is a good correlation between the enhancement of ELF/VLF waves and the CME events. Immidately following the enhanced wave activity driven by CMEs during the initial phase, the wave intensity decreases temporarily at the beginning of storm main phase. The strongest waves predominantly occur from the late main phase to early recovery phase. The ELF waves below 3 kHz are significantly intensified during the whole storm time, while the high-frequency waves above 3 kHz seem strengthened only during the late main and early recovery phase. The ELF waves below 3 kHz can exist in a wide L-shell range, with the intensity peaking at L ~ 3-4. High-frequency waves at f > 9 kHz exist mostly outside the plasmapause. The stronger ELF/VLF waves on the dayside can last longer time than those on the nightside.
Lightning-generated whistlers profoundly affect the energetic particle population in Earth’s radiation belts, influencing space weather and endangering astronauts and satellites. We report the discovery of specularly reflected (SR) whistler in which the lightning energy injected into the ionosphere at low latitudes reaches the magnetosphere after undergoing a specular reflection in the conjugate ionosphere, contradicting previous claims that lightning energy injected at low latitudes cannot escape the ionosphere. SR whistlers provide a low-latitude channel to transport lightning energy to the magnetosphere. We calculate the relative contributions of SR, magnetospherically reflected, subprotonospheric, and ducted whistlers to the lightning energy reaching the magnetosphere. When SR whistlers are considered, the global lightning energy contribution to the magnetosphere doubles, implying that the previous estimates of the impact of lightning energy on radiation belts may need substantial revisions. Whistler dispersion and intensity analyses quantitatively confirm our results and suggest new remote-sensing methods of the magnetosphere, ionosphere, Earth-ionosphere waveguide, and lightning flashes.
We evaluate average wave intensities at frequencies up to 10 kHz measured by two ground stations in Canada and two others in Finland at auroral and subauroral latitudes over a full year, as well as by the low‐altitude DEMETER spacecraft during the years 2004–2010. Lightning location and energy data obtained by the World Wide Lightning Location Network, along with geomagnetic activity characterized by the Kp index, are further used. Latitudinal, diurnal, and annual variations are analyzed, and the global intensities measured on the ground and by the spacecraft are systematically compared for the first time. We show that lightning‐generated waves often dominate the measured wave intensities, particularly during the night, in summer, and at higher frequencies. DEMETER observations, supported by ray‐tracing analysis, reveal a significant role of nonducted lightning‐generated whistler propagation between the hemispheres. Finally, the wave intensity response to geomagnetic activity variations is quite different on the ground compared to in space. While spacecraft‐measured wave intensities are considerably larger during periods of enhanced geomagnetic activity, the ground‐based intensities are only sporadically enhanced during geomagnetically active periods.
We analyze continuous multi‐point measurements of electromagnetic field waveforms onboard the Cluster spacecraft in order to contribute to the discussion on sources of plasmaspheric hiss, known as a shaping agent for the Earth radiation belts. In our case study we aim at finding sources of hiss observed close to the geomagnetic equator in the outer plasmasphere on the dayside. We find hiss to be triggered from whistlers of different spectral properties. Whistlers with the lowest observed dispersion arrive to different spacecraft with time delays indicating their origin in the northern hemisphere. Positions of source lightning discharges are then found using the time coincidences with the Word Wide Lightning Location Network data from three active thunderstorm regions in Europe. We find that subionospheric propagation of lightning atmospherics is necessary to explain the observations. Geographic locations of their ionospheric exit points then determine spectral properties of resulting unducted whistlers and triggered hiss. By this well documented chain of events starting with a lightning discharge in the atmosphere we confirm that magnetospherically reflecting whistlers and hiss triggered from them are among possible sources of plasmaspheric hiss.
This study reports the temporal and spatial distributions of the extremely/very low
frequency (ELF/VLF) wave activities, as well as the energetic electron fluxes in the ionosphere during an intense storm (Dst ~ -174 nT) occurred on Aug. 26, 2018, based on the observations by a set of detectors onboard the China Seismo-Electromagnetic Satellite (CSES). A good correlation of the ionospheric ELF/VLF wave activities with energetic electron precipitations during the various storm evolution phases is revealed. The strongest ELF/VLF emissions at a broad frequency band extending up to 20 kHz occurred from the near-end main phase to the early recovery phase of the storm, while
during other phases the wave activities mainly appeared at the frequency range below 6 kHz. Variations in the precipitating fluxes were also spotted in correspondence with changing geomagnetic activity, with the max values primarily appearing outside of the plasmapause during active conditions. The energetic electrons at energies below 1.5 MeV got strong enhancements during the whole storm time on both the day and night side. The examinations based on the half-orbit data show that under quiet condition, CSES is able to well depict the outer/inner radiation belt and slot region, whereas under disturbed conditions, such regions become less sharply defined. The regions poleward from geomagnetic latitudes over 50° host the most robust electron precipitation irrespective
of quiet or active conditions, in the regions equatorward below 30°, fluxes enhancements were mainly observed during storm time, and only occasionally in quiet time. The nightside ionosphere also shows remarkable temporal variability along with storm evolution process but with a relatively weaker wave activities, but similar level of fluxes enhancement compared to the ones on dayside ionosphere. The ELF/VLF whistler-mode waves recorded by CSES mainly include structure-less VLF waves, structured VLF quasi-periodic emissions, structure-less ELF hiss waves etc. Wave vector analysis shows that during storm time these ELF/VLF whistler-mode waves obliquely propagate
mostly likely from the radiation belt toward the Earth direction. We suggest that energetic electrons in the high latitude ionosphere most likely transport from the outer radiation belt as a consequence of their interactions with ELF/VLF waves.
The Communication/Navigation Outage Forecast System (C/NOFS) satellite's VEFI payload performed frequent recordings of the vector electric field in the band 0–16 kHz during the epoch 2008–2014. The Vector Electric Field Instrument (VEFI) was supported by ion‐composition data from the Coupled Ion Neutral Dynamics Investigation (CINDI) instrument. We focus here on statistics of these “burst‐mode” recordings, of which 6,890 (mostly ~12‐s duration) records meet stringent quality‐control criteria, allowing inference of the wave vector k and its orientation relative to the Earth's magnetic field B0. The 6,890 records occur between ±13° (geographic) latitude and between ~ 400‐ and 850‐km altitude, mostly in the topside ionosphere. The wave activity is dominated by terrestrial lightning. We analyze the whistler‐wave intensity and polarization for each pixel in the time‐frequency spectrogram for each record. We then gather weighted statistics on wave polarization, naturally weighted by wave intensity. In this manner we arrive at statistical results that represent the bulk of the energy flow due to whistler waves. Despite rather nonstationary statistics, we can reach three empirical results. We see no evidence of a low‐latitude suppression of whistler‐wave activity, in contrast to the predictions of models of transmission through a laminar ionosphere.
The wave vector polar angle is always in the range 40° to 90° from parallel to B0. This indicates that the propagation at low latitudes is dominated by oblique, not ducted, whistlers.
At the lowest magnetic latitudes, the wave vector polar angle with respect to B0 becomes nearly 90°.
Previously published statistics based on Cluster spacecraft measurements surprisingly show that in the outer radiation belt, lower band whistler mode waves predominantly propagate unattenuated parallel to the magnetic field lines up to midlatitudes, where ray tracing simulations indicated highly attenuated waves with oblique wave vectors. We explain this behavior by considering a large fraction of ducted waves. We argue that these ducts can be weak and thin enough to be difficult to detect by spacecraft instrumentation while being strong enough to guide whistler mode waves in a cold plasma ray tracing simulation. After adding a tenuous hot electron population, we obtain a strong effect of Landau damping on unducted waves, while the ducted waves experience less damping or even growth. Consequently, the weighted average of amplitudes and wave normal angles of a mixture of ducted and unducted waves provides us with strong quasi‐parallel waves, consistent with the observations.
Due to its polar orbit Cluster spacecraft crossed plasmaspheric plumes out of the magnetic equatorial plane. We study the occurrence of broadband, narrowband, and rising tone emissions in the plume vicinity, below the local proton gyrofrequency. Based on a database of 935 Cluster plumes crossings, reduced to 189 unique plumes, we find that broadband activity is the most common case. We confirm result from a previous study showing that plume vicinity is not a preferred place for observing narrowband emissions. Rising tones are the less frequently observed of these three kinds of emissions. Nevertheless, ElectroMagnetic Ion Cyclotron (EMIC) rising tone occurrence rate is high compared to the narrowband one: Tones are seen in six of 30 plume events (20%) when narrowband emissions are observed. Rising tones are observed at absolute magnetic latitudes larger than 17° and up to 35°. We detail the 16 August 2005 plume crossing when a rising tone is observed. Results of a ray tracing analysis agree with a tone triggering process taking place above 15° of magnetic latitude.
We use the measurements performed by the DEMETER (2004–010) and the Van Allen Probes (2012–016, still operating) spacecraft to investigate the longitudinal dependence of the intensity of whistler mode waves in the Earth's inner magnetosphere. We show that a significant longitudinal dependence is observed inside the plasmasphere on the nightside, primarily in the frequency range 400 Hz to 2 kHz. On the other hand, almost no longitudinal dependence is observed on the dayside. The obtained results are compared to the lightning occurrence rate provided by the Optical Transient Detector/Lightning Imaging Sensor mission normalized by a factor accounting for the ionospheric attenuation. The agreement between the two dependencies indicates that lightning-generated electromagnetic waves may be responsible for of the observed effect, thus substantially affecting the overall wave intensity in the given frequency range. Finally, we show that the longitudinal dependence is most pronounced for waves with oblique wave of normal angles.
During the Juno perijove explorations from 27 August 2016 through 1 September 2017, strong electromagnetic impulses induced by Jupiter lightning were detected by the Microwave Radiometer (MWR) instrument in the form of 600-MHz sferics and recorded by the Waves instrument in the form of Jovian low-dispersion whistlers discovered in waveform snapshots below 20 kHz. We found 71 overlapping events including sferics, while Waves waveforms were available. Eleven of these also included whistler detections by Waves. By measuring the separation distances between the MWR boresight and the whistler exit point, we estimated the distance whistlers propagate below the ionosphere before exiting to the magnetosphere, called the coupling distance, to be typically one to several thousand of kilometers with a possibility of no subionospheric propagation, which gives a new constraint on the atmospheric whistler propagation.
The Slovak Organisation for Space Activities (SOSA) has been active in various types of space-related activities since its establishment in 2009. They include launching stratospheric balloons, the development of the first Slovak suborbital rocket ARDEA, a spaceflight simulator and many others. SOSA, together with the Žilina University in Žilina, the Slovak Technical University in Bratislava, the Technical University Košice and a handful of private companies, have designed and created the first Slovak satellite skCUBE, which is due to be launched into space in 2017. The numerous unique technologies, which have been developed during the creation of skCUBE, are now being sold as products internationally. Recently, an effort to create a new space vision and strategy for Slovakia has been initiated by SOSA, together with several Slovak universities and international partners, including the Technion – Israel Institute of Technology and the ETH Zurich University. Their plan is to develop and launch a CubeSat fleet to detect gamma ray bursts from space. Part of this initiative is also the establishment of space incubators by SOSA across Slovakia to train and involve students directly space-related technologies.
Automated wave-feature detection is required to efficiently analyze large archives of broadband Very-Low-Frequency recordings for discrete-whistler identification and feature extraction. We describe a new method to do this, even in the presence of simultaneous, multiple whistler phase dispersions. Previous techniques of whistler identification were unable to deal with simultaneous, multiple phase dispersions. We demonstrate the new method with data from the VEFI payload on the C/NOFS satellite, from the mission years 2008 - 2014
Equatorial noise (often phenomenologically described as magnetosonic waves in the literature) is a natural electromagnetic emission which is generated by instability of ion distributions, and which can interact with electrons in the Van Allen radiation belts. We use multicomponent electromagnetic measurements of the DEMETER spacecraft to investigate if equatorial noise propagates inward down to the Earth. Analysis of a selected event recorded under disturbed geomagnetic conditions shows that equatorial noise can be observed at an altitude of 700 km, while propagating radially downward as a superposition of spectral lines from different distant sources observed at frequencies both below and above the local proton cyclotron frequency. Changes in the local ion composition encountered by the waves during their inward propagation disconnect the identified wave mode from the low frequency magnetosonic mode. The local ion composition also induces a cutoff which prevents the waves from propagating down to the ground.
Lightning-generated whistler waves are electromagnetic plasma waves in the Very Low Frequency (VLF) band, which play an important role in the dynamics of radiation belt particles. In this paper, we statistically analyze simultaneous waveform data from the Van Allen Probes (RBSP), and global lightning data from the World Wide Lightning Location Network (WWLLN). Data were obtained between July to September 2013 and between March to April 2014. For each day during these periods, we predicted the most probable 10 minutes for which each of the two RBSP satellites would be magnetically conjugate to lightning producing regions. The prediction method uses integrated WWLLN stroke data for that day obtained during the three previous years. Using these predicted times for magnetic conjugacy to lightning activity regions, we recorded high time resolution, burst mode waveform data. Here we show that whistlers are observed by the satellites in more than 80% of downloaded waveform data. About 22.9% of the whistlers observed by RBSP are one-to-one coincident with source lightning strokes detected by WWLLN. About 40.1% more of whistlers are found to be one-to-one coincident with lightning if source regions are extended out 2000 km from the satellites footpoints. Lightning strokes with far-field radiated VLF energy larger than about 100 J are able to generate a detectable whistler wave in the inner magnetosphere. One-to-one coincidences between whistlers observed by RBSP and lightning strokes detected by WWLLN are clearly shown in the L-shell range of L = 1 - 3. Nose whistlers observed in July 2014 show that it may be possible to extend this coincidence to the region of L ≥ 4.
When the Akebono (EXOS-D) satellite passed through the plasmasphere, a series of lightning whistlers was observed by its analog wideband receiver (WBA). Recently, we developed an intelligent algorithm to detect lightning whistlers from WBA data. In this study, we analyzed two typical events representing the clear dispersion characteristics of lightning whistlers along the trajectory of Akebono. The event on March 20, 1991 was observed at latitudes ranging from 47.83° (47,83°N) to ~11.09° (11.09°S) and altitudes between ~2232 and ~7537 km. The other event on July 12, 1989 was observed at latitudes from 34.94° (34.94°N) and ~41.89° (41.89°S) and altitudes ~1420-~7911 km. These events show systematic trends; hence, we can easily determine whether the wave packets of lightning whistlers originated from lightning strikes in the northern or the southern hemispheres. Finally, we approximated the path lengths of these lightning whistlers from the source to the observation points along the Akebono trajectory. In the calculations, we assumed the dipole model as a geomagnetic field and two types of simple electron density profiles in which the electron density is inversely proportional to the cube of the geocentric distance. By scrutinizing the dipole model we propose some models of dispersion characteristic that proportional to the electron density. It was demonstrated that the dispersion D theoretically agrees with observed dispersion trend. While our current estimation is simple, it shows that the difference between our estimation and observation data is mainly due to the electron density profile. Furthermore, the dispersion analysis of lightning whistlers is a useful technique for reconstructing the electron density profile in the Earth's plasmasphere. Copyright © 2012 The Institute of Electronics, Information and Communication Engineers.
We statistically analyzed lightning whistlers detected from the analog waveform data below 15 kHz observed by the VLF instruments onboard AKEBONO. We examined the large amount of data obtained at Uchinoura Space Center in Japan for 22 years from 1989 to 2010. The lightning whistlers were mainly observed inside the L-shell region below 2. Seasonal dependence of the occurrence frequency of lightning whistlers has two peaks around July to August and December to January. As lightning is most active in summer in general, these two peaks correspond to summer in the northern and southern hemispheres, respectively. Diurnal variation of the occurrence frequency showed that lightning whistlers begin to increase in the early evening and remain at a high occurrence level through the night with a peak around 21 in MLT. This peak shifts toward night side compared with lightning activity, which begins to rise around noon and peaks in the late afternoon. This trend is supposed to be caused by attenuation of VLF wave in the ionosphere in the daytime. Comparison study with the ground-based observation revealed consistent results, except that the peak of the ground-based observation appeared after midnight while our measurements obtained by AKEBONO was around 21 in MLT. This difference is explained qualitatively in terms of that lightning whistlers measured at the ground station passed through the ionosphere twice above both source region and the ground station. These facts provide an important clue to evaluate quantitatively the absorption effect of lightning whistler in the ionosphere.
[1] Satellites in the Earth's magnetosphere can be used to record the rich electromagnetic wave activity due to terrestrial lightning, typically up to several tens of kHz. With simultaneous recordings of the three components of wave electric field E and of the three components of wave magnetic field B, the entire wave field, polarization, and wavevector can be specified without any appeal to a priori assumptions about the wave mode, and without any reliance on the validity of a dispersion relation. However, some satellites lack such a complete suite of measurements. We develop a method which assumes the theoretical dispersion relation for whistler waves, then uses recordings of the three components of wave electric field E but no magnetic components, to derive the wave polarization and the wavevector (up to a sign ambiguity on the latter). The method can work only because the dispersion relation, which is assumed, already contains information from the full Maxwell's Equations. We illustrate the method with 12-sec-duration simultaneous recordings, at 32 kilosamples/sec, of three orthogonal components of wave electric field E from the C/NOFS satellite in low-Earth orbit. Our particular example in this article is shown to contain two broadband whistler features in the range 4-15 kHz, whose wavevectors differ both according to their polar angles from the geomagnetic field B0, and according to their azimuth around the geomagnetic field B0.
[1] A climatology of VLF (very low frequency) wave intensity from lightning in the plasmasphere is constructed. Starting from Optical Transient Detector/Lightning Imaging Sensor (OTD/LIS) lightning data representing 1995–2005, a climatology of strikes is assembled with 1° × 1° latitude-longitude spatial resolution, averaged into 2 h bins for each month of the year. Assuming a linear relationship between optical flash rate and VLF power flux, and that the VLF amplitude drops off as one over distance, a proxy for VLF power is developed. A typical lightning spectrum is applied and the values are scaled by appropriate transionospheric absorptions for each time and place. These values are mapped along geomagnetic field lines in order to compare them to E-field spectral densities measured by the DEMETER satellite between 2005 and 2009. An overview of the DEMETER survey mode data is presented which leads to the best scaling of the lightning VLF climatology in LEO (low earth orbit). The resulting data set represents a monthly, 2-hour, solar minimum climatology of VLF wave intensity from lightning in LEO. Finally, the E-field spectral densities are converted to Poynting flux, mapped to the plasmasphere, and converted to B-field spectral densities. Good overall agreement is found with previous observations and estimates. This new climatology is expected to have a significant impact on calculations of pitch-angle diffusion for relativistic electrons in the inner radiation belt.
We present a novel technique designed to calculate the detailed differential number flux signature (as a function of energy and time) of precipitating radiation-belt electrons, driven by a magnetospherically reflecting (MR) whistler wave, initiated by a single cloud-to-ground lightning discharge. Our model consists of several stages. First, we calculate the MR whistler wave characteristics at 1° latitude intervals along a given field-line. This is accomplished using an extensive ray tracing and interpolation algorithm involving ~120 million rays, and accounting for the effects of Landau damping, spatial, and temporal dispersion. We then use these wave characteristics to compute the pitch angle changes of resonant electrons by assuming that the interactions are linear, and independent between adjacent latitude and wave frequency bins. The pitch angle changes are transformed to precipitating flux using a novel convolution method and displayed as a function of particle energy and time at the feet of a given field line. We have calculated and compared the differential number flux signatures at the northern and southern feet of the L = 2.3 and L = 3 field lines, and found that precipitation onset and duration times increase with latitude (consistent with previous work). The precipitation consists of suprathermal Landau resonance electrons (E 10 keV) which are intense but contribute little to the total energy flux, a flux gap (10 keV E 80 keV) corresponding to a change in coupling mechanism from Landau resonance to gyroresonance, and a series of precipitation swaths (E 80 keV) corresponding to gyroresonance interactions. The swaths result in periodic maxima in the precipitated energy flux, which correspond to the equatorial traversals of the underlying MR whistler wave energy. Global precipitation signatures were computed for a number of lightning discharge latitudes and are presented in a companion paper (Bortnik et al., 2006).
A new whistler phenomenon has been identified through measurements at ground stations, on an Aerobee rocket between 100 and 200 km, and on the Alouette satellite at 1000 km. The new phenomenon is called the 'subprotonospheric' or 'SP' whistler, since most of its path appears to be restricted to the region below about 1000 km. The first example of an SP whistler was reported by Barrington and Belrose. In the present report a large number of observations are summarized, and the basic characteristics of the new phenomenon are de- scribed. Experimental results are presented which suggest that the whistler ray path is confined to the region between roughly 100- and 1000-km altitude, and that the whistler energy can echo back and forth between these levels. The SP phenomenon occurs mostly at night, typically within a few hours after sunset. SP events are often observed over a period of one or two hours in duration and, for a single Alouette pass, have been observed over a north-south range as great as 2000 km in extent. The evidence. suggests that the SP phenomenon occurs mostly near sunspot minimum and at dipole latitudes greater than 45 degrees.
The paper, which is in two parts, describes an investigation of the nature and origin of the 'whistling atmospherics' or 'whistlers' which are sometimes observed at frequencies below 15 kc/s. The first part describes an experimental study of their properties, in the course of which a considerable number of whistlers were recorded and analyzed, and the law of the variation of their frequency with time determined. Some whistlers are heard to follow impulsive atmospherics, and these are found to be produced in the normal way by lightning strokes taking place within a distance of about 2000 km. Other whistlers are unaccompanied by atmospherics; they differ from the former type in several further respects. The diurnal and annual variations of the properties of both types of whistler have also been studied. In the second part of the paper a theory of the origin of the whistling atmospherics, originally due to Barkhausen (1930) and Eckersley (1935), is developed in detail. The theory proposes that they are due to waves which originate in normal impulsive atmospherics and travel through the outer ionosphere, following the lines of force of the earth's magnetic field and crossing over the equator at a great height. During their journey they become dispersed so as to arrive as 'whistlers'. They may be reflected from the earth's surface back along the same path, one or more times, to produce whistlers with increased dispersions. The effects responsible for the guiding of the waves along the lines of the geomagnetic field provide sufficient focusing action to prevent the energy from being spread unduly. Measurements of the degree of dispersion of the whistlers have been interpreted to yield information about the density of electrons in the atmosphere at very great heights. The density required seems considerably larger than could reasonably have been expected. If the free electrons are produced by ionization of the terrestrial atmosphere its temperature in these regions must be at least 7200 degrees K. The results might alternatively be explained on the assumption that the electrons are falling in from outside, and if this were so it might account for the relationship between the occurrence of whistlers and magnetic activity.
We report observations of waves near the local proton cyclotron frequency and its lowest harmonics, made by the Plasma Wave Instrument on board the Polar spacecraft, on orbits for which the perigee (at an altitude of 1 Earth radius) was in or near the southern auroral zone. The electromagnetic nature of these waves was revealed by measuring their magnetic components simultaneously with two independent antenna systems, one a single loop and the other a set of triaxial search coils. Waves of this kind were found in the southern auroral zone on about a third of the orbits examined. Peaks in the magnetic field power spectra occurred at frequencies both below and above the fundamental cyclotron frequency. The ratio of the amplitudes of the electric and magnetic fluctuations was usually greater than the speed of light, suggesting that we observe a magnetic component related to the wave mode customarily called electrostatic ion cyclotron waves. The fluctuating vector of the wave magnetic field was in or close to the plane perpendicular to the static magnetic field. Our main result is that its polarization, which covered a broad range from left-hand elliptic to right-hand elliptic, can be explained by the superposition of many linearly polarized waves.
1] DEMETER spacecraft detects short bursts of lightning-induced electron precipitation (LEP) simultaneously with newly-injected upgoing whistlers, and sometimes also with once-reflected (from conjugate hemisphere) whistlers. For the first time causative lightning discharges are definitively geo-located for some LEP bursts aboard a satellite. The LEP bursts occur within <1 s of the causative lightning and consist of 100– 300 keV electrons. First in-situ observations of large regions of enhanced background precipitation are presented. The regions are apparently produced and maintained by high rate of lightning within a localized thunderstorm.
Energetic electrons (E > 100 keV) in the Earth's radiation belts undergo Doppler-shifted cyclotron resonant interactions with a variety of whistler mode waves leading to pitch angle scattering and subsequent loss to the atmosphere. In this study we assess the relative importance of plasmaspheric hiss and lightning-generated whistlers in the slot region and beyond. Electron loss timescales are determined using the Pitch Angle and energy Diffusion of Ions and Electrons (PADIE) code with global models of the spectral distributions of the wave power based on CRRES observations. Our results show that plasmaspheric hiss propagating at small and intermediate wave normal angles is a significant scattering agent in the slot region and beyond. In contrast, plasmaspheric hiss propagating at large wave normal angles and lightning-generated whistlers do not contribute significantly to radiation belt loss. The loss timescale of 2 MeV electrons due to plasmaspheric hiss propagating at small and intermediate wave normal angles in the center of the slot region (L = 2.5) lies in the range 1-10 days, consistent with recent Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) observations. Wave turbulence in space, which is responsible for the generation plasmaspheric hiss, thus leads to the formation of the slot region. During active periods, losses due to plasmaspheric hiss may occur on a timescale of 1 day or less for a wide range of energies, 200 keV < E < 1 MeV, in the region 3.5 < L < 4.0. Plasmaspheric hiss may thus also be a significant loss process in the inner region of the outer radiation belt during magnetically disturbed periods.
1] Using ray tracing and Landau damping calculations based on recent data on suprathermal particle fluxes from the HYDRA instrument aboard the POLAR satellite, we estimate the energy distribution and the lifetimes of 0.2–10 kHz whistler mode waves in the plasmasphere. The rays are injected at 1000 km altitude and latitudes of 25°, 35°, 45°, and 55° to represent whistler wave energy originating in lightning discharges occurring in middle to low-latitude thunderstorms. The lifetime is defined as the time at which the wave power is reduced by 10 dB. Results indicate that the lifetime of whistler waves at lower frequencies is dramatically larger than those at higher frequencies and that rays injected at lower latitudes generally persist longer than those injected at higher latitudes in agreement with previous studies. An important characteristic of magnetospherically reflected (MR) whistlers is the strong tendency for whistler wave components at each frequency to eventually migrate to and settle into a multiply reflecting pattern at a specific (determined only by the wave frequency) L-shell, at which the wave energy would persist indefinitely in the absence of Landau damping and other losses. Consideration of this behavior together with the typical power spectrum of a single, vertical, cloud-to-ground lightning stroke, allows the estimation of the relative MR whistler wave energy in the inner magnetosphere as a function of L-shell. Results indicate that MR whistler energy deposition is maximized at the location of the slot region, suggesting that such MR whistlers launched by lightning discharges may be responsible for the enhanced diffusion rates and may play a more significant role than previously assumed in the formation and maintenance of the slot region between the inner and outer radiation belts.
1] Lightning generated whistlers are ubiquitous within the plasmasphere at both high and low altitudes, and these waves propagate efficiently in both ducted and nonducted modes. On the other hand, in the magnetospheric region outside the plasmasphere, lightning-generated whistlers are commonly observed at low altitudes (up to $6000 km) but only rarely at higher altitudes near the magnetic equatorial plane. The reasons for the lack of these waves at higher altitudes are not well understood. In the present paper we use data from the Wide Band Plasma (WBD) instruments on the four Cluster spacecraft to study the characteristics of lightning-generated whistlers observed on 4 separate days in 2001 at L shells ranging from L = 4 to L = 5, magnetic latitudes ranging from À20° to 10°, and Kp indices ranging from 3 to 6. The propagation paths of the lightning-generated whistlers are determined using a two-dimensional ray-tracing model to calculate the ray paths and group delays from the lower ionosphere to each of the four Cluster spacecraft over a range of frequencies (1 kHz < f < 8 kHz). The electron density distributions used for the ray-tracing calculations are derived from measurements with the Whisper relaxation sounder instrument. Our new results indicate that whistlers are observed outside the plasmasphere in the low-density regions only in the presence of large-scale irregularities within which the waves are ''ducted.'' This conclusion is sustained by an exhaustive search of whistlers outside the plasmasphere using all the Cluster passes during 2001 and 2002. In all the cases we found that dispersion characteristics are matched by ray-tracing simulations only if the whistlers are ducted. In some cases, whistler wave energy injected by an individual lightning discharge appears with significant smearing in time. The new results presented in this paper support a possible explanation of why whistlers outside the plasmasphere are rarely observed, based on wave conversion electromagnetic whistler mode to quasi-electrostatic lower hybrid mode (Bell et al., 2004).
1] We interpret observations of low-altitude electromagnetic ELF hiss observed on the dayside at subauroral latitudes. A divergent propagation pattern has been reported between 50° and 75° of geomagnetic latitude. The waves propagate with downward directed wave vectors which are slightly equatorward inclined at lower magnetic latitudes and slightly poleward inclined at higher latitudes. Reverse ray tracing using different plasma density models indicates a possible source region near the geomagnetic equator at a radial distance between 5 and 7 Earth radii by a mechanism acting on highly oblique wave vectors near the local Gendrin angle. We analyze waveforms received at altitudes of 700–1200 km by the Freja and DEMETER spacecraft and we find that low-altitude ELF hiss contains discrete time-frequency structures resembling wave packets of whistler mode chorus. Emissions of chorus also predominantly occur on the dawnside and dayside and have recently been considered as a possible source of highly accelerated electrons in the outer Van Allen radiation belt. Detailed measurements of the Cluster spacecraft at radial distances of 4–5 Earth radii show chorus propagating downward from the source region localized close to the equator. The time-frequency structure and frequencies of chorus observed by Cluster along the reverse raypaths of ELF hiss are consistent with the hypothesis that the frequently observed dayside ELF hiss is just the low-altitude manifestation of natural magnetospheric emissions of whistler mode chorus.
This paper is related to the presentation of the magnetic field experiment IMSC, and to the description of the electronic block named BANT onboard the micro-satellite DEMETER. The main scientific objective of the DEMETER mission is related to the study of the ionospheric perturbations in relation with the seismic and volcanic activities. There are several hypotheses to explain these ionospheric perturbations and one goal is to understand the right generation mechanism. The same scientific payload will allow us to study the ionospheric perturbations in relation with the anthropogenic activities. In total, DEMETER will perform a comprehensive study of the Earth electromagnetic environment at the altitude of the satellite. Onboard DEMETER, all scientific experiments are first processed by the electronic module BANT which is connected to an onboard memory. The functions of BANT will be described, and the first results of IMSC will be presented.
The objective of the ICE (Instrument Champ Electrique) experiment on board DEMETER is to provide a nearly continuous survey of the electromagnetic and/or electrostatic waves that may arise from the coupling of seismic activity with the upper atmosphere and ionosphere. To this aim it makes use of 4 spherical electrodes with embedded preamplifiers that are deployed by stacer booms at approximately 4 m from the satellite. Measurements are made over a wide frequency range from DC to 3.175 MHz, subdivided in the signal processing unit in four frequency channels DC/ULF, ELF, VLF and HF. Three axis measurements are available in the DC/ULF range for all modes of operation of DEMETER and in the ELF range in the DEMETER Burst modes. In the VLF and HF ranges and in ELF during DEMETER Survey modes only one axis of measurement is available that can be selected by telecommand. We present in this paper a general description of the instrument and its modes of operation and in-flight performances. The sensitivity is ∼0.1–0.2 μV/m Hz1/2 from ∼100 Hz through the HF range and the dynamical range is >80 dB in ELF and VLF and about 42 dB in HF. In order to illustrate the instrument capabilities, we briefly describe a number of observations from the first months of operation in various regions along the orbit from the equator to high latitudes.
Whistler waves caused by lightning are known to penetrate the ionosphere with significant wave amplitudes. Recent rocket experiments have shown that lightning from sources as far as 1000 km from the ionospheric rocket subtrack are easily seen. The spectral energy density of lightning in the ionosphere often has significant power at frequencies near and below 1 kHz. Such low-frequency whistler waves can propagate all the way to the magnetopause boundary layer if they begin in the cusp or polar cap. Lightning is common over high-latitude continental land masses in the summer months which places this wave source function well within range of the cusp and polar cap. Ray tracing studies using a global three-dimensional two-fluid code are conducted to investigate the propagation of these whistler waves into the high latitude magnetosphere. The estimated, mapped lightning whistler wave amplitude is compared to wave intensity measurements from ISEE and AMPTE and supports the argument that falling tone whistler waves, seen by Geotail near the dayside magnetopause, may have been from intense lightning. It is found that the mapped whistler wave amplitude from lightning is comparable to the in situ wave amplitudes measured in the outer dayside magnetosphere and low latitude boundary layer.
Ray tracing for whistler-mode propagation has been performed with the effects of ions included. The method is similar. to that employed by Yabroff but for the modification in the refractive index due to ions. Outstanding characteristics of the ray paths in such a medium result from the existence of purely transverse propagation at the lower frequencies. The main purpose of this study is to confirm the Smith interpretation of the “subprotonospheric” whistlers. It is found that an enhancement of electron density at the latitude of interest can support the ray path of the fractional hop as Smith suggested, as well as a possibility of successive echoes through a single path. The effect of collisions on propagation is also discussed.
The behavior of high-energy electrons trapped in the Earth's Van Allen
radiation belts has been extensively studied, through both experimental
and theoretical techniques. While the evidence for whistler induced
electron precipitation (WEP) from the radiation belts is overwhelming,
and the mechanisms behind WEP are well understood, the overall
significance of WEP on radiation belt loss rates has not been clear. In
this paper we investigate the L-shell variation and significance of
WEP-driven loss of Van Allen belt electrons by combining in situ
measurements of electron precipitation, local WEP rates determined from
Trimpi perturbations, and global lightning distributions. Our modeling
suggests that long-term WEP driven losses are more significant than all
other inner radiation belt loss processes for electron kinetic energies
in the range ~50-150 keV in the L-shell range L = 2-2.4. These
calculated lifetimes are comparable to the observed decay rates of
artificially injected high-energy electrons. The upper energy limit of
the WEP significance range increases with decreasing L to ~225 keV at L
= 2. For electron energies above this range manmade VLF transmitters and
plasmaspheric hiss should dominate over all other loss processes.
However, as our lifetimes are based on rather conservative parameter
estimates, these conclusions should represent the lower bounds for the
energy ranges over which WEP losses are significant. For lower L-shells
the coupling of lightning activity to the production of WEP events
rapidly decreases, such that by L ~ 1.7 WEP will be unimportant in the
overall loss processes.
An experimental study of the proton whistler, a new VLF phenomenon observed in satellite data, is presented, and an explanation of this new effect is given. The proton whistler appears on a frequency-time spectrogram as a tone which starts immediately after the reception of a short fractional-hop whistler at the satellite and initially shows a rapid rise in frequency, asymptotically approaching the gyrofrequency for protons in the plasma surrounding the satellite. It is proposed that the proton whistler is simply a dispersed form of the original lightning impulse and that the dispersion can be explained by considering the effect of ions on the propagation of an electromagnetic wave in the ionosphere. The propagation of a wave in a multicomponent plasma for frequencies of the order of the ion gyrofrequencies is discussed. In the ionosphere it is found that, in addition to the right-hand polarized whistler mode, the left-hand polarized mode (ion cyclotron wave) is also a possible mode of propagation for certain ranges of frequencies and altitudes. Between each two adjacent ion gyrofrequencies there is a frequency for which both modes of propagation are linearly polarized. These frequencies are called the crossover frequencies. A wave propagating in the ionosphere changes polarization at the altitude where the wave frequency is equal to a crossover frequency. This polarization reversal provides the mechanism by which an upgoing whistler can become an ion cyclotron wave. We show that the proton whistler is an ion cyclotron wave which occurs via this polarization reversal process. The crossover frequency can be measured from spectrograms of proton whistlers and is used to determine the fractional concentration of H+ in the plasma surrounding the satellite. Near the altitude and frequencies for which polarization reversal occurs, it is shown that the right-hand polarized wave and the ion cyclotron wave may be strongly coupled. For frequencies of the order of the ion gyrofrequencies, this coupling process plays an important part in determining what regions of the ionosphere are accessible to waves from a given source location.
Whistlers generated by lightning have been observed in the magnetosphere of Saturn by the Cassini Radio and Plasma Wave Science Investigation (RPWS). Two whistlers were observed as the spacecraft flew over the rings on July 1, 2004, and the third was observed on October 28, 2004, during the inbound pass of orbit A at a radial distance of 6.19 RS (Saturn radii). Of the three, the third has the best signal-to-noise ratio and is the main subject of this presentation. The whistler has a good fit to the well-known Eckersley law for the dispersion of whistlers, with a dispersion constant of 81.3 Hz1/2 sec. Since to a first approximation the whistler energy follows the planetary magnetic field line, the lightning that caused the whistler must be located at a relatively high latitude, roughly 66 degrees in this case. It is not known whether the causative lightning was located in the northern or southern hemisphere. However, the location of the spacecraft at 12.4 degrees north latitude and the relatively large dispersion suggest that the whistler passed through the dense equatorial plasma torus. If so, the lightning would be located in the southern hemisphere. Based on the measured dispersion and local electron density at the spacecraft, which was about 6.5 cm-3, the effective path length through the torus can be determined and is about 2.17 RS. Although not completed at the present time, we plan to use a Gaussian scale height model of the plasma torus to determine the north-south thickness of the torus from the measured dispersion. This model can then be compared with other estimates of the torus thickness, such as can be obtained from the plasma temperature using a centrifugal potential model. As more whistlers are detected during the Cassini mission, we should eventually be able to determine the north-south thickness of the torus as a function of radial distance.
Simultaneous observations of times of occurrence of whistlers were made at Seattle, Washington, and Stanford, California, two hours every week from October 1951 to October 1952. Times were measured to an accuracy of about ±1 second. The objective was to determine the percentage of whistlers received at either station which were coincident at both. A total of 318 whistlers was received at Stanford and 283 at Seattle during simultaneous observations. The occurrence rate of whistlers (during a two-hour period) varied from 0 to roughly 55 per hour at Stanford and from 0 to 70 per hour at Seattle. The correlation between the occurrence rates was poor. The number of true coincidences was found by subtracting the number of chance coincidences from the number of total coincidences. A method for computing the number of chance coincidences from a knowledge of the time intervals between whistlers at the one station was derived. The analysis showed that approximately 22 per cent were observed simultaneously at both stations. This result is examined in relation to possible theories of whistler origin and propagation, and is shown to support the Storey-Eckersley theory.
Whistler waves caused by lightning are known to penetrate the ionosphere with significant wave amplitudes. Recent rocket experiments have shown that lightning from sources as far as 1000 km from the ionospheric rocket subtrack are easily seen. The spectral energy density of lightning in the ionosphere often has significant power at frequencies near and below 1 kHz. Such low-frequency whistler waves can propagate all the way to the magnetopause boundary layer if they begin in the cusp or polar cap. Lightning is common over high-latitude continental land masses in the summer months which places this wave source function well within range of the cusp and polar cap. Ray tracing studies using a global three-dimensional two-fluid code are conducted to investigate the propagation of these whistler waves into the high latitude magnetosphere. The estimated, mapped lightning whistler wave amplitude is compared to wave intensity measurements from ISEE and AMPTE and supports the argument that falling tone whistler waves, seen by Geotail near the dayside magnetopause, may have been from intense lightning. It is found that the mapped whistler wave amplitude from lightning is comparable to the in situ wave amplitudes measured in the outer dayside magnetosphere and low latitude boundary layer.
Methods of diagnostics of magnetospheric parameters based on the analysis of ground-based whistler dynamic spectra and the results of these diagnostics are reviewed. Particular emphasis is placed on the methods of extrapolation of whistler dynamic spectra and on the application of ground-based whistlers to the estimation of magnetospheric electron temperature.
We use the methodology presented in a companion paper to calculate the temporal and spatial precipitation signatures of energetic radiation-belt electrons due to pitch-angle scattering by magnetospherically reflecting (MR) whistler waves generated by lightning discharges at geomagnetic source latitudes of λs = 25°, 35°, 45°, and 55°. We show precipitated energy and number fluxes, as well as average precipitated energy as a function of time and L-shell for all four λs cases, and then extrapolate the results of the λs = 35° case in longitude to produce a time-sequence of geographic ‘hot spots’ which are affected by the MR whistler induced electron precipitation. We then discuss the total precipitation in both hemispheres due to the various λs cases. Our major findings are that the precipitation region moves to higher L-shells as a function of time, on short (0.1 sec, at the start of the event) and long (10 sec) timescales, corresponding to the first hop of the wave, and the MR portion of the whistler wave, respectively. There is also structure within the long-timescale precipitation on the order of ∼1 sec, reflecting the periodic MR of the underlying whistler wave. As λs increases, an additional precipitated flux signature which is more incoherent and discontinuous, begins to form at higher L-shells and later times, due to MR whistler wave reflections from the plasmapause. At lower L-shells, a pronounced maximum occurs in the number flux of ∼1 keV electrons at L ∼ 2–3 due to the Landau resonance. The geographic hot spot affected by the precipitation can split into two separate regions per hemisphere, and occur simultaneously in both hemispheres so that up to four distinct precipitation hot spots can occur on the Earth at any instant, driven by a single lightning discharge. We also discuss potential sources of error, and comparison to related modeling efforts, and to observations using a either satellite-borne instruments or ground-based techniques.
The characteristics of subionospheric propagation of magnetospheric whistlers have been investigated by means of both digital spectral analysis with high resolution and our field-analysis direction finding measurement for whistlers observed in South China. It is found that very low latitude whistlers, propagated in the Earth-ionosphere waveguide over a distance of the order of 1000km, exhibit very clear additional dispersion effects near the cutoff frequencies of the subionospheric 1st- and higher- order modes. The additional traces are also found to be left-handed polarized, and when the frequency becomes very close to exactly left-handed circular and the incident angle becomes nearly vertical. While, the whistler components at frequencies away from the cutoff frequencies are linearly polarized. Finally, we emphasize the important use of the subionospheric propagation characteristics in the study of the whistler duct structure, ionospheric transmission of a downgoing whistler and its excitation of subionospheric modes.
We present an evaluation of transport of sulfur hexafluoride (SF6) in the two-way nested chemistry-transport model ``Tracer Model 5'' (TM5). Modeled SF6 values for January 2000 to November 2003 are compared with NOAA CMDL observations. This includes new high-frequency SF6 observations, frequent vertical profiles, and weekly flask data from more than 60 sites around the globe. This constitutes the most extensive set of SF6 observations used in transport model evaluation to date. We find that TM5 captures temporal variability on all timescales well, including the relatively large SF6 signals on synoptic scales (2-5 days). The model overestimates the meridional gradient of SF6 by 19%, similar to previously used transport models. Vertical profiles are reproduced to within the standard error of the observations, and do not reveal large biases. An important area for future improvements is the mixing of the planetary boundary layer which is currently too slow, leading to modeled SF6 mixing ratios that are too large over the continents. Increasing the horizontal resolution over North America from 6×4°, to 3×2°, to even 1×1° (lon×lat) does not affect the simulated global scale SF6 distribution and potentially minimizes representation errors for continental sites. These results are highly relevant for future CO2 flux estimates with TM5, which will be briefly discussed.
Study of a new whistler phenomenon shows that the magnetospheric ; ionization profile often exhibits a knee,' that is, a region at several earth ; radii in which the ionization density drops rapidly from a ralatively normal ; level to a substantially depressed one. The new whistler phenomenon (called, for ; convenience, the knee whistler') is compared with ordinary whistlers and is ; illustrated by a number of examples recorded at middle- and high-latitude ; stations, It is suggested that the knee exists at all times in the magnetosphere, ; and that its position varies, moving inward with increasing magnetic activity. ; There are indications that conditions of whistler-mode propagation may be ; unusually favorable on the low-latitude side of the knee and that the region on ; the high-latitude side may be favorable for the production of triggered ; ionospheric noise. It is pointed out that knee whistlers account for a ; substantial number of the observations of deep density depressions during ; magnetic storms. Several questions of interpretation are raised, and the ; direction of future investigations is indicated. (auth);
Numerical simulation based upon three-dimensional ray-tracing from a single source point located in the equatorial region is used to explain the wave distribution functions (WDF) found for ELF/VLF hiss detected on the GEOS satellite. Emissions at very oblique k vectors and in all azimuthal directions are shown to produce the GEOS-1 multipeaked WDFs at the longitudes and L values close to the longitudes and L values of a chosen single source point. Inverse ray-tracings, started with the parameters of the WDF's observed emissions, are used to locate the sources.
The measurement of the wave normal direction of short whistlers is made in the ionosphere by means of a three-dimensional loop antenna on a rocket. The results of the whistler wave normal direction are presented, and some relationships to the propagation characteristics in the magnetosphere and also to the penetration through the ionosphere are discussed. Since the proposal of trapped propagation by Smith [1961] there has never been any direct evidence for the presence of field-aligned ducts for whistler propagation. However, a steady accumulation of indirect evidence for ducted propagation of ground-based whistlers enables us to deduce the electron density profile of the magnetosphere [Helliwell, 1965]. Carpenter [1968] ha s obtained evidence for ducted propagation that is inferred from the fact that the phenomena are consistent with the half-gyrofrequency propagation effect predicted by Smith. Then Angerami [1970] found in situ evidence for the existence of ducts, 'using the simultaneous detection of ducted whistlers as well as the high-frequency leaked ones on board the Ogo 3 satellite, and also got information on duct properties, such as the enhancement factor, the width, and the interduct spacing. Furthermore, Scarf and Chappell [1973] have studied the association of whistler dispersions with changes in local plasma density and have found that'groups of whistlers with relatively constant dispersions tend to be detected in regions where the local ion concentration is significantly enhanced. These studies on ducts are concerned with the high-latitude cases. At low latitudes no observational results suggesting the existence of ducts have been obtained, whether they are direct or indirect, probably because compared with the high-latitude whistlers, the low-latitude whistlers have the inevitable disadvantage that their path latitudes are difficult to determine. However, Hayakawa [1973] and Hayakawa and Ohtsu [1973] in simultaneous observations at multiple stations have found some results to remove such a difficulty that imply strongly ducted propagation of low-latitude whistlers. An analysis of ray paths in the inner magnetosphere has led us to conclude that the observed results are satisfactorily interpreted in terms of ducted propagation. Although these recent studies are indirect, they are strong evidence for low-latitude ducts. In order to confirm the actual existence of ducts for low-latitude whistlers we attempt to measure the wave normal directions of short whistlers in the ionosphere by means of a rocket. The information on wave normal directions is a conclusive factor in knowing whether the whistlers are ducted or nonducted. In this paper, we report preliminary results based on in situ measurement of the wave normal direction of short whistlers made with the K-9M-41 rocket. The experimental results on the wave normal direction seem to be consistent
Subprotonospheric (SP) whistlers consist of a series of low-dispersion components that result from repeated reflections between the base of the ionosphere and altitudes up to ~1000 km. We have used wave-normal angles and plasma characteristics measured by the DEMETER microsatellite as an input for a three-dimensional ray tracing technique. For several SP whistlers we have also succeeded in finding the causative lightning located at relatively large distances from the satellite footprint along the geomagnetic field line. We show that the reflections and formation of the SP whistlers take place owing to an oblique propagation, with respect to the magnetic field, in the waveguide formed by a profile of the increasing lower hybrid resonance frequency in the upper ionosphere and the base of the ionosphere. We have observed propagation across the magnetic meridian planes. We conclude that the individual components of the SP whistler propagate along different raypaths. The reflected components enter the ionosphere at relatively large distances from the satellite footprint and experience a spread of wave-normal angles during this entry. Depending on the initial wave-normal angle, these waves undergo a different number of reflections before reaching the satellite, thus arriving with different time delays. However, the first component observed of a SP whistler is formed by waves entering the ionosphere at relatively small distances from the satellite footprint and at relatively small wave-normal angles. These waves do not reflect above the satellite but propagate to the opposite hemisphere.
A method is outlined for obtaining F sub N and T sub N, t e nose frequency and the time delay to the nose, for whistlers that do not exhibit a nose on the spectrographic records. The method is based upon a universal whistler dispersion function which is a function of f/F sub N only. Two time-delay measurements, at widely separated frequencies, are made on a whistler trace, and from this information and the universal dispersion function, F sub N and T sub N are obtained. Results of applying the method to actual nose whistlers are tabulated and show good agreement between calculated and measured values of F sub N and T sub N. A method of whistlersource identification is outlined which utilizes approximate information on nose frequency and is particularly useful for middle-latitude whistlers. (Author)
Presents the theory of the effect of the ionosphere and magnetosphere on
radio waves and incorporates recent findings from space science and
plasma physics. Also includes accounts of some of the mathematical
techniques now used.