No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Ionospheric Source for Low-Frequency Broadband Electromagnetic Signatures
Abstract
A linear eigenmode analysis shows that small-scale static electric field structures commonly found in the auroral ionosphere are natural generators of low-frequency ( ω≤Ωi, where Ωi is the ion cyclotron frequency) electromagnetic waves with broadbanded frequency spectra. These waves can account for the observed upward Poynting flux and distinctive polarization signatures in the ionosphere. The ratio of wave electric to magnetic field strength can be much larger than the Alfvén velocity. The local Doppler shift is important for facilitating resonant energization of ions and electrons.
... In a set of papers [81,82,83], Peñano and Ganguli, derive a system of eigenvalue equations describing electromagnetic waves in a collisionless, magnetized plasma in the presence of a localized transverse inhomogeneous dc electric field. They numerically solve the resulting dispersion relation for typical conditions in the F region of the ionosphere in two regimes: very low frequencies and for frequencies near the ion cyclotron frequency. ...
... Using negative energy wave considerations, it was shown that these sheared transverse flows can lead to an instability with the Doppler-shifted frequency resonant with the ion cyclotron frequency. In subsequent papers, the theory was extended to arbitrary flow profiles [36], cylindrical geometry [84], and electromagnetic instabilities [81,82,83]. ...
... In this section we present the Peñano and Ganguli [81] non-local, collisionless model for electromagnetic waves in the presence of an inhomogeneous electric field transverse to the background magnetic field. What follows is an outline of the derivation, however, a thorough treatment can be found in Appendix A. A Cartesian coordinate system is used with the background magnetic field along the z axis, such that B 0 = B 0ẑ . ...
Laboratory observations of electromagnetic ion-cyclotron waves generated by a localized transverse dc electric field are reported. Experiments indicate that these waves result from a strong E×B flow inhomogeneity in a mildly collisional plasma with subcritical magnetic field-aligned current. The wave amplitude scales with the magnitude of the applied radial dc electric field. The electromagnetic signatures become stronger with increasing plasma β, and the radial extent of the power is larger than that of the electrostatic counterpart. Near-Earth space weather implications of the results are discussed.
... (48) However, since some power is also found in the electromagnetic regime, the derivation has also been generalized to the electromagnetic regime using a fluid model for the ions (Peñano and Ganguli 1999, 2000. ...
... Subsequently, demonstrated that by increasing the magnitude of the transverse electric field and virtually eliminating the axial current with biased ring electrodes (Fig. 39), the electrostatic ion cyclotron waves could be sustained by a sheared transverse flow alone. These experiments were later followed up by Tejero et al. (2011) to confirm the electromagnetic IEDDI (Peñano and Ganguli 1999). These waves, besides validating the theory, were shown to be efficient in ion heating as was expected . ...
Plasma in the earth’s magnetosphere is subjected to compression during geomagnetically active periods and relaxation in subsequent quiet times. Repeated compression and relaxation is the origin of much of the plasma dynamics and intermittency in the near-earth environment. An observable manifestation of compression is the thinning of the plasma sheet resulting in magnetic reconnection when the solar wind mass, energy, and momentum floods into the magnetosphere culminating in the spectacular auroral display. This phenomenon is rich in physics at all scale sizes, which are causally interconnected. This poses a formidable challenge in accurately modeling the physics. The large-scale processes are fluid-like and are reasonably well captured in the global magnetohydrodynamic (MHD) models, but those in the smaller scales responsible for dissipation and relaxation that feed back to the larger scale dynamics are often in the kinetic regime. The self-consistent generation of the small-scale processes and their feedback to the global plasma dynamics remains to be fully explored. Plasma compression can lead to the generation of electromagnetic fields that distort the particle orbits and introduce new features beyond the purview of the MHD framework, such as ambipolar electric fields, unequal plasma drifts and currents among species, strong spatial and velocity gradients in gyroscale layers separating plasmas of different characteristics, etc. These boundary layers are regions of intense activity characterized by emissions that are measurable. We study the behavior of such compressed plasmas and discuss the relaxation mechanisms to understand their measurable signatures as well as their feedback to influence the global scale plasma evolution.
... which, in conjunction with the quasi-neutrality condition or the Poisson equation, provides the electrostatic dispersion eigenvalue condition in the form of a second order differential equation. Using the fluid model for ions the derivation has also been generalized to the electromagnetic regime [53,54,55]. ...
... Subsequently, Amatucci et al. [110] demonstrated that by increasing the magnitude of the transverse electric field and virtually eliminating the axial current with biased ring electrodes (Fig. 39), the electrostatic ion cyclotron waves could be sustained by a sheared transverse flow alone. These experiments were later followed up by Tejero et al. [111] to confirm the electromagnetic IEDDI [53]. These waves, besides validating the theory, were shown to be efficient in ion heating [110] as was expected [108]. ...
Plasma in the earth's magnetosphere is subjected to compression during geomagnetically active periods and relaxation in subsequent quiet times. Repeated compression and relaxation is the origin of much of the plasma dynamics and intermittency in the near-earth environment. An observable manifestation of compression is the thinning of the plasma sheet resulting in magnetic reconnection when the solar wind mass, energy, and momentum floods into the magnetosphere culminating in the spectacular auroral display. This phenomenon is rich in physics at all scale sizes, which are causally interconnected. This poses a formidable challenge in accurately modeling the physics. The large-scale processes are fluid-like and are reasonably well captured in the global magnetohydrodynamic (MHD) models, but those in the smaller scales responsible for dissipation and relaxation that feed back to the larger scale dynamics are often in the kinetic regime. The self-consistent generation of the small-scale processes and their feedback to the global plasma dynamics remains to be fully explored. Plasma compression can lead to the generation of electromagnetic fields that distort the particle orbits and introduce new features beyond the purview of the MHD framework, such as ambipolar electric fields, unequal plasma drifts and currents among species, strong spatial and velocity gradients in gyroscale layers separating plasmas of different characteristics, \textit{etc.} These boundary layers are regions of intense activity characterized by emissions that are measurable. We study the behavior of such compressed plasmas and discuss the relaxation mechanisms to understand their measurable signatures as well as their feedback to influence the global scale plasma evolution.
... EMIC waves are potentially even more important to these processes since they can propagate far from the wave generation region, heating ions along their path. Electromagnetic instabilities due to such localized transverse electric field structures have also been predicted by [19], but they have not previously been experimentally investigated. Such locally generated EMIC waves could affect broader ionospheric and magnetospheric dynamics by heating remote ions and convecting energy far from the region of wave creation. ...
... Our experiment establishes that strongly localized dc electric fields perpendicular to the ambient magnetic field can behave as a radiation source for EMIC waves, which can transport the energy away from the region of wave generation. The results are consistent with the theory of Peñano and Ganguli [19]. The EMIC wave amplitude increases with increasing plasma β and the Doppler-shifted frequency is resonant with a harmonic of the ioncyclotron frequency. ...
Laboratory observations of electromagnetic ion cyclotron waves generated by a localized transverse dc electric field are reported. Experiments indicate that these waves result from a strong E×B flow inhomogeneity in a mildly collisional plasma with sub-critical magnetic field-aligned current. The wave amplitude scales with the magnitude of the applied radial dc electric field. The electromagnetic signatures become stronger with increasing plasma β, and the radial extent of the power is larger than that of the electrostatic counterpart. Near-Earth space weather implications of the result are discussed.
... In addition, inhomogeneities in the plasma flow are ever present in the auroral zone which can act as an additional source of free energy for wave generation [Amatucci, 1999]. This shear-driven mode, known as inhomogeneous energy-density-driven instability (IEDDI), is considered to be a generation mechanism for BBELF waves Penano and Ganguli, 1999]. A full kinetic analysis of the IEDDI has been carried out , and the existence of IEDD waves has been verified in the laboratory Amatucci et al., 1998 Amatucci et al., , 1994 DuBois et al., 2013b] . ...
Transversely accelerated ions and the associated heating of the high-latitude ionosphere have been attributed to broadband extremely low-frequency (BBELF) turbulence. Controlled laboratory tests of the hypotheses on the formation mechanism of BBELF waves have involved only a few examples, e.g., current-driven and shear-driven instabilities. In this work, electrostatic fluctuations in the ion-cyclotron frequency range have been excited by inhomogeneous energy-density driven instability (IEDDI). This was achieved using the interpenetrating plasma method with a much larger electric field scale size LE comparable to the ion gyroradius ρi, which was challenging earlier because of plasma conditions. The peak frequency of the IEDDI spectrum falls as low as ω≈0.3ωci, where ωci is ion cyclotron frequency. This is an interesting result because the previous attempts could not produce such low frequency IEDDI, although it was known theoretically to be possible. The observations made by FAST, Freja, and THEMIS satellites might be explainable in terms of the reported experimental results.
... We expect this to be a possible mechanism responsible for the ion heating. It has been shown that velocity shears (including those generated by KHI) can generate a broad spectra of electromagnetic waves [38][39][40][41][42] . The macroscopic velocity shear originating from the interaction between the solar wind and the Earth's magnetosphere provides free energy that generates KHI on MHD scales. ...
The solar wind is a supersonic magnetized plasma streaming far into the heliosphere. Although cooling as it flows, it is rapidly heated upon encountering planetary obstacles. At Earth, this interaction forms the magnetosphere and its sub-regions. The present paper focuses on particle heating across the boundary separating the shocked solar wind and magnetospheric plasma, which is driven by mechanisms operating on fluid, ion and electron scales. The cross-scale energy transport between these scales is a compelling and fundamental problem of plasma physics. Here, we present evidence of the energy transport between fluid and ion scales: free energy is provided in terms of a velocity shear generating fluid-scale Kelvin–Helmholtz instability. We show the unambiguous observation of an ion-scale magnetosonic wave packet, inside a Kelvin–Helmholtz vortex, with sufficient energy to account for observed ion heating. The present finding has universal consequences in understanding cross-scale energy transport, applicable to environments experiencing velocity shears during comparable plasma regimes.
... Besides this, the inhomogeneous plasmas can be source for electrostatic potential (i.e., spatially localized dc electric fields) that are likely to accompany steep density gradients and contribute to the inhomogeneity. The localized transverse electric fields can significantly affect wave dispersion properties and can be a source for Alfv enic waves [25][26][27] which may amplify the antennalaunched waves. Scime et al. 28 have observed high speed flows that are believed to be due to this potential structure. ...
The propagation of linear Kinetic Alfv�en waves (KAWs) in inhomogeneous magnetized plasma has been studied while including inhomogeneities in transverse and parallel directions relative to the background magnetic field. The propagation of KAWs in inhomogeneous magnetized plasma is expected to play a key role in energy transfer and turbulence generation in space and laboratory
plasmas. The inhomogeneity scale lengths in both directions may control the nature of fluctuations
and localization of the waves. We present a theoretical study of the localization of KAWs,
variations in magnetic field amplitude in time, and variation in the frequency spectra arising from
inhomogeneities. The relevance of the model to space and laboratory observations is discussed.
... Tejero et al. (2011) reported on laboratory observations of EMIC waves generated by a localized transverse direct-current electric field. Peñano and Ganguli (1999Ganguli ( , 2002) argue that electromagnetic IEDDI could account for the Poynting flux that originates from the ionosphere, being observed by satellites at a higher altitude. As already mentioned, in the present study it is assumed that the frequencies in the satellite frame are Doppler-shifted frequencies, resulting from satellite traversing spatial structures . ...
We examine the effectiveness of nonuniform, quasistatic, transverse electric fields that are often observed
in the auroral region in destabilization of inhomogeneous energy-density-driven (IEDD) waves. Specifically, the IEDD dispersion relation of Ganguli et al. (1985a, b) is evaluated for an electric field structure observed by the FAST satellite in the auroral ionosphere at 1000 km altitude. The background field-aligned current, plasma density and ion composition are derived from FAST observations. Other input parameters adopted in the calculations are varied in pertinent ranges. Unstable solutions are obtained that indicate a variety of frequencies and perpendicular wavelengths. These can manifest as a broadband spectrum of IEDD waves.
... v A . While these events have been interpreted by Ivchenko et al. [1999] according to the Alfvén resonator model [Stasiewicz et al., 1997], the shear-driven eigenmodes discussed in this study and in previous articles [Peñano and Ganguli, 1999;Peñano and Ganguli, 2000] are also consistent with these observations and should be considered as a possible alternative. ...
Localized dc electric fields directed perpendicular to the geomagnetic field are studied as a possible ionospheric source of electromagnetic ion cyclotron waves. The eigenvalue formulation of the problem is solved numerically and a class of unstable electromagnetic ion cyclotron eigenmodes are found that have properties consistent with some observations of broadband waves near auroral arcs. The frequency range of instability is comparable to the spectral width measured from photometer observations of broadband auroral flickering. Eigenmodes are associated with electric field polarization reversals and E/B ratios similar to that encountered by satellites and sounding rockets traversing auroral arcs. The dc electric field affects ion resonance conditions and allows for resonant interaction within the extent of the eigenfunction.
... Ganguli et al. [1985] showed that transverse sheared flow can drive EIC instability through the inhomogeneous energy-density driven (IEDD) mechanism [Gavrishchaka et al., 1996]. Peñ ano and Ganguli [1999Ganguli [ , 2000Ganguli [ , 2002a extended the study to the electromagnetic regime and found that the transverse sheared flow can also drive unstable eigenmodes at subcyclotron frequencies. Peñano and Ganguli [2002b] further confirmed that electromagnetic ion cyclotron eigenmodes can be generated as well. ...
Three-dimensional electromagnetic particle-in-cell (PIC) simulations are performed to study the properties of current shear-driven (CSD) instabilities, which are driven by the free energy stored in the inhomogeneous transverse magnetic field and associated with the transverse gradient of the field-aligned current. Current shear-driven instabilities are unstable in both a static field-aligned current equilibrium and the sheared field-aligned current sheet embedded in a transversely finite Alfvén wave. The simulation results demonstrate that the electromagnetic fluctuations generated by CSD instabilities have characteristics similar to the broadband ELF (BBELF) fluctuations observed in the topside auroral region and at higher altitudes. It is also shown that CSD instabilities have the capacity to accelerate ions transversely. Comparison of the PIC simulations with the satellite observations and three-dimensional, two-fluid MHD simulations supports CSD instabilities as potential candidates for the generation of BBELF fluctuations and the correlated transverse acceleration of ions.
Laboratory experiments provide a valuable complement to explore the fundamental physics of space plasmas without the limitations inherent to spacecraft measurements. Specifically, experiments overcome the restriction that spacecraft measurements are made at only one (or a few) points in space, enable greater control of the plasma conditions and applied perturbations, can be reproducible, and are orders of magnitude less expensive than launching spacecraft. Here I highlight key open questions about the physics of space plasmas and identify the aspects of these problems that can potentially be tackled in laboratory experiments. Several past successes in laboratory space physics provide concrete examples of how complementary experiments can contribute to our understanding of physical processes at play in the solar corona, solar wind, planetary magnetospheres, and outer boundary of the heliosphere. I present developments on the horizon of laboratory space physics, identifying velocity space as a key new frontier, highlighting new and enhanced experimental facilities, and showcasing anticipated developments to produce improved diagnostics and innovative analysis methods. A strategy for future laboratory space physics investigations will be outlined, with explicit connections to specific fundamental plasma phenomena of interest.
In a laboratory experiment, we demonstrate the substantial effects that collisions between charged and neutral particles have on low-frequency (Ω i ≪ ω ≪ Ω e ) shear-driven electrostatic lower hybrid waves in a plasma. We establish a strong (up to 2.5 kV/m) highly localized electric field with a length scale shorter than the ion gyroradius, so that the ions in the plasma, unlike the electrons, do not develop the full E × B drift velocity. The resulting shear in the particle velocities initiates the electron-ion hybrid (EIH) instability, and we observe the formation of strong waves in the vicinity of the shear with variations in plasma densities of 10% or greater. Our experimental configuration allows us to vary the neutral background density by more than a factor of two while holding the charged particle density effectively constant. Not surprisingly, increasing the neutral density decreases the growth rate/saturation amplitude of the waves and increases the threshold electric field necessary for wave formation, but the presence of neutrals affects the dominant wave frequency as well. We show that a 50% increase in the neutral density decreases the wave frequency by 20% while also suppressing the electric field dependence of the frequency that is observed when fewer neutrals are present. The majority of these effects, as well as the values of the frequencies we observe, closely match the predictions of previously developed linear EIH instability theory, for which we present the results of a numerical solution.
For a wide variety of laboratory and space plasma environments, theoretical predictions state that plasmas are unstable to inhomogeneous flows over a very broad frequency range. Such sheared flows are generated in the Earth's magnetosphere and intensify during active periods. Specifically, for a velocity shear oriented perpendicular to a uniform background magnetic field, the shear scale length (LE) compared to the ion gyro-radius (ρi) determines the character of the shear-driven instability that may prevail. An interpenetrating plasma configuration is used to create a transverse velocity shear profile in a magnetized plasma column, a condition similar to that found in the natural boundary layers. The continuous variation of ρi / LE and the associated transition of the instability regimes driven by the shear flow mechanism are demonstrated in a single laboratory experiment. Broadband wave emission correlated to increasing/decreasing stress (i.e., ρi / LE), a characteristic signature of a boundary layer crossing, is found under controlled and repeatable conditions. This result holds out the promise for understanding the cause and effect of the in-situ observation of broadband electrostatic noise.
It is shown that a magnetized plasma layer with a velocity gradient in the flow perpendicular to the ambient magnetic field is unstable to waves in the Very Low Frequency band that spans the ion and electron gyrofrequencies. The waves are formally electromagnetic. However, depending on wave vector k̄=kc/ω (normalized by the electron skin depth) and the obliqueness, k/k, where k are wave vectors perpendicular and parallel to the magnetic field, the waves are closer to electrostatic in nature when k̄⪢1 and k⪢k and electromagnetic otherwise. Inhomogeneous transverse flows are generated in plasma that contains a static electric field perpendicular to the magnetic field, a configuration that may naturally arise in the boundary layer between plasmas of different characteristics.
It is commonly believed that Alfvénic waves observed in the ionosphere originated at trans ionospheric altitudes. However, recent observations of waves localized over skin depth scales, which are prone to Landau damping, and upward going Poynting flux suggest an ionospheric source. This theoretical study establishes the generation of electromagnetic waves at subcyclotron frequencies by localized static electric fields of the type commonly observed in the auroral ionosphere. The problem is formulated in terms of an eigenvalue system of equations which can describe all cold plasma normal modes, ion acoustic waves, and the various mode couplings induced by nonuniform electric fields. It is found that the velocity shear associated with the background electric field can significantly affect the observable properties of Alfvén waves. In particular, Alfvén waves can be destabilized when the magnitude of the velocity shear frequency exceeds the ion cyclotron frequency. Velocity shear can also significantly modify the ratio of E/B for these waves. The analysis further reveals electromagnetic Kelvin-Helmholtz instabilities, which are non-Alfvénic, with lower velocity shear thresholds. We model conditions encountered by several rockets and satellites, for example, FAST, Freja, AMICIST, AT2, to analyze wave properties and find that for typical ionospheric parameters the Kelvin-Helmholtz instabilities can generate electromagnetic waves with broadbanded spectra and other physical characteristics similar to observations. It is also shown that these waves are capable of resonant interaction with electrons over localized regions and hence may be important to understanding the generation of suprathermal electron bursts.
The effects of a transverse gradient in the plasma flow velocity parallel to the ambient magnetic field are analyzed. A transverse velocity gradient in the parallel ion flow, even in small magnitude, can increase the parallel phase speed of the ion-acoustic waves sufficiently to reduce ion Landau damping. This results in a significantly lower threshold current for the current driven ion acoustic instability. Ion flow gradients can also give rise to a new class of ion cyclotron waves via inverse cyclotron damping. A broadband wave spectrum with multiple cyclotron harmonics is possible. A combination of the multiple cyclotron harmonic waves can result in spiky electric field structures with their peaks separated by an ion cyclotron period. A spatial gradient in the parallel electron flow is also considered but it is found to play a minimal role in the low frequency regime. Relevance of these to natural plasma environments is discussed. © 2002 American Institute of Physics.
Electrostatic shocks (ESs) and small-scale, field-aligned currents
(FACs) are known to be associated with low-altitude auroral
acceleration. We show that this phenomenon can be consistently described
by including the transverse wave-guide for shear Alfvén waves
(TAWG) and the density dip in the Alfvén wave model. The TAWG is
the density depletion in a low β plasma at h ≤ 2 RE.
Paired, anti-parallel FACs are found to be its eigenmodes. The
ne-dip in the centre, caused by a steep gradient of the
electric field, promotes formation of the strong double layer (SDL).
This in turn yields a narrow field-aligned electron beam at the edge of
an arc or the black aurora, depending upon whether it is the region of
the upward or downward Birkeland current, respectively.
Direct-current and ac plasma-density and vector-electric-field detectors on a polar orbiting satellite have measured spatially confined regions of extremely large electric fields in the auroral zone at altitudes below 8000 km. Such regions frequently have double structures of opposing electric fields containing characteristic and different wave spectra internal and external to themselves. These structures are identified as paired electrostatic shocks which are associated with electrostatic ion-cyclotron wave turbulence.
Bursts of large amplitude impulsive electric and magnetic field oscillations are a common feature observed from FAST when crossing the dayside auroral oval at altitudes from 1500–2500 km. The oscillations have transverse amplitudes of up to 1 V/m and 100nT and exhibit a parallel electric field component with amplitudes which may be as large as 100mV/m. Calculation of E1/B1 over 100 events yields an average value four times the local Alfven speed. The wave period is usually less than 0.25s with ‘perpendicular wavelengths’ which average to 7.1 electron skin depths (c/ωpe∼80m). Poynting flux calculations indicate predominately downward fluxes with magnitude up 10−2 Wm−2 usually accompanied by a smaller upwards component. Invariably these waves are accompanied by field-aligned fluxes of down going and sometimes counterstreaming suprathermal electrons. Comparison with theoretical studies indicate that these observations are consistent with the characteristics of a shear Alfven wave with k⟂∼ωpe/c propagating in the inertial dispersive regime and interfering with a reflected component. However the observed large parallel electric field component, if real, has yet to be explained
This paper discusses results from a statistical study of narrowband auroral electromagnetic ion cyclotron (EMIC) waves recorded by the Freja magnetic field experiment from December 1, 1993, to November 30, 1994. The study involves data collected at all magnetic local times (MLT), invariant latitudes between 40 ø and 75 ø, and altitudes between 1200 and 1750 km. Wave events have been identified using a two-step process. First, magnetic field spectra in the frequency range between 5 and 256 Hz were surveyed using an automated procedure which detects spectral peaks. Second, the results of this automated procedure were confirmed by visually inspecting wave spectrograms which contained these spectral peaks. A total of 316 auroral EMIC wave events were identified. The occurrence of these waves peak at auroral latitudes in the premidnight sector (1800-0100 MLT). The events have a strong seasonal dependence with peak occurrence in the winter. These properties are the same as those of auroral electron precipitation, which is suggested to be the free energy source of these waves. The spectral properties of the waves were also analyzed. The wave usually contained either one or two spectral peaks. In the cases where two spectral peaks were observed one peak was below the local O + gyrofrequency (fo +) and one was above the He + gyrofrequency (fi-i+). The observations suggest that auroral EMIC waves can be generated below fo +. The wave spectra were consistent with wave generation at altitudes near or above Freja's altitude. Analysis of the spectra suggest that the auroral EMIC waves originated from a source altitude between 1500 and 5500 km.
On March 9, 1978, a sounding rocket launched from Poker Flat, Alaska, at 2200 LT, made a four-component measurement of a 0.5 Hz hydromagnetic wave as the payload crossed the poleward boundary of a quiet homogeneous auroral arc. An energy flux of about 10 to the -6th W/sq m was observed propagating upward with a left-handed polarization within the arc, and a flux 6 times greater was observed propagating downward with a right-handed polarization on the arc boundary. The waves were identified as shear mode Alfven waves. Various models for the source of the free energy are discussed with the conclusion that the most likely production mechanism was either the electromagnetic or electrostatic Kelvin-Helmholtz instability.
The Polar Satellite carries the first three-axis electric field detector flown in the magnetosphere. Its direct measurement of electric field components perpendicular and parallel to the local magnetic field has revealed new classes and features of electric field structures associated with the plasma acceleration that produces discrete auroras and that populates the magnetosphere with plasma of ionospheric origin. These structures, associated with the hydrogen ion cyclotron mode, include very large solitary waves, spiky field structures, wave envelopes of parallel electric fields, and very large amplitude, nonlinear, coherent ion cyclotron waves.
The TEAMS instrument on the FAST satellite has detected events in which He+ ions are resonantly accelerated perpendicular to the magnetic field to energies of several keV. The events occur in association with electromagnetic ion cyclotron (EMIC) waves and conic distributions of up to a few hundred eV in H+ and a few keV in O+. Concentrations of He+ can be significantly elevated during the events. Our interpretation is that the He+ ions are accelerated through a cyclotron resonance with the waves. This acceleration is similar to a proposed mechanism for selective ion acceleration in impulsive solar flares.
Preliminary studies from FAST observations indicate that auroral ion cyclotron waves (ICWs) above the helium cyclotron frequency (ΩHe+) occur at all MLTs have amplitudes sometimes >1 V/m and often have a magnetic component even above the proton cyclotron frequency (Ωp) with amplitudes up to 2nT. Waves with transverse amplitudes greater than 100 mV/m often exhibit large spiky downward pointing field-aligned components with amplitudes that may exceed that of transverse fluctuation. The ratio of electric to magnetic fluctuation amplitude (E1/B1) is close to c. Using the dimensional electric and magnetic field measurements available from FAST it is demonstrated that these waves are in most cases polarized transverse to the ambient magnetic field (B0) and have linear polarization with wavelengths perpendicular to B0 ranging from hundreds of meters to kilometers. Calculations of the Poynting vector associated with these ICWs indicates Poynting fluxes directed downwards along B0 for waves below the local Ωp and generally upwards for waves observed close to and above this frequency with fluxes which may exceed 10% of the total field-aligned electron beam energy flux.
A new phenomenon was found at the polar edge
of the auroral oval in the postmidnight-morning sectors: field-aligned (FA)
high-energy upward electron beams in the energy range 20–40 keV at altitudes
about 3RE, accompanied by bidirectional electron FA beams of keV
energy. The beam intensity often reaches more than 0.5·103
electrons/s·sr·keV·cm2, and the beams are observed for a
relatively long time (~3·102–103s), when the satellite at
the apogee moves slowly in the ILAT-MLT frame. A qualitative scenario of the
acceleration mechanism is proposed, according to which the satellite is within a
region of bidirectional acceleration where a stochastic FA acceleration is
accomplished by waves with fluctuating FA electric field components in both
directions.Key words. Ionosphere (particle acceleration;
wave-particle interactions) · Magnetospheric physics (magnetosphere-ionosphere
interactions)
More than one mechanism is causing ion energization perpendicular to the geomagnetic field in the auroral region. Observations by the Freja satellite obtained during 20 months at altitudes around 1700 km are used to study the most common O+ energization mechanisms. Ion energization can be due to broadband low-frequency waves, causing resonant energization by interaction at frequencies around the ion gyrofrequency. Energization can also be caused by resonant energization by electromagnetic ion cyclotron waves at about half the proton gyrofrequency, or by emissions near the lower hybrid frequency. About 90% of all O+ heating events and about 95% of the total O+ upflow are caused by ion energization associated with broadband low-frequency waves. The dependence of the different energization types on magnetic local time, magnetic latitude, magnetic activity, and season is studied. We find that the prenoon auroral region is a major source of O+ ions energized by broadband low-frequency waves.
Electromagnetic ion cyclotron waves provide an important mechanism, in
addition to parallel electric fields, for accelerating electrons in the
aurora. Such waves may be responsible for flickering aurora and for the
intense field-aligned electron component of the aurora. Electrons are
accelerated by the small parallel electric field component of the wave
at the Landau resonance. In an inhomogeneous magnetic field the phase
velocity of the wave can increase and electrons that are near resonance
can be accelerated to large energies. Test particle simulations using
these waves show that electron conics, intense field-aligned electron
fluxes, and the trapped electron distribution can all be explained using
a single wave together with a parallel electric field. Specifically, it
is shown that ˜100 Hz electromagnetic ion cyclotron waves can
produce the observed ˜100 Hz oscillations in the 30 keV electron
flux in intense aurora. These results show the importance of both waves
and parallel electric fields in accelerating auroral particles.
The authors present measurements made from the double-probe electric field instrument on the Freja satellite of a very intense electric field event seen in the auroral oval. The electric field measurements are correlated with potential, charged particle, and wave activity measurements. They see two very narrow electric field events separated by approximately 5 km, having field strengths near 1 V/m. These structures are seen to be associated with an excess of positive charge, to be associated with no electron precipitation, with slight plasma depletions, and with wave activity. The authors suggest these structures are black aurorae, with a total absence of auroral emissions.
Recent auroral sounding rocket data illustrate the relative significance of two transverse acceleration of ion (TAI) mechanisms for initiating nightside auroral ion outflow. First, the new data from this two payload mission show clearly that for lower-hybrid solitary wave events, a) these individual events are spatially localized to scales approximately 100 m wide perpendicular to B, and b) the probability of occurrence of the events is greatest at times of maximum VLF wave intensity. Second, ion acceleration by broadband, low frequency electrostatic waves is observed in a 30 km wide region at the poleward edge of the arc. The fluxes from this and other sounding rockets are shown to be consistent with DE-1 and Freja outflow measurements, indicating that the AMICIST observations show the low altitude, microphysical signatures of nightside auroral outflow from TAI.
Using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft, we make a detailed comparison between the observed polarization properties of electromagnetic ion cyclotron (EMIC) waves and those predicted by theory. The polarization can be described by three parameters: the ellipticity ε, the ratio of parallel (to the background magnetic field B0) magnetic fluctuations deltaBz to the major axis component of the elliptical perturbation in the perpendicular plane deltaBmajor, and the phase angle between deltaBz and deltaBmajor. On the basis of the plasma parameters observed during EMIC events, we have calculated the linear properties of the theoretical modes and compared these to the observations. The result is that two and in some cases, three of the observed polarization properties are inconsistent with the assumption that the waves result from a single linear mode. We use a simple model with two constituent waves in various azimuthal orientations (around B0) and temporal phase relations and show that the distribution of observed polarization properties can be understood as resulting from the superposition of more than one mode. When there is superposition, the instantaneous polarization characteristics of the fluctuations do not reliably reflect the constituent wave properties and the minimum variance direction cannot be associated with a wave vector direction. Nonetheless, we have shown that the constituent wave properties can be inferred from the distribution of observed properties. For superposition of two waves with only slightly dissimilar characteristics, the constituent wave E is approximately the median observed E, E, and the constituent thetakB (angle between the wave vector k and B0) is approximately given by tanthetakB=deltaBz/deltaBmajor/E, with the overbar on deltaBz/deltaBmajor again indicating a median value.
We present high speed photometric measurements of the auroral intensity of flickering aurora. These measurements reveal the existence of intensity modulations in the discrete aurora above the Nyquist frequency of standard 30 frame per second TV cameras. The intensity fluctuations observed are primarily below 80 Hz, although frequencies up to 180 Hz have been detected. Changes in the spectral characteristics from essentially band-limited to broadband intensity fluctuations and then to narrowband fluctuations are seen in individual discrete arcs within a few minutes. The center frequency of the observed fluctuations also change during this period. Data obtained from a standard narrow field TV camera observing the same arc show that only the low ( ≈ 10 Hertz) narrowband modulations would be considered standard flickering aurora.
Auroral satellites and sounding rockets frequently ob- serve large electric fields perpendicular to the magnetic field that have a narrow scale length perpendicular to the magnetic field if they are interpreted as spatial structures. These fields have been variously attributed to electrostatic shock structures or to kinetic Alfv6n waves. These two models can be distinguished by con- sidering the ratio of the magnetic field perturbation to the electric field. This ratio is calculated within the context of the electro- static approximation, the fully kinetic Alfv6n wave dispersion relation considered by Lysak and Lotko (1996), and the cold fluid model including ionospheric reflection presented by Lysak ( 1991, 1993). Results for this model show that the ratio of the electric to magnetic field is not always equal to the Alfv6n speed, especially for structures that are very narrow in the direction perpendicular to the magnetic field. These narrow structures have electric fields that are enhanced with respect to the Alfv6nic value, and thus may appear as electrostatic.
Using De 2 data of ion drift velocities and magnetic fields, we have calculated the field-aligned Poynting flux (Sâ¥) for 576 orbits over the satellite lifetime. This is the first application over an extended data set of Poynting flux observations from in situ measurements. The data has been sorted by interplanetary magnetic field conditions (northward or southward IMF) and geomagnetic activity (Kp3) and binned by invariant latitude and magnetic local time. Our general results may be summarized as (1) the averaged S⥠is everywhere directed into the ionosphere, indicating that electric fields of magnetospheric origin generally dominate, and (2) the distribution of S⥠for southward IMF can be well explained in terms of an average two cell convection pattern, while for northward IMF a multiple cell convection pattern may be inferred. We have addressed the interesting question of the distribution of upward Poynting flux by binning only upward observations and found that average upward Poynting flux of less than 3 mW/M² may occur anywhere across the high latitude ionosphere. We have also observed a region at high latitudes in the predawn sector where the average upward Poynting flux is of significant size and occurrence frequency during southward IMFR and high Kp conditions. 11 refs., 3 figs.
The Kelvin-Helmholtz instability has often been invoked to describe transport at the magnetopause. However, recent observations of vortices in the postnoon sector of the auroral zone [Elphinstone et al., 1993] have indicated that structure of these vortices is inconsistent with the evolution of the standard Kelvin-Helmholtz instability, and is instead consistent with the theory of auroral spirals proposed by Hallinan [1976]. Models of the Kelvin-Helmholtz instability coupled to the ionosphere have generally neglected the effect of the field-aligned current which is generated by the velocity shear. This current must close in the ionosphere, and leads to the presence of a magnetic shear in addition to the velocity shear. This magnetic shear is susceptible to a dynamic current sheet instability, which has many of the same properties as the Kelvin-Helmholtz instability but produces vortices which are wound in a sense consistent with spiral observations. This instability, which may grow much faster than Kelvin-Helmholtz, may provide a basis for a dynamic theory of auroral spirals.
Kinetic Alfvén waves have been invoked in association with auroral currents and particle acceleration since the pioneering work of Hasegawa [1976]. However, to date, no work has considered the dispersion relation including the full kinetic effects for both electrons and ions. Results from such a calculation are presented, with emphasis on the role of Landau damping in dissipating Alfvén waves which propagate from the warm plasma of the outer magnetosphere to the cold plasma present in the ionosphere. It is found that the Landau damping is not important when the perpendicular wavelength is larger than the ion acoustic gyroradius and the electron inertial length. In addition, ion gyroradius effects lead to a reduction in the Landau damping by raising the parallel phase velocity of the wave above the electron thermal speed in the short perpendicular wavelength regime. These results indicate that low-frequency Alfvén waves with perpendicular wavelengths greater than the order of 10 km when mapped to the ionosphere will not be significantly affected by Landau damping. While these results, based on the local dispersion relation, are strictly valid only for short parallel wavelength Alfvén waves, they do give an indication of the importance of Landau damping for longer parallel wavelength waves such as field line resonances.
The linear and nonlinear kinetic properties of electrostatic oblique waves below the lower hybrid frequency are investigated. For propagation angles v = k∥/k⊥ < √me/mi the waves are damped by either parallel electron Landau or ion cyclotron damping. For Ti/Te ≫ 1 the waves are only weakly damped and can propagate. These waves are called slow ion cyclotron (SIC) and slow ion acoustic (SIA) waves. A fluid-kinetic model, comprised of hot linear kinetic ions and cold nonlinear fluid electrons, is proposed to describe a nonlinear wave breaking process of small-scale Alfvén waves resulting in broadband extremely low-frequency (ELF) wave emission. Numerical solutions of the fluid-kinetic model are compared to the electric and magnetic fields of solitary kinetic Alfvén waves and broadband ELF waves observed by the Freja satellite within a hot ion environment. The agreement in waveform morphology and amplitude between the fluid-kinetic simulations and the observed waves provides support for the theory that observed SIA waves are the result of a nonlinear emission process from SIC waves.
When an electron distribution drifts relative to the ions along a d.c. magnetic field, it is known that, above some critical drift velocity, a nearly field-aligned electromagnetic ion cyclotron instability may be excited. We extend the study of this instability over wide variations in plasma parameters, ion β in particular, and beyond marginal stability.Above threshold the most unstable waves propagate very obliquely to the ambient d.c. magnetic field at wavenumbers of the order of an inverse ion Larmor radius. At low ion β the critical electron drift normalized to the ion thermal velocity scales inversely as β i- ½ while, for β i>10-2, the critical drift scales as the ion thermal velocity. For the Te≈Tielectromagnetic ion cyclotron instabifity begins to have a lower threshold than the corresponding electrostatic instability at β i≈me/Mi. In a moderately high β i, homogeneous, collisionless plasma the electromagnetic ion cyclotron instability appears to have the lowest threshold of any current driven instability.
We present Freja Cold Plasma Analyzer (CPA) measurements from an encounter with the low altitude (approximately 1750 km) polar cusp during which the CPA measured 2-D images of the thermal (0-16 eV) particle distributions at 1.2 s time resolution, and simultaneously made rapid estimates (600/s) of integrated thermal particle flux into the instrument. The high resolution data show bursty ion flux enhancements of the order of tens of percent on time scales of tens of ms, or alternatively, hundreds of m spatial scales. The flux of electrons from 0-16 eV also varied by tens of percent and on temporal/spatial scales comparable to those in the ion cases. There is some evidence that the thermal particle flux variations are associated with intense low-frequency electromagnetic fluctuations with temporal/spatial scales identical to those seen by the CPA (tens of ms, hundreds of m).
Extremely low-frequency (ELF) magnetic and electric field plasma wave emissions were recorded on 2 October 1993 on auroral field lines by the Magnetic Field Experiment during Freja orbit 4770. The ELF wave frequencies were below the local oxygen gyrofrequency (25 Hz) and between the helium and proton gyrofrequencies (100 to 400 Hz). The ELF waves, interpreted as electromagnetic ion cyclotron (EMIC) waves, were observed in a region of inverted-V-type electron precipitation. The EMIC waves were correlated over time with auroral and lower energy (almost-equal-to 100 eV) electrons, which are both possible sources of free energy, and also with transversely accelerated oxygen ions. The waves above the helium gyrofrequency were more closely correlated with the transverse oxygen ion acceleration than the waves below the oxygen gyrofrequency. These observations are consistent with a scenario in which electron beams generate EMIC waves, which then produce transverse oxygen ion acceleration through a gyroresonant interaction.
Using an image orthicon television system, measurements have been made of the thicknesses of 581 auroral structures appearing in the magnetic zenith over College, Alaska during 5 nights in 1966 and 1967. Assuming a lower border height of 100 km for all structures, the thickness values ranged from the instrumental limit at 70 m to 4440 m, the median thickness being 230 m. Rayed auroral structures were found to be thinner than homogeneous structures, and all types exhibited a strong tendency to become thinner as their brightness increased.
Magnetized test ions are subjected to acceleration through a numerically simulated oblique double layer in order to determine whether they emerge with velocity vectors aligned with or oblique to the ambient magnetic field. A criterion for oblique alignment, depending on the double-layer parameters and on the external magnetization, is obtained. When it is applied to observed and theoretical auroral double layers, this criterion predicts that accelerated heavy ions will be substantially less magnetic field aligned than will accelerated hydrogen ions, thus suggesting auroral double layers as a source of high-energy ion conics. Test particle simulations are also used to investigate the perpendicular heating of ions at low altitudes by the electric fields associated with moving auroral arcs. The rapid motion of small-scale structures in the arcs is suggested as a source of low-energy conical ion distributions, and the slow drifts of the entire arc forms are inferred to heat ionospheric ions.
It is noted that small-scale regions of large electric fields have been observed above the auroral zone by the S3-3 satellite. The data from five such electrostatic shocks are examined in great detail. The three higher altitude shocks (all above 5,700 km) are found to be associated with upward-going ion beams, indicating that the potential associated with the shock closed below the satellite to give rise to the parallel electric field required for the acceleration of the ion beam. In all these cases, electrostatic ion cyclotron waves are found to be adjacent to the shock and to extend throughout the upward-going ion beam region. The lack of noticeable Doppler shift in the electrostatic ion cyclotron waves in association with large convective drift velocities is seen as indicating that the wavelength of the electrostatic ion cyclotron wave can be several kilometers and that the potential difference within the wave can be on the order of 100 V.
A kinetic theory in the form of an integral equation is provided to study the electrostatic oscillations in a collisionless plasma immersed in a uniform magnetic field and a nonuniform transverse electric field. In the low temperature limit the dispersion differential equation is recovered for the transverse Kelvin-Helmholtz modes for arbitrary values of K parallel, where K parallel is the component of the wave vector in the direction of the external magnetic field assumed in the z direction. For higher temperatures the ion-cyclotron-like modes described earlier in the literature by Ganguli, Lee and Plamadesso are recovered. In this article, the integral equation is reduced to a second-order differential equation and a study is made of the kinetic Kelvin-Helmholtz and ion-cyclotron-like modes that constitute the two branches of oscillation in a magnetized plasma including a transverse inhomogeneous dc electric field.
Simultaneous measurements of energetic particles and ac electric fields made by the javelin sounding rocket NASA 8:56 during the late expansion phase of a magnetic storm have revealed an intense shear in plasma flow of magnitude 20 (m/s)/m at the edge of an auroral arc. Structure with two characteristic scales sizes is displayed in the region of shear. Larger structures are of the order of several kilometers in size. Intense irregularities with characteristic wavelengths smaller than the scale size of the shear have also been detected. The large-scale changes in the orientation of the charge sheet at the edge of the arc may be due to the Kelvin-Helmholtz branch; shorter-wavelength modes may be related to the shear driven resistive drift wave. Observations are consistent with the suggestion that velocity shear instabilities may play a role in the formation of high-latitude irregularities.
Central plasma sheet (CPS) ion conics are oxygen-dominated, with peak energies ranging from tens to hundreds of eV centered around pitch-angles between 115 and 130 degrees. Because of the lack of correlation between the CPS conics and the observed currents and/or electron beam-like structures, it is not likely that all of these conics are generated by interactions with electrostatic ion cyclotron waves or lower hybrid waves. Instead, it is suggested that the observed intense broad band electric field fluctuations in the frequency range between 0 and 100 Hz can be responsible for the transverse energization of the ions through cyclotron resonance heating with the left-hand polarized electromagnetic waves. This process is much more efficient for heating the oxygen ions than hydrogen ions, thus providing a plausible explanation of the oxygen dominance in CPS conics. Simple algebraic expressions are given from which estimates of conic energy and pitch angle can be easily calculated. This suggested mechanism can also provide some preheating of the oxygen ions in the boundary plasma sheet (BPS) where discrete aurorae form.
A new mechanism for exciting the kinetic ion cyclotron waves in the presence of a nonuniform electric field perpendicular to the external magnetic field is given. Application of this instability to various space plasmas is discussed. The new instability mechanism may provide a more efficient agent for perpendicular ion heating than other EIC generation processes, since the linear growth rate is insensitive to the temperature ratio.
The wave experiment of the Viking satellite frequently detected dynamic small-scale (\ensuremath{\simeq} 100 m), large-amplitude, rarefactive (|\frac{\ensuremath{\Delta}n}{n}|\ensuremath{\lesssim}50%) solitary waves of negative potential (|\ensuremath{\varphi}|\ensuremath{\lesssim}2 V) moving upwards along the magnetic field lines (\ensuremath{\upsilon}=5 \mathrm{to} >50 km/s). The structures, which resemble ion holes, often have an upward-directed net potential drop (\ensuremath{\lesssim} 1 V) and are then interpreted as weak double layers.