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Propagation of whistler mode chorus to low altitudes: Spacecraft observations of structured ELF hiss
... To evaluate frequency spacing more accurately, we applied the Hilbert transform method (Santolík et al., 2014) and the corresponding variance values (solid red) for these frequency spacings. At high latitudes (Figure 2g) To further understand wave propagation features, we applied the Singular Value Decomposition method (Santolík et al., 2003), which has been widely used in the analysis of ELF/VLF space plasma waves (e.g., Němec et al., 2007;Santolík et al., 2006;Wei et al., 2007). The SVD method is computed under the Magnetic_Field_Aligned_Coordinate (MFAC) system by Santolík et al. (2003), in which the Z-axis is directed along with the ambient magnetic field B 0 , the Y-axis is directed along the cross-product of the Z-axis and the position vector of the satellite (so that the +Y-axis is nominally eastward at the equator), and the X-axis completes the right-handed system. ...
... It is admitted that the interpretation of such a puzzle is beyond our current knowledge, and we will leave this complicated topic to future study. A ray-tracing study by Santolík et al. (2006) suggests that MLR's sources might be located in the equatorial region outside the plasmapause. The conjugate observations between Cluster and DEMETER also confirmed that MLR waves could propagate from the inner magnetosphere to the upper ionosphere with a significant azimuthal extent from the source . ...
This paper reports a large‐scale magnetospheric line radiation (MLR) event during a moderate geomagnetic storm on 11 September 2018, which was well recorded by the China‐Seismo‐Electromagnetic Satellite (CSES) in the upper ionosphere. The event shows a symmetrical propagation feature at the conjugated locations between the two hemispheres, exhibiting a large spatial extension roughly from the latitudes 54°N to 53°S. The parallel structures are visible both in the electric and magnetic fields at a frequency band ranging from the local proton cyclotron frequency to ∼1.6 kHz. The wave intensity of parallel spectral lines was primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings between neighboring spectral lines roughly vary from ∼80 to 110 Hz at the high latitudes and ∼80–130 Hz at the low latitudes, suggesting a slight variation feature with latitude. The parallel spectral structures of MLR drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. The wave vector analysis shows that the MLR waves are right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, where the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. The other large‐scale MLR events all exhibit similar parallel structures and polarization characteristics, suggesting the universality of such a phenomenon. However, the azimuthal angles differ among different events, showing complex features.
... The central position of the chorus source fluctuates at time scales of minutes within few thousands of km of the magnetic equator with a typical speed of 100 km s À 1 . Results show that chorus can propagate to low attitudes towards the Earth if it is generated with earthward inclined wave vectors (Santolik et al., 2006;Bortnik et al. 2007). Chorus waves propagate with slightly equatorward inclined downward wave vectors at lower MLAT and slightly poleward inclined at higher latitudes (Santolik et al., 2006). ...
... Results show that chorus can propagate to low attitudes towards the Earth if it is generated with earthward inclined wave vectors (Santolik et al., 2006;Bortnik et al. 2007). Chorus waves propagate with slightly equatorward inclined downward wave vectors at lower MLAT and slightly poleward inclined at higher latitudes (Santolik et al., 2006). Bortnik et al. (2008) have studied the propagation of chorus using numerical ray tracing for a set of rays initiated at the geomagnetic equator and consistent with observations. ...
The plasmasphere sandwiched between the ionosphere and the outer magnetosphere is populated by up flow of ionospheric cold ($ 1 eV) and dense plasma along geomagnetic field lines. Recent observations from various instruments onboard IMAGE and CLUSTER spacecrafts have made significant advances in our understanding of plasma density irregularities, plume formation, erosion and refilling of the plasmasphere, presence of thermal structures in the plasmasphere and existence of radiation belts. Still modeling work and more observational data are required for clear understanding of plasmapause formation, existence of various sizes and shapes of density structures inside the plasma-sphere as well as on the surface of the plasmapause, plasmasphere filling and erosion processes; which are important in understanding the relation of the process proceeding in the Sun and solar wind to the processes observed in the Earth's atmosphere and ionosphere.
... The downward magnetospheric emission includes chorus and plasmaspheric hiss. The downward chorus is widely considered as a possible source for the ionospheric hiss (Parrot, Benoist, et al., 2006;Parrot, Buzzi, et al., 2006;Parrot et al., 2016;Santolík, Chum, et al., 2006;Santolík, Němec, et al., 2006), and the plasmapheric hiss is another source (if not a major source) recently proposed by Chen et al. (2017). The generation mechanism of the downward magnetospheric emission for ionospheric hiss has recently received increasing attention because it explains most of the features seen in the ionosphere with frequencies from~100 Hz to 1 kHz. ...
... The intense emissions with broadband spectra at high-latitude ionosphere (MLAT:~30°to~60°) at both the Northern and Southern Hemisphere are hiss, too (Chen et al., 2017), which are very common in the upper ionosphere (Parrot et al., 2016;Santolík, Chum, et al., 2006;Zhima et al., 2013). For this type of high-latitude ionospheric hiss, a burst mode waveform from 14:49 to 14:52 UT was used to examine the wave propagation parameters. ...
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.
... To verify if these scenarios are realistic for both types of whistlers we performed a schematic ray-tracing simulation, based on the procedure of Cerisier (1970) with a diffusive equilibrium model of the plasma density distribution, modified to include an adaptive integration step by Santolík et al. (2006. The model in our case was set to an exospheric temperature of 700 K and calibrated to a plasma density of 10 4 cm −3 at a reference altitude of 700 km. ...
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 model opens new possibilities for the explanation of the characteristics of the generation of these chorus waves such as their high growth rates or their relationship with the ELF hiss. This phenomenon is widely studied in [97]. It is a broadband electromagnetic emission in the frequency range from a few hundred Hz to about 2 kHz. ...
This paper describes the characteristics of contributions made by researchers worldwide in the field of ELF (extremely low frequency) waves from 1957 to 2019. The data were collected through the Scopus database and processed with analytical and bibliometric techniques. The selection of the keywords is an essential step, because ELF has a very different meaning in some areas of medicine, where it is associated with a gene. A total of 12,436 documents were worked on in 12 thematic communities according to their collaborative relationships between authors and documents. Studies included authors publishing in the different thematic areas and the country where the USA stands first with more researchers in this theme than China and Japan. Documents were analyzed from the temporal perspective, their overall contribution, means of publication, and the language of the publication. Research requires extra effort and multidisciplinary collaboration to improve the knowledge, the application, and influence of these fields.
... These signals can be used to analyse the Earth's ionosphere, magnetosphere and terrestrial lightning. Emissions of a whistler-mode chorus or hiss [16], [17] at higher geomagnetic latitudes (above 65°) might be observable by this receiver as well. Space-born detectors with magnetic loop antennas have the advantage of being sensitive in the VLF radio band, which is very interesting from a scientific perspective. ...
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.
... Chorus is believed to be generated near the geomagnetic equator, where the first derivative of the magnetic field strength along the field line is nearly zero (Lauben et al. 2002). The analysis of multi-satellite Cluster data of chorus emissions (Parrot et al. 2003;Santolík et al. 2005Santolík et al. , 2006 shows that the generation region is localized near the equatorial cross-section of the L ∼ 4 magnetic flux tube, and has a typical scale size of ∼2000 km along the magnetic field lines. The source centre is defined by the balance of the Poynting flux parallel and antiparallel to the field lines Santolík 2008). ...
The space between the two spherical conducting shells, Earth surface and the lower boundary of the ionosphere, behaves as a spherical cavity in which some electromagnetic signals can propagate a long distance and is called Earth-ionosphere waveguide. Through this waveguide ultra low frequency (ULF), extremely low frequency (ELF) and very low frequency (VLF) signals can propagate efficiently with low attenuation. Resonances which occur for ELF waves due to round-the-world propagation interfering with phase difference are called Schumann resonances. Lightnings are the main sources of energy continuously producing these electromagnetic radiations from the troposphere. Some fixed frequency signals are also transmitted through the waveguide from different stations for navigation purposes. The intensity and phase of these signals at a particular position depend on the waveguide characteristics which are highly influenced by different natural events. Thus the signatures of different geophysical and extra-terrestrial events may be investigated by studying these signals through proper monitoring of the time series data using suitable techniques. In this article, we provide a review on ULF, ELF and VLF signals within the waveguide in terms of different geophysical and extra-terrestrial events like lightning, earthquakes, Leonid meteor shower, solar flares, solar eclipse, geomagnetic storms, and TLEs etc.
We use conjugate observations of magnetospheric whistler‐mode waves at frequencies up to 16 kHz by the DEMETER spacecraft (at an altitude of approximately 660 km) and the ground‐based Kannuslehto station in Finland (L≈5.38) to investigate the wave propagation to the ground and their characteristic spatial scales. For this purpose, we evaluate correlations between the wave intensities measured by the spacecraft and the ground‐based station at various frequencies as a function of their longitudinal and L‐shell separations. Two different approaches are used: (a) direct correlation of wave intensities measured at the same times and (b) correlation of wave intensities within corresponding frequency‐time windows, focusing on the identification of the same frequency‐time wave signatures. We show that the characteristic longitudinal scales of the investigated waves are between about 60° and 90° . We further demonstrate that, while the wave intensities measured by DEMETER are generally larger during periods of enhanced geomagnetic activity, wave intensities measured on the ground during increased activity are only slightly larger during the daytime and decrease during the nighttime.
Whistler‐mode hiss waves are crucial to the dynamics of Earth's radiation belts, particularly in the scattering and loss of energetic electrons and forming the slot region between the inner and outer belts. The generation of hiss waves involves multiple potential mechanisms, which are under active research. Understanding the role of hiss waves in radiation belt dynamics and their generation mechanisms requires analyzing their temporal and spatial evolutions, especially for strong hiss waves. Therefore, we developed an Imbalanced Regressive Neural Network (IR‐NN) model for predicting hiss amplitudes. This model addresses the challenge posed by the data imbalance of the hiss data set, which consists of predominantly quiet‐time background samples and fewer but significant active‐time intense hiss samples. Notably, the IR‐NN hiss model excels in predicting strong hiss waves (>100 pT). We investigate the temporal and spatial evolution of hiss wave during a geomagnetic storm on 24–27 October 2017. We show that hiss waves occur within the nominal plasmapause, and follow its dynamically evolving shape. They exhibit intensifications with 1 and 2 hr timescale similar to substorms but with a noticeable time delay. The intensifications begin near dawn and progress toward noon and afternoon. During the storm recovery phase, hiss intensifications may occur in the plume. Additionally, we observe no significant latitudinal dependence of the hiss waves within |MLAT| < 20°. In addition to describing the spatiotemporal evolution of hiss waves, this study highlights the importance of imbalanced regressive methods, given the prevalence of imbalanced data sets in space physics and other real‐world applications.
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.
Plain Language Summary
In near‐Earth space, whistler‐mode chorus waves are typically observed in the plasma trough. The wave vector direction of chorus waves, which is an important parameter in quantifying wave‐particle interactions, has been shown to become oblique near the boundaries of plasmaspheric plumes. In this study, we explain this phenomenon by using ray tracing to simulate an observation of chorus waves near a plume boundary with a steep density gradient. Ray tracing shows that the density gradient at the plume boundary can strongly refract the chorus wave, contributing to oblique wave vectors. With a steep density gradient, the wave can be reflected at the plume boundary and thus cannot enter the plume. Our ray‐tracing results reproduce the observed wave power distribution in different wave vector directions. We also established an analytical criterion to determine if the chorus wave can enter the plume region near the equatorial region. Our results give insight into wave‐particle interactions by chorus waves with the influence of azimuthal density structures such as plume boundaries.
Quasi-periodic (QP) emissions are a type of magnetospheric ELF/VLF waves characterized by a periodic intensity modulation ranging from tens of seconds to several minutes. Here, we present 63 QP events observed between January 2017 and December 2018. Initially detected at the VLF receiver in Kannuslehto, Finland (KAN, MLAT = 67.7°N, L = 5.5), we proceeded to check whether these events were simultaneously observed at other subauroral receivers. To do so we used the following PWING stations: Athabasca (ATH, MLAT = 61.2°N, L = 4.3, Canada), Gakona (GAK, MLAT = 63.6°N, L = 4.9, Alaska), Husafell (HUS, MLAT = 64.9°N, L = 5.6, Iceland), Istok (IST, MLAT = 60.6°N, L = 6.0, Russia), Kapuskasing (KAP, MLAT = 58.7°N, L = 3.8, Canada), Maimaga (MAM, MLAT = 58.0°N, L = 3.6, Russia), and Nain (NAI, MLAT = 65.8°N, L = 5.0, Canada). We found that: (1) QP emissions detected at KAN had a relatively longer observation time (1–10 h) than other stations, (2) 11.3% of the emissions at KAN were observed showing one-to-one correspondence at IST, and (3) no station other than IST simultaneously observed the same QP emission as KAN. Since KAN and IST are longitudinally separated by 60.6°, we estimate that the maximum meridional spread of conjugated QP emissions should be close to 60° or 4 MLT. Comparison with geomagnetic data shows half of the events are categorized as type II, while the rest are mixed (type I and II). This study is the first to clarify the longitudinal spread of QP waves observed on the ground by analyzing simultaneous observations over 2 years using multiple ground stations.
Graphical Abstract
Hiss waves play an important role in removing energetic electrons from Earth’s radiation belts by precipitating them into the upper atmosphere. Compared to plasmaspheric hiss that has been studied extensively, the evolution and effects of plume hiss are less understood due to the challenge of obtaining their global observations at high cadence. In this study, we use a neural network approach to model the global evolution of both the total electron density and the hiss wave amplitudes in the plasmasphere and plume. After describing the model development, we apply the model to a storm event that occurred on 14 May 2019 and find that the hiss wave amplitude first increased at dawn and then shifted towards dusk, where it was further excited within a narrow region of high density, namely, a plasmaspheric plume. During the recovery phase of the storm, the plume rotated and wrapped around Earth, while the hiss wave amplitude decayed quickly over the nightside. Moreover, we simulated the overall energetic electron evolution during this storm event, and the simulated flux decay rate agrees well with the observations. By separating the modeled plasmaspheric and plume hiss waves, we quantified the effect of plume hiss on energetic electron dynamics. Our simulation demonstrates that, under relatively quiet geomagnetic conditions, the region with plume hiss can vary from L = 4 to 6 and can account for up to an 80% decrease in electron fluxes at hundreds of keV at L > 4 over 3 days. This study highlights the importance of including the dynamic hiss distribution in future simulations of radiation belt electron dynamics.
Plain Language Summary
The wave electric field on Van Allen Probes has recently been corrected for sheath impedance effects, which can affect both the amplitude and phase of the measurements. Here, for the first time, we directly quantify the impact of this correction factor on chorus wave measurements of electric field wave power. We also investigate the impact on the Poynting vector direction and magnitude, which is itself determined from electric field observations and magnetic field measurements. It is found that after applying the sheath correction to chorus wave observations, the electric field wave power typically increases by a factor between two and nine, whereas the Poynting flux typically increases by a factor of ∼2. The relationship between the wave vector and the Poynting vector is compared to that expected from cold plasma theory, for both the corrected and uncorrected observations. Significant differences between observations and theory are apparent when using the uncorrected measurements, however the sheath‐corrected observations agree well with the theoretical predictions.
Plasmaspheric hiss is an electromagnetic wave mode that occurs ubiquitously in the high‐density plasmasphere and contributes crucially to the dynamic behavior of the Earth's Van Allen radiation belts. While plasmaspheric hiss is commonly considered to be a broadband emission with frequencies from ∼100 Hz to several kHz, here we report Van Allen Probes measurements of unambiguous banded signatures of plasmaspheric hiss, uniquely characterized by an upper band above ∼200 Hz, a lower band below ∼100 Hz and a power gap in between. Banded plasmaspheric hiss occurs with the probability ∼8% in the postnoon sector within 2.5–5.0 Earth radii, showing strong dependence on geomagnetic and solar wind conditions. Observations also suggest that banded hiss waves result possibly from two combined sources, the upper band originating from the transformation of chorus waves propagating from outside the plasmasphere, and the lower band from localized excitation inside the plasmasphere, which however requires future investigation. The banded hiss waves shed new light on the evolution of the Earth's radiation belts and have implications for understanding whistler‐mode waves in planetary magnetospheres.
We present observations on the local proton cyclotron band emissions (PCBEs), which is one type of hiss wave in the upper ionosphere, recorded by the China Seismo‐Electromagnetic Satellite (CSES). The PCBEs are characterized by narrow‐band electromagnetic emissions along the local proton cyclotron frequency (fcp ), exhibiting a sharp cutoff effect. The lower cutoff frequency (fcut−low ) is closely along the local fcp , decreasing with magnetic latitude, while the upper cutoff frequency (fcut−up ) is diffuse and deviates from the local fcp . We developed an algorithm to automatically extract PCBEs from CSES's massive electromagnetic data and statistically examined their occurrence and bandwidth's dependence on location and season. The global morphology of PCBEs in 2019 shows that they primarily occur in the mid‐high magnetic latitude (approximately 20°–55°) dayside ionosphere, with the maximum occurrence happening around the South Atlantic Anomaly region. Their bandwidth shows a decreasing tendency with magnetic latitude and strong seasonal dependence, with a wider bandwidth in local winter and a narrower one in local summer. The ratio of fcut−low and fcp also shows a seasonal dependence, with greater values in local summer and smaller values in local winter. This characteristic of fcut−low is similar to that of the H⁺‐He⁺ cutoff frequency (fcut−ion ). This study provides statistical characteristics of occurrence and the cutoff features of PCBEs based on the CSES operating in the upper ionosphere.
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.
Plain Language Summary
Chorus wave properties are typically parameterized spatially in magnetic coordinates. Here, for the first time, we specifically study chorus wave vector characteristics near density enhancements known as plasmaspheric plumes, which have been identified as an access region for chorus waves to enter the plasmasphere. It is found that chorus waves propagate at larger angles with respect to the background magnetic field directly on the boundary of high‐density plume structures. This behavior is reported on both the Eastward and Westward plume edges, yet the variations in the distribution of wave vector angles is reported to be different between the Eastwards and Westwards edge. In general, it is found that the wave vector, k, is distributed symmetrically about the anti‐Earthwards direction, however, near the Eastwards plume boundary an Eastwards skew is reported. These results shed new light on the propagation of chorus waves near plasmaspheric plumes, which is important for modeling the propagation of waves into the plasmasphere, as well as for modeling interactions between waves and particles in this region.
Dynamical variations of radiation belt trapped electron fluxes are examined to better understand the variability of enhancements linked to substorm clusters. Analysis is undertaken using the Substorm Onsets and Phases from Indices of the Electrojet substorm cluster algorithm for event detection. Observations from low earth orbit are complemented by additional measurements from medium earth orbit to allow a major expansion in the energy range considered, from medium energy energetic electrons up to ultra‐relativistic electrons. The number of substorms identified inside a cluster does not depend strongly on solar wind drivers or geomagnetic indices either before, during, or after the cluster start time. Clusters of substorms linked to moderate (100 nT < AE ≤ 300 nT) or strong AE (AE ≥ 300 nT) disturbances are associated with radiation belt flux enhancements, including up to ultra‐relativistic energies by the strongest substorms (as measured by strong southward Bz and high AE). These clusters reliably occur during times of high speed solar winds streams with associated increased magnetospheric convection. However, substorm clusters associated with quiet AE disturbances (AE ≤ 100 nT) lead to no significant chorus whistler mode intensity enhancements, or increases in energetic, relativistic, or ultra‐relativistic electron flux in the outer radiation belts. In these cases the solar wind speed is low, and the geomagnetic Kp index indicates a lack of magnetospheric convection. Our study clearly indicates that clusters of substorms occurring outside of high speed wind streams are not by themselves sufficient to drive acceleration, which may be due to the lack of pre‐cluster convection.
The whistler mode chorus emissions are pervasively detected by the Juno satellite in Jupiter’s magnetospheric environment. This article pays particular attention to a sample observation made by the Juno on 3 November 2019, where typical whistler mode chorus waves are measured. The emission is characterized by a broad range of wave frequencies from below fce/2, where fce denotes the local electron cyclotron frequency, down to the lower‐hybrid frequency, with a gradually downshifting frequency over time. The excitation appears to coincide with the detection of a “butterfly” pitch‐angle distribution and the expected loss‐cone feature associated with the energetic electrons. These anisotropic features, especially the butterfly pitch‐angle distribution, gradually disappear as the waves are excited and the electron phase space distribution becomes isotropic. The aim of this article is to model the characteristics by means of quasilinear kinetic theory of the whistler instability driven by a loss‐cone electron distribution function with a narrow loss‐cone angle, which is to be expected from low‐latitude regions of the Jovian magnetosphere. It is shown that the theoretically constructed dynamic wave spectrum is consistent with the observation made on 3 November 2019. The present finding demonstrates that the quasilinear theory can be a powerful theoretical tool for interpreting various Jovian plasma wave emissions, which includes the whistler waves, but also other wave modes.
Whistler‐mode chorus waves are critical for driving resonant scattering and loss of radiation belt relativistic electrons into the atmosphere. The resonant energies of electrons scattered by chorus waves increase at increasingly higher magnetic latitudes. Propagation of chorus waves to middle and high latitudes is hampered by wave divergence and Landau damping but is promoted otherwise if ducted by density irregularities. Although ducting theories have been proposed since the 1960s, no conjugate observation of ducted chorus propagation from the equatorial magnetosphere to the ionosphere has been observed so far. Here we provide such an observation, for the first time, using conjugate spacecraft measurements. Ducted chorus waves maintain significant wave power upon reaching the ionosphere, which is confirmed by ray‐tracing simulations. Our results suggest that ducted chorus waves may be an important driver for relativistic electron precipitation.
Electromagnetic waves observed in the inner magnetosphere at frequencies between about 0.5 and 4 kHz sometimes exhibit a quasiperiodic (QP) time modulation of the wave intensity with modulation periods from tens of seconds up to a few minutes. Such waves are typically termed “QP emissions” and their origin is still not fully understood. We use a large set of more than 2,000 of these events identified in the low‐altitude DEMETER spacecraft data to check for energetic electron flux variations matching the individual QP wave elements. Altogether, seven such events are identified and their detailed analysis is performed. Energetic electron fluxes are found to be modulated primarily at energies lower than about 250 keV. While the waves may propagate unducted across L‐shells, the energetic particles follow magnetic field lines from the interaction region down to the observation point. This is used to estimate the locations of anticipated generation regions to L‐shells between about 4 and 6, and the respective source radial dimensions to about 0.6–1.2 Earth radii. The frequencies of the events are confined below half of the equatorial electron gyrofrequency in the determined source regions. Finally, it is shown that individual QP elements exhibit a fine inner structure corresponding to the wave bouncing between the hemispheres.
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.
We present, for the first time, a plasmaspheric hiss event observed by the Van Allen probes in response to two successive interplanetary (IP) shocks occurring within an interval of ∼2 h on December 19, 2015. The first shock arrived at 16:16 UT and caused disappearance of hiss for ∼30 min. Combined effect of plasmapause crossing, significant Landau damping by suprathermal electrons and their gradual removal by magnetospheric compression led to the disappearance of hiss. Calculation of electron phase space density and linear wave growth rates showed that the shock did not change the growth rate of whistler waves within the core frequency range of plasmaspheric hiss (0.1–0.5 kHz) during this interval making conditions unfavorable for the generation of hiss. The recovery began at ∼16:45 UT which is attributed to an enhancement in local plasma instability initiated by the first shock‐induced substorm and additional possible contribution from chorus waves. This time, the wave growth rate peaked within the core frequency range (∼350 Hz). The second shock arrived at 18:02 UT and generated patchy hiss persisting up to ∼19:00 UT. It is shown that an enhanced growth rate and additional contribution from shock‐induced poloidal Pc5 mode (periodicity ∼240 s) ultralow frequency (ULF) waves resulted in the excitation of hiss waves during this period. The hiss wave amplitudes were found to be additionally modulated by background plasma density and fluctuating plasmapause location. The investigation highlights the important roles of IP shocks, substorms, ULF waves, and background plasma density in the variability 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.
Magnetospheric Extremely Low‐Frequency/Very Low‐Frequency (ELF/VLF) waves have an important role in the acceleration and loss of energetic electrons in the magnetosphere through wave‐particle interaction. It is necessary to understand the spatiotemporal development of magnetospheric ELF/VLF waves to quantitatively estimate this effect of wave‐particle interaction, a global process not yet well understood. We investigated spatiotemporal development of magnetospheric ELF/VLF waves using 6 PWING ground‐based stations at subauroral latitudes, Exploration of energization and Radiation in Geospace and RBSP satellites, POES/MetOp satellites, and the RAM‐SCB model, focusing on the March and November 2017 storms driven by corotating interaction regions in the solar wind. Our results show that the ELF/VLF waves are enhanced over a longitudinal extent from midnight to morning and dayside associated with substorm electron injections. In the main to early storm recovery phase, we observe continuous ELF/VLF waves from ∼0 to ∼12 MLT in the dawn sector. This wide extent seems to be caused by frequent occurrence of substorms. The wave region expands eastward in association with the drift of source electrons injected by substorms from the nightside. We also observed dayside ELF/VLF wave enhancement, possibly driven by magnetospheric compression by solar wind, over an MLT extent of at least 5 h. Ground observations tend not to observe ELF/VLF waves in the post‐midnight sector, although other methods clearly show the existence of waves. This is possibly due to Landau damping of the waves, the absence of the plasma density duct structure, and/or enhanced auroral ionization of the ionosphere in the post‐midnight sector.
We have conducted an extensive survey of whistler mode hiss emission at Saturn using nearly the complete data set from the Cassini Radio and Plasma Wave Science investigation. We distinguish the hiss emission by frequency and intensity threshold criteria. The survey includes all longitudes and latitudes >25° for r < 10 RS, limited to the availability of plasma density data. The results indicate that hiss intensity is highest at high latitude, and the level of intensity is larger than chorus by at least an order of magnitude. After a rapid increase in intensity starting near L = 13, the hiss intensity remains relatively constant extending to L > 100. The intensity of the hiss as a function of frequency has a shallow peak near 35 Hz but remains always in the range ~5 × 10⁻⁵ nT² < I < ~10⁻³ nT². These survey results will be important in modeling the scattering interactions of whistler mode emissions with electrons in the Saturn magnetosphere. Based on recent particle and field observations, sources of Saturn hiss occur on and near the footprints of field‐aligned currents, plasma injections, and reconnected field lines.
Whistler mode chorus waves have recently been established as the most likely candidate for scattering relativistic electrons to produce the electron microbursts observed by low altitude satellites and balloons. These waves would have to propagate from the equatorial source region to significantly higher magnetic latitude in order to scatter electrons of these relativistic energies. This theoretically proposed propagation has never been directly observed. We present the first direct observations of the same discrete rising tone chorus elements propagating from a near equatorial (Van Allen Probes) to an off‐equatorial (Arase) satellite. The chorus is observed first on the more equatorial satellite and is found to be more oblique and significantly attenuated at the off‐equatorial satellite. This is consistent with the prevailing theory of chorus propagation and with the idea that chorus must propagate from the equatorial source region to higher latitudes. Ray tracing of chorus at the observed frequencies confirms that these elements could be generated parallel to the field at the equator, and propagate through the medium unducted to Van Allen Probes A and then to Arase with the observed time delay, and have the observed obliquity and intensity at each satellite.
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.
We study quasiperiodic very low frequency (VLF) emissions observed simultaneously by Van Allen Probes spacecraft and Kannuslehto and Lovozero ground‐based stations on 25 December 2015. Both Van Allen Probes A and B detected quasiperiodic emissions, probably originated from a common source, and observed on the ground. In order to locate possible regions of wave generation, we analyze wave‐normal angles with respect to the geomagnetic field, Poynting flux direction, and cyclotron instability growth rate calculated by using the measured phase space density of energetic electrons. We demonstrate that even parallel wave propagation and proper (downward) Poynting flux direction are not sufficient for claiming observations to be in the source region. Agreement between the growth rate and emission bands was obtained for a restricted part of Van Allen Probe A trajectory corresponding to localized enhancement of plasma density with scale of 700 km. We employ spacecraft density data to build a model plasma profile and to calculate ray trajectories from the point of wave detection in space to the ionosphere and examine the possibility of their propagation toward the ground. For the considered event, the wave could propagate toward the ground in the geomagnetic flux tube with enhanced plasma density, which ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.
Key Points
The rising‐tone quasiperiodic waves with simultaneous energetic precipitations were observed in the dayside ionosphere
The wave spectral properties show dynamic structures with clear cutoff frequencies
The quasiperiodic modulations are probably related to ULF waves or the wave particle cyclotron instability
We investigate the propagation characteristics of low‐altitude hiss in the ionosphere by numerical simulation with a two‐dimensional full‐wave model. The simulation results demonstrate that linear mode conversion from whistler to H⁺ band electromagnetic ion cyclotron wave and polarization reversal occur simultaneously where wave frequency matches the H⁺−He⁺ crossover frequency. This mode conversion efficiency shows sensitive dependence on wave normal angle and plays a significant role in the propagation of whistler emission near the local proton gyro‐frequency in the ionosphere by redistributing the wave energy below and above the H⁺−He⁺ cutoff frequency, which can explain the low‐altitude hiss observed by the Freja and Detection of Electromagnetic Emissions Transmitted from Earthquake Regions satellites, respectively. The energy of whistler‐mode low‐altitude hiss emission can be transferred to reflected left‐hand polarized electromagnetic ion cyclotron through mode conversion and the efficiency reaches a maximum for intermediate incident wave normal angle (of 45°).
Low frequency (LF) ~22 Hz to 200 Hz plasmaspheric hiss was studied using a year of Polar plasma wave data occurring during solar cycle minimum. The waves are found to be most intense in the noon and early dusk sectors. When only the most intense LF (ILF) hiss was examined, they are found to be substorm dependent and most prominent in the noon sector. The noon sector ILF waves were also determined to be independent of solar wind ram pressure. The ILF hiss intensity is independent of magnetic latitude. ILF hiss is found to be highly coherent in nature. ILF hiss propagates at all angles relative to the ambient magnetic field. Circular, elliptical, and linear/highly elliptically polarized hiss have been detected, with elliptical polarization the dominant characteristic. A case of linear polarized ILF hiss that occurred deep in the plasmasphere during geomagnetic quiet was noted. The waveforms and polarizations of ILF hiss are similar to those of intense high frequency hiss. We propose the hypothesis that ~10–100 keV substorm injected electrons gradient drift to dayside minimum B pockets close to the magnetopause to generate LF chorus. The closeness of this chorus to low altitude entry points into the plasmasphere will minimize wave damping and allow intense noon‐sector ILF hiss. The coherency of ILF hiss leads the authors to predict energetic electron precipitation into the mid latitude ionosphere and the electron slot formation during substorms. Several means of testing the above hypotheses are discussed.
We model lower band chorus observations from the DEMETER satellite using daily and hourly autoregressive‐moving average transfer function (ARMAX) equations. ARMAX models can account for serial autocorrelation between observations that are measured close together in time and can be used to predict a response variable based on its past behavior without the need for recent data. Unstable distributions of radiation belt source electrons (tens of keV) and the substorm activity (SMEd from the SuperMAG array) that is thought to inject these electrons were both statistically significant explanatory variables in a daily ARMAX model describing chorus. Predictions from this model correlated well with observations in a hold‐out test data set (validation correlation of 0.675). Source electron flux was most influential when observations came from the same day or the day before the chorus measurement, with effects decaying rapidly over time. Substorms were more influential when they occurred on previous days, presumably due to their injecting source electrons from the plasma sheet. A daily ARMAX model with interplanetary magnetic field (IMF)|B|, IMF Bz, and solar wind pressure as inputs instead of those given above was somewhat less predictive of chorus (r=0.611). An hourly ARMAX model with only solar wind and IMF inputs was even less successful, with a validation correlation of 0.502.
In this study, we use the observations of electromagnetic waves by Detection of Electromagnetic Emissions Transmitted from Earthquake Regions satellite to investigate propagation characteristics of low‐altitude ionospheric hiss. In an event study, intense hiss wave power is concentrated over a narrow frequency band with a central frequency that decreases as latitude decreases, which coincides to the variation of local proton cyclotron frequency fCH. The wave propagates obliquely to the background magnetic field and equatorward from high latitude region. We use about ∼6 years of observations to statistically study the dependence of ionospheric hiss wave power on location, local time, geomagnetic activity, and season. The results demonstrate that the ionospheric hiss power is stronger on the dayside than nightside, under higher geomagnetic activity conditions, in local summer than local winter. The wave power is confined near the region where the local fCH is equal to the wave frequency. A ray tracing simulation is performed to account for the dependence of wave power on frequency and latitude.
Plain Language Summary
Plasmaspheric hiss waves are typically observed inside a high‐density region of geospace known as the plasmasphere. Chorus waves are typically observed at higher altitudes, beyond the plasmasphere region, where the density is substantially lower. Despite the differences between these two wave types, it has been proposed that chorus waves may propagate in such a way that they enter the plasmasphere, where they become a source of plasmaspheric hiss. However, this mechanism can only occur if chorus waves have a specific set of initial conditions. In this study, we find that chorus waves are rarely observed with these required conditions. Only in a spatially limited region close to the edge of plasmaspheric plume structures, where chorus wave power is typically weaker, do we observe a significant fraction of chorus waves that exist with the conditions required to propagate into the plasmasphere. This result qualitatively indicates that chorus waves may not be a substantial source of plasmaspheric hiss.
Plain Language Summary
Highly oblique chorus can provide efficient energy diffusion for the rapid acceleration of electrons. The electron pitch angle scattering rates driven by them could be stronger than those by quasi‐parallel chorus with the same wave amplitudes. Recent studies have indicated that highly oblique lower band chorus could be excited by a low‐energy electron beam or electron plateau. Here we present another excitation mechanism of highly oblique lower and upper band chorus by using the simultaneous observations and numerical modeling. A highly oblique chorus waves event was observed by Van Allen Probe A on 3 July 2016. The waves occurred in both lower and upper bands during 23:08–23:25 UT and then only in the lower band during 23:25–24:00 UT. We calculated chorus growth rates based on the observed data. At 23:18:11 UT, the growth rate has peaks ≈1.3 × 10⁻⁴ and 1.6 × 10⁻⁴ at 0.32fce and 0.64fce, respectively. At 23:54:07 UT, the growth rate has a peak ≈1.2 × 10⁻⁴ at 0.34fce in the lower band and a peak below 1 × 10⁻⁵ in the upper band. Our new results provide a reliable evidence that highly oblique chorus waves can be excited by the energetic electrons with a loss cone distribution and distinct temperature anisotropy.
Intense ~300-Hz to 1.0-kHz plasmaspheric hiss was studied using Polar plasma wave data. It is found that the waves are coherent in all local time sectors with the wave coherency occurring in approximately three- to five-wave cycle packets. The plasmaspheric hiss in the dawn and local noon time sector are found to be substorm (AE*) and storm (SYM-H*) dependent. The local noon sector is also solar wind pressure dependent. It is suggested that coherent chorus monochromatic subelements enter the plasmasphere (as previously suggested by ray tracing models) to explain these plasmaspheric hiss features. The presence of intense, coherent plasmaspheric hiss in the local dusk and local midnight time sectors is surprising and more difficult to explain. For the dusk sector waves, either local in situ plasmaspheric wave generation or propagation from the dayside plasmasphere is possible. There is little evidence to support substorm generation of the midnight sector plasmaspheric hiss found in this study. One possible explanation is propagation from the local noon sector. The combination of high wave intensity and coherency at all local times strengthens the suggestion that the electron slot is formed during substorm intervals instead of during geomagnetic quiet (by incoherent waves). Plasmaspheric hiss is found to propagate at all angles relative to the ambient magnetic field, θkB. Circular, elliptical, and linear polarized plasmaspheric hiss have been detected. No obvious, strong relationship between the wave polarization and θkB was found. This information of hiss properties should be useful in modeling wave-particle interactions within the plasmasphere.
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.
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.
Van Allen Probes observations are used to statistically investigate plasmaspheric hiss wave properties. This analysis shows that the wave normal direction of plasmaspheric hiss is predominantly field aligned at larger L shells, with a bimodal distribution, consisting of a near-field aligned and a highly oblique component, becoming apparent at lower L shells. Investigation of this oblique population reveals that it is most prevalent at L < 3, frequencies with f/fce> 0.01 (or f> 700 Hz), low geomagnetic activity levels, and between 1900 and 0900 MLT. This structure is similar to that reported for oblique chorus waves in the equatorial region, perhaps suggesting a causal link between the two wave modes. Ray tracing results from HOTRAY confirm that is feasible for these oblique chorus waves to be a source of the observed oblique plasmaspheric hiss population. The decrease in oblique plasmaspheric hiss occurrence rates during more elevated geomagnetic activity levels may be attributed to the increase in Landau resonant electrons causing oblique chorus waves to be more substantially damped outside of the plasmasphere. In turn, this restricts the amount of wave power that can access the plasmasphere and evolve into oblique plasmaspheric hiss. These results confirm that, despite the difference in location of this bimodal distribution compared to previous studies, a direct link between oblique equatorial chorus outside of the plasmasphere and oblique hiss at low L shells is plausible. As such, these results are in keeping with the existing theory of chorus as the source of plasmaspheric hiss.
We present results of simultaneous observations of VLF chorus elements at the ground based station Kannuslehto in Northern Finland and on board Van Allen Probe A. Visual inspection and correlation analysis of the data reveal one-to-one correspondence of several (at least 12) chorus elements following each other in a sequence. Poynting flux calculated from electromagnetic fields measured by the EMFISIS instrument on board Van Allen Probe A shows that the waves propagate at small angles to the geomagnetic field and oppositely to its direction, i.e., from Northern to Southern geographic hemisphere. The spacecraft was located at L≃4.1 at a geomagnetic latitude of −12.4∘ close to the plasmapause and inside a localized density inhomogeneity with about 30% density increase and a transverse size of about 600 km. The time delay between the waves detected on the ground and on the spacecraft is about 1.3 s, with ground-based detection leading spacecraft detection. The measured time delay is consistent with the wave travel time of quasi-parallel whistler mode waves for a realistic profile of the plasma density distribution along the field line. The results suggest that chorus discrete elements can preserve their spectral shape during a hop from the generation region to the ground followed by reflection from the ionosphere and return to the near-equatorial region.
Whistler-mode chorus waves are a naturally occurring electromagnetic emission observed in Earth's magnetosphere. Here, for the first time, data from NASA's Magnetospheric Multiscale (MMS) mission were used to analyze chorus waves in detail, including the calculation of chorus wave normal vectors, k. A case study was examined from a period of substorm activity around the time of a conjunction between the MMS constellation and NASA's Van Allen Probes mission on 07 April 2016. Chorus wave activity was simultaneously observed by all six spacecraft over a broad range of L-shells (5.5 < L < 8.5), magnetic local time (06:00 < MLT < 09:00), and magnetic latitude (-32° < MLat < -15°), implying a large chorus active region. Eight chorus elements and their substructure were analyzed in detail with MMS. These chorus elements were all lower band and rising tone emissions, right-handed and nearly circularly polarized, and propagating away from the magnetic equator when they were observed at MMS (MLat ~ -31°). Most of the elements had “hook” like signatures on their wave power spectra, characterized by enhanced wave power at flat or falling frequency following the peak, and all the elements exhibited complex and well organized substructure observed consistently at all four MMS spacecraft at separations up to 70 km (60 km perpendicular and 38 km parallel to the background magnetic field). The waveforms in field-aligned coordinates also demonstrated that these waves were all phase coherent allowing for the direct calculation of k. Error estimates on calculated k revealed that the plane wave approximation was valid for six of the eight elements and most of the subelements. The wave normal vectors were within 20-30° from the direction anti-parallel to the background field for all elements and changed from subelement to subelement through at least two of the eight elements. The azimuthal angle of k in the perpendicular plane was oriented earthward and was oblique to that of the Poynting vector, which has implications for the validity of cold plasma theory.
We present simulations of the loss of radiation belt electrons by resonant pitch angle diffusion caused by whistler mode hiss waves for March 2013. Pitch angle diffusion coefficients are computed from the wave properties and the ambient plasma data obtained by the Van Allen Probes with a resolution of 8 hours and 0.1 L-shell. Loss rates follow a complex dynamic structure, imposed by the wave and plasma properties. Hiss effects can be strong, with minimum lifetimes (of ~1 day) moving from energies of ~100 keV at L~5 up to ~2 MeV at L~2, and stop abruptly, similarly to the observed energy-dependent inner belt edge. Periods when the plasmasphere extends beyond L~5 favor long-lasting hiss losses from the outer belt. Such loss rates are embedded in a reduced Fokker-Planck code and validated against MagEIS observations of the belts at all energy. Results are complemented with a sensitivity study involving different radial diffusion and lifetime models. Validation is carried out globally at all L-shells and energies. The good agreement between simulations and observations demonstrates that hiss waves drive the slot formation during quiet times. Combined with transport, they sculpt the energy-structure of the outer belt into an "S-shape". Low energy electrons (<0.3 MeV) are less subject to hiss scattering below L=4. In contrast, 0.3-1.5 MeV electrons evolve in a environment that depopulates them as they migrate from L~5 to L~2.5. Ultra-relativistic electrons are not affected by hiss losses until L~2-3.
We analyze the propagation properties of low altitude hiss emission in the ionosphere observed by DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions). There exist two types of low altitude hiss; type I emission at high latitude is characterized by vertically downward propagation and broadband spectra while type II emission at low latitude is featured with equatorward propagation and a narrower frequency band above ∼fcH+. Our ray tracing simulation demonstrates that both types of the low altitude hiss at different latitude are connected and they originate from plasmaspheric hiss and in part chorus emission. Type I emission represents magnetospheric whistler emission that accesses the ionosphere. Equatorward propagation associated with type II emission is a consequence of wave trapping mechanisms in the ionosphere. Two different wave trapping mechanisms are identified to explain the quatorial propagation of Type II emission; one is associated with the proximity of wave frequency and proton cyclotron frequency while the other occurs near the ionospheric density peak.
A study of dayside plasmaspheric hiss at frequencies from ~22 Hz to ~1.0 kHz was carried out using one year of Polar data. It is shown that intense, dayside plasmaspheric hiss is correlated with solar wind pressure with P > 2.5 nPa. The dayside effect is most prominent in the ~300 to ~650 Hz range. Intense dayside waves are also present during SYM-H < -5 nT. The latter is centered at local noon, with the greatest intensities in the L =2 to 3 region. Assuming drift of ~25 keV electrons from midnight to the wave MLT, plasmaspheric hiss is shown to be highly correlated with precursor AE* and SYM-H* indices, indicating that the hiss is associated with substorms and small injection events. Our hypothesis is that both sets of waves originate as outer zone (L = 6 to 10) chorus and then propagate into the plasmasphere. Fourteen high intensity dayside plasmaspheric hiss events were analyzed to identify the wave k, polarization and the degree of coherency. The waves are found to be obliquely propagating, elliptically polarized and quasi-coherent (~0.5 to 0.8 correlation coefficient). It is hypothesized that the dayside plasmaspheric hiss is quasi-coherent because the chorus has been recently generated in the outer magnetosphere and have propagated directly into the plasmasphere. It is possible that the quasi-coherency of the dayside hiss at L = 2 to 3 may be an alternate explanation for the generation of the energetic particle slot region.
We present new results on wave vectors and Poynting vectors of chorus rising and falling tones on the basis of 6 years of THEMIS (Time History of Events and Macroscale Interactions during Substorms) observations. The majority of wave vectors is closely aligned with the direction of the ambient magnetic field (B0). Oblique wave vectors are confined to the magnetic meridional plane, pointing away from Earth. Poynting vectors are found to be almost parallel to B0. We show, for the first time, that slightly oblique Poynting vectors are directed away from Earth for rising tones and towards Earth for falling tones. For the majority of lower band chorus elements, the mutual orientation between Poynting vectors and wave vectors can be explained by whistler mode dispersion in a homogeneous collisionless cold plasma. Upper band chorus seems to require inclusion of collisional processes or taking into account azimuthal anisotropies in the propagation medium. The latitudinal extension of the equatorial source region can be limited to ±6∘ around the B0-minimum, or approximately ±5000 km along magnetic field lines. We find increasing Poynting flux and focusing of Poynting vectors on the B0-direction with increasing latitude. Also wave vectors become most often more field-aligned. A smaller group of chorus generated with very oblique wave normals tends to stay close to the whistler mode resonance cone. This suggests that close to the equatorial source region (within ∼20∘ latitude), a wave guidance mechanism is relevant, for example in ducts of depleted or enhanced plasma density.
The twin Van Allen Probes spacecraft were launched on 30 August 2012 to study the Earth's Van Allen radiation belts. The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) investigation includes the Waves instrument that simultaneously measures three orthogonal components of the wave magnetic field from ~10 hertz (Hz) to 12 kilohertz (kHz) and, with the support of the Electric Fields and Waves (EFW) sensors, three components of the wave electric field from ~10 Hz to 12 kHz, and a single electric component up to ~500 kHz. Since launch, a variety of plasma waves have been detected that are believed to play a role in the dynamics of the radiation belts, including whistler mode chorus, plasmaspheric hiss, and magnetosonic equatorial noise. Lightning produced whistlers, electron cyclotron harmonic emission, quasi-periodic (QP) whistler mode emission, and the upper hybrid resonance (UHR) are also often detected. The UHR is used to determine the local electron plasma density (an important parameter of the plasma required for various modeling and simulation studies). Measuring all six components simultaneously allows the wave propagation parameters of these plasma wave emissions, including the Poynting flux, wave normal vector, and polarization, to be obtained. We will summarize the EMFISIS wave observations and discuss their role in the Van Allen radiation belt dynamics.
This chapter focuses on the origin of the plasmaspheric hiss (PH) emission, and in particular, considers whether chorus could be the embryonic source of PH. It describes the ray-tracing model used to study chorus wave propagation. The chapter mainly focuses on more recent observations that still raise puzzling questions regarding the origin of PH. It then shows that PH has a lower frequency boundary, and that, at least in some instances, chorus could not be the source of the PH power and that it would necessarily need to be excited locally. The chapter also shows a coincident observation between THEMIS-E and one of the newly launched Van Allen Probes (RB-B) that showed simultaneous chorus and hiss emissions. These emissions were very well correlated but occurred at much higher L-shells than predicted by our ray-tracing model.
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