FAST observations of VLF waves in the auroral zone: Evidence of very low plasma densities (original) (raw)
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FAST satellite wave observations in the AKR source region
Geophysical Research Letters, 1998
The Fast Auroral Snapshot (FAST) satellite has made observations in the Auroral Kilometric Radiation (AKR) source region with unprecedented frequency and time resolution. We confirm the AKR source is in a density depleted cavity and present examples in which cold electrons appeared to have been nearly evacuated (ttho t > ticold ). Electron distributions were depleted at lowenergies and up-going ion beams were always present. Source region amplitudes were far greater than previously reported, reaching 2x10 '4 (V/m)2/Hz (300 mV/m) in short bursts with bandwidths generally < 1 kHz. Intense emissions were often at the edge of the density cavity. Emissions were near or below the cold plasma electron cyclotron frequency in the source region, and were almost entirely electromagnetic. The IEI/IBI ratio was constant as a function of frequency and rarely displayed any features that would identify a cold plasma cutoff or resonance.
Modulated electron-acoustic waves in auroral density cavities: FAST observations
Geophysical Research Letters, 1999
We report on FAST observations of large amplitude (up to 500 mV m −1) envelope solitary waves at the edges of the AKR source region. These edges are characterized by the presence of two electron populations: a dominant hot (∼ keV) component and a minority cold (< 60 eV) component. The nonlinear waves are recorded when the spacecraft passes the base of the parallel auroral acceleration region. They form intense packets of electron acoustic waves. The modulation is due to ion acoustic waves. These structures are electrostatic and propagate along the magnetic field at speeds of a few 100 km s −1. They may play a crucial role in the acceleration processes taking place in these regions.
The Fast Auroral SnapshoT (FAST) Mission
Geophysical Research Letters, 1998
The FAST satellite mission investigates plasma electron beams, electrostatic shocks, ion beams and conics, processes occurring in the low altitude auroral acceleration density cavities, and numerous wave modes including auroral region, where magnetic field-aligned currents couple global kilometric radiation, ion cyclotron waves and VLF auroral hiss magnetospheric current systems to the high latitude ionosphere. and saucers. The time resolution of these measurements was In the transition region between the hot tenuous magnetospheric typically limited first by the spacecraft spin period, and ultimately plasma and the cold, dense ionosphere, these currents give risc to by telemetry capacity. In recent years, numerous sounding rocket parallel electric fields, particle beams, plasma heating, and a host experiments have revealed a broad range of important of wave-particle interactions. FAST instruments provide microphysical processes occurring in the aurora on short temporal observations of plasma particles and fields in this region, with and spatial scales [e.g., . excellent temporal and spatial resolution combined with high The Frcja satellite, launched in 1992, carried out auroral quantitative accuracy. The spacecraft data system performs onboard evaluation of the measurements to select data "snapshots" that are stored for later transmission to the ground. New measurements from FAST show that upward and downward current regions in the auroral zone have complementary field and particle features defined by upward and downward directed parallel electric field structures and corresponding electron and ion beams. Direct measurements of wave particle interactions have led to several discoveries, including Debye-scale electric solitary waves associated with the acceleration of upgoing electron beams and ion heating, and the identification of electrons modulated by ion cyclotron waves as the source of flickering aurora. Detailed quantitative measurements of plasma density, plasma waves, and electron distributions associated with auroral kilometric radiation source regions yield a consistent explanation for AKR wave generation.
Journal of Geophysical Research, 1997
The results of simultaneous observations of charged particle fluxes within the energy range 50 eY to 20 keY (plasma energy-angle spectrometer, PEAS experiment) and plasma waves within the frequency range 0.1 to 10 MHz (Plasma radio spectrometer, PRS 3 experiment) on board the APEX spacecraft are presented. The data were obtained at polar and auroral latitudes in the dawn-dusk and noon-midnight time sectors. The low-frequency (LF) sporadic emission mainly on frequencies lower than the local gyrofrequency was commonly observed within a comparatively narrow latitude interval (_5°_ 6j of the auroral oval. The maximum of spectral wave intensity was revealed at a frequency of -200 kHz. The sporadic character of the observed emissions, the spectra, are very similar to those reported for auroral kilometric radiation (AKR), and the temporal and spatial occurrences imply its relation to the source of AKR emissions measured at the higher altitudes, with regards to our wave measurements as LF AKR-type emissions. Comparative analysis of spatial distributions of charged particle energy spectra and variations of wave spectra at auroral oval latitudes showed that several conditions should be fulfilled for LF AKR generation. These conditions are related both to the intensity and energy of precipitating ion and electron fluxes, and to albedo electrons and the ionospheric plasma parameters. The wideband HF emission was observed together with the LF AKR-type emission during the morning auroral oval crossing. At spacecraft apogee altitudes, in the polar cap zone intense electron precipitations and an increase of plasma noise intensity at frequencies equal or below /He were observed at the period when the IMF was northward These precipitations and emissions were accompanied by the horizontal currents crossing the polar cap, the so-called theta structure. Recently, a lot of experimental results have reported relating whenJ;, < /He-However, according to the calculations of Zarka et the low-altitude records of the AKR-type emission [Benson, al. [1986], to reproduce the observed spectral intensity of the 1985; Benson and Wong, 1987]. The low altitudes present a AKR, with the maser mechanism, the precipitating electrons special interest because the intensive fluxes of the electrons which are reflected from the magnetic mirror and which have the loss-cone distribution have been poorly investigated up to now.
A coherent nonlinear theory of auroral kilometric radiation 1. Steady state model
Journal of Geophysical Research, 1980
A theory of auroral kilometric radiation due to the nonlinear interaction between negative energy electromagnetic waves and coherent electrostatic ion cyclotron waves is developed. The theory predicts that such radiation produced must have X mode polarization and must have a frequency that lies in a narrow band between the right-hand cutoff and the Doppler-shifted beam cyclotron frequency for each local point of origin. The basic requirement for free space accessibility is the presence of high-energy beams in the inverted V events and a density-depleted cavity of the type observed by Isis I measurements. Beam densities of the order of 10-3 of the (depleted) background density appear to be necessary for the instability producing the radiation. Under ideal conditions, paths of wave growth of the order of 100 km may be adequate to produce observed radiation levels from electromagnetic noise at the appropriate frequency in the source region. 1. INTRODUCTION There have been several observational studies made in recent years of high-intensity kilometric radiation of terrestrial origin. Measurements have been made by satellite from the source region in the auroral zone at R • 2-3 Re by Isis 1 [Benson and Calvert, 1979] all of the way out to R • 100 Re by Voyager 1 [Kaiser et al., 1978]. A number of interesting features of the auroral kilometric radiation (AKR) have been deduced as a result of these satellite measurements. The first measurements were made on Ogo 1 [Dunckel et al., 1970]. More detailed measurements were made on Imp 6 and Imp 8, which were analyzed by Gurnett [1974]. Gurnett found the spectrum to lie in 50 kHz < f < 500 kHz, with peak intensity at f ~ 200 kHz. The peak intensity power emission was estimated to be about 10 • W. He noted that the radiation appears to originate at low altitudes (R-2-3 R•) in the auroral region and to be closely associated with the occurrence of discrete auroral arcs, which are believed to be generated by intense inverted V electron precipitation bands. When arcs do not occur, only a small band of diffuse aurora is present, and the radiation disappears. The radiated power was estimated to be close to 1% of the maximum energy dissipated by the auroral charged particle precipitation. Recent observations in the source region from Isis I reveal and establish further properties of the AKR that were not well-established from the far field measurements [Benson and Calvert, 1979]. Those observations showed that the radiation was generated in the X mode just above the local cutoff frequency and propagated almost perpendicular to the background magnetic field. It was found to be generated within density-depleted regions with peak density such that %,, <
Journal of Geophysical Research, 1984
Recent observations at low altitudes made in the source region of the auroral kilometric radiation (AKR hereinafter) strongly support the maser synchrotron instability (MSI) as the relevant generation process for the AKR. This was first realized by Wu and Lee (1979). In a recent work (Le Qu•au et al. (1984), hereinafter called reference 1) we have proposed an analytical treatment of the MSI. The present work is a continuation of this analytical study. First we investigate the physical process of the MSI, and simple analytical expressions, valid for any distribution functions of nonthermal electrons are given. It is shown that supraluminous X mode waves (o•/kllc >> 1) can resonate with relatively low energy electrons 2 2 provided that the cold plasma parameter, ec = o• /o•c , is much smaller than unity. The proper frame of reference for studying the resonant coupling between the X mode and nonthermal electrons moves at a parallel velocity kllc2/o•. Resonance curves are then circles centered around this value and electrons diffuse with almost constant parallel velocities. The difference between the MSI and the standard cyclotron theory (where resonance curves are straight lines while diffusion curves are circles centered around o•/kll ) is stressed. A parametric study of the instability conditions for such a supraluminous wave is conducted. We investigate the role played by 0, the propagation angle %, the normalized density of energetic electrons g, the energy width of the part of the distribution function where an inversion of population (Of/0%. > 0) occurs, and finally u, the parallel velocity which characterizes the maximum of Of/0%. integrated along a resonance circle. In the case of the shifted loss cone distribution used here, u is simply its parallel bulk velocity. Largest growth rates are obtained for kllc2/o• = u. nett, 1974; Gurnet• and Green, 1978]. It is also strongly correlated with intense electronic precipitation, the so-called "inverted V" occurring along auroral field lines [Green et al., 1979; Benson et al., 1980]. For these reasons, Gurnett suggested to call it the auroral kilometric radiation (AKR hereinafter). Gurnett and Green [1978], using Hawkeye data, gave indirect evidence of an X mode generation for the AKR. This conclusion was also supported by direct measurements of AKR polarization on Voyager 1 and 2, when these spacecraft left the earth's environment [Kaiser et al., 1978] and, more recently, on DE-1 [Shawhan and Gumerr, 1982]. Moreover, investigations performed close to or even within the AKR source region [Benson and Calvert, 1979] also led to the same conclusion. On the basis of the low-altitude ISIS satellite, Benson and Calvert [19793 concluded that the AKR is generated just above the X mode cutoff, within cold plasma depleted regions (e.g., •c•/2= C%e/CO c < 0.2). Similar conclusions were drawn by James [19803, who showed from a ray tracing analysis that the emission is initially almost perpendicular to the magnetic field line, with a small earthward component. Calvert [19813, using Hawkeye data (at higher altitudes than ISIS), has indirectly determined an averaged cold plasma density profile. According to this result, at R •-1.8 Re, which corresponds to an electron gyrofrequency close to the peak frequency of the AKR, namely 250 kHz [Gurnett, 1974' Kaiser and Alexander, 19773, the ratio ec •/2 can be as low as 3x10 -2. The distribution function of both electrons at•d ions has been studied, inside the so-called inverted V regions, on the basis of S3.3 data. Mizera and Fennel [1977] and Mizera et al. [1981] have shown that both downgoing and upgoing electrons can exhibit pronounced nonthermal features at altitudes R-• 5000-8000 km above the neutral region. Upgoing electrons exhibit a large loss cone; downgoing are also characterized by nonthermal features in form of a "hole," which implies that c•f/Ov•_ > 0 and c•f/OVll > 0 in a localized region of phase space.
Journal of Geophysical Research, 1984
Emissions that appear to have been electrostatic ion cyclotron (EIC) waves have been observed at low altitude in the diffuse aurora by a sounding rocket payload. The rocket was launched from Poker Flat, Alaska, at •2030 MLT. The flight successively traversed •70 km of the diffuse aurora, a dark region, and a quiet 40 kR auroral arc. In the diffuse aurora, peaks were observed in the power spectrum of the electric field at frequencies near the hydrogen and oxygen ion cyclotron frequencies. Doppler shift and polarization analyses have been performed using EIC wave spectrum parameters derived from linear theory. Both analyses indicated that these emissions had properties consistent with those VLF covering a frequency range from 2.5 Hz to 8 kHz. Emphasis in the analysis and discussion will be placed on emission features that had properties similar to those expected for electrostatic hydrogen and oxygen cyclotron waves [Bering, 1983b]. Electrostatic ion cyclotron (EIC) waves are considered to be one of the more important plasma wave modes present in and near auroral arcs. First discussed in detail by Drummond and Rosenblurb [1962], the EIC wave mode has the lowest threshold for instability among the various possible current driven instabilities to which auroral Birkeland currents might be subject [Kindel and Kennel, 1971]. Among other effects, expected for H + and O + EIC waves. Taken together, EIC waves have been suggested as the mechanism the two analyses indicated that both emission bands were due to waves propagating both up and down the field line and eastward parallel to the poleward boundary of the diffuse aurora. The large local cold plasma density and resulting large Landau damping require that the source be responsible for the selective perpendicular ion heating which produces "ion conics." Ion conic is the term used to describe ions flowing up into the magnetosphere with a minimum in their distribution function at 180 ø pitch angle and broad maximum between 90 ø and 130 ø pitch angle local. Free energy for the waves was apparently [Sharp et al., 1977, 197q; Ghielmetti et al., available in the 5 BA/m 2 downward parallel current 1978; Croley et al., 1978; Whalen et al., 1978; density which was inferred from the magnetometer data. The presence of the waves indicates that this current was being carried by less than 2% of the plasma, presumably in the form of a field aligned beam of electrons with energies of a few eV. Introduct ion This paper is the first in a series of three papers about an extensive set of electric field Klumpar,
Auroral zone plasma waves detected at polar: PCBL waves
Advances in Space Research, 2001
Polar Cap Boundary Layer waves are ELFNLF electric and magnetic waves detected on field lines adjacent to the polar cap, thus their name. Waves are present at this location 96% of the time. The wave latitude-local time distribution is shown to be the same as that of the aurora1 oval. The most intense waves are detected coincident with the strongest magnetic field gradients (field-aligned currents). Specific frequency bands of whistler mode-waves are identified:-200 Hz, 1-2 kHz and-5 kHz. Two types of intense electric waves are present: solitary bipolar pulses (electron holes) and-kHz electric turbulence. The PCBL waves are most likely a consequence of aurora1 zone field-aligned current instabilities. The currents have in turn been ascribed to be due to magnetospheric convection driven by the solar wind.
Microstructure of the auroral acceleration region as observed by FAST
Journal of Geophysical Research, 1999
The Fast Auroral Snapshot (FAST) explorer satellite was designed to investigate the microscale structure of the auroral acceleration region that was unresolved by previous satellites. This paper will present highlights from the first 2 years of the FAST mission and compare them with previous observations and auroral models. In particular, we find good agreement with the overall field-aligned current systems previously discovered; however, we present evidence that the downward currents are often carried by energetic upgoing electrons. These upgoing electrons are correlated with diverging electrostatic shocks, indicating that quasi-static parallel electric fields are responsible for their energization. Some of these field-aligned fluxes contain large-amplitude fast solitary waves which produce strong modulations of the electrons. Observations in inverted-V arcs show that the parallel acceleration region contains narrow (-10 kin) fingers of potential that extend along the magnetic field. On these narrow kilometer scales, ion beam energies are found to agree with the inferred potential determined from the perpendicular electric field or from the widening of the electron loss cone, implying acceleration is typically quasi-static on ion transit times. We also find evidence that both the ion and electron upgoing beams produced by the parallel electric fields have plateaued parallel distribution functions. Fast solitary waves are a prime candidate to stabilize the electron beams and may provide the resistance that allows the downward directed parallel electric fields to form in the highly conducting ionosphere. Intense ion cyclotron waves and ion solitary waves are often observed during ion beams, but the stabilizing waves have not been identified. In addition, intense electromagnetic ion cyclotron waves are also observed in inverted-V arcs, along with strongly modulated electron fluxes, indicating that turbulent acceleration is occurring in addition to simple acceleration by a static potential drop. 1. Introduction In situ measurements of particles and fields in the Earth's auroral zones have been made for decades. These measurements have revealed a rich variety of plasma phenomena on multiple spatial and temporal scales, with greater detail and new phenomena revealed as instrumentation has improved. Until recently, limited spacecraft resources, simple instrument designs, and high spacecraft velocity have forced satellite missions to concentrate on the weak double layers [Temerin et al., 1982], and field-aligned counterstreaming electron beams [Sharp et al., 1980] and their association with energetic ion conics and downward currents [Klumpar and Heikkila, 1982]. These large-scale satellite measurements have been complemented by sounding rockets, which have provided brief, high-resolution measurements below the primary auroral acceleration region. These observations were used to identify fine spectral features such as the monoenergetic electron spectral peak [Mcllwain, large-scale structure of the auroral zone. These missions were able 1960; Evans, 1968], indicating parallel electric fields, and narrow to identify the large-scale field-aligned currents [Iijima and field-aligned downward electron fluxes [Totbert and Carlson, Potemra, 1978], the convection flows [Cauffman and Gurnett, 1980], indicating ionospheric source populations for accelerated 1971], and inverted-V arcs which are the primary sources of auroral light [Frank and Ackerson, 1971]. Observations of large converging electric fields perpendicular to B [Mozer et al., 1977], upgoing ion beams [Shelley et al., 1976], and density cavities [Persoon et al., 1988] were used to infer regions containing upward directed parallel electric fields. Observations of ion outflow in the form of conics [Ghielmetti et al., 1978; Co#in et al., 1984] and ion beams [Shelley et al., 1976] provided measurements of the ionospheric plasma input into the magnetosphere. Other observations include auroral kilometric radiation [Gurnett, 1974], VLF hiss and saucers (resonance cone whistler wave signatures) and their correlation with downward-current regions [Gurnett and Frank, 1972],
Auroral kilometric radiation sources - In situ and remote observations from Viking
Journal of Geophysical Research, 1993
The present paper is a summary of studies carried out from Viking measurements on the propagation and the generauon of the auroral kilometric radiation (AKR hereafter). Advantage is taken of the spin modulation of the AKR observed as Viking was rotating in the cartwheel mode. This, together with the study of the cutoff of the various spectral components, confirms that low-amplitude Z and O modes are generated at the same time as a larger-amplitude X mode. Hence Z, O and X mode AKR is all generated by the same sources. The spectrum of the dominant polarization, the X mode, usually contains several spectral peaks. An AKR source crossing is characterized by a minimum in the frequency of the lowest-frequency peak (fpeak) and by a maximum of its amplitude. About 50 AKR source crossings are used to demonstrate that fpeak approaches fce, the electron gyrofrequency: (fpeak-fce)/fce •-0.025 in AKR sources. Similarly, the lo•v-frequency cutoff of the AKR is found, on average, to coincide with fce. The density inside AKR sources is determined from four sets of independent measurements, namely (1) the upper frequency cutoff of the hiss, (2) the relaxation sounder, (3) the Langmuir probe, and (4) particle measurements. It is shown that an AKR source coincides with a strong depletion in the density of the cold/cool electrons that becomes comparable to or less than the density of energetic electrons (E > 1 keV). The total density inside AKR sources is of the order of 1 cm -3, typically a factor 5 to 10 below that of the surrounding regions. AKR sources are found to coincide also with an acceleration region characterized by a potential drop of > 1 kV, both below and above the spacecraft. Evidence for this comes from the observation of electrons accelerated above the spacecraft and ions accelerated below it. In addition to a strong depletion in the density of the cool electrons, particle measurements on Viking give evidence of several possible free energy sources that could drive unstable the AKR, namely (1) a loss cone, (2) a hole for parallel velocities smaller than that of the observed downgoing electron beam, and (3) a trapped electron component for a pitch angle a •-90% The trapped electron component, bounded at low perpendicular energies (a few keVs) by an enhanced loss cone, is observed inside, and only inside, AKR sources. It is therefore concluded that the corresponding o•fio•v.L > 0, for small parallel velocities, is the free energy source that drives unstable the cyclotron maser. 11,657 11,658 Roux ET AL.: AURORAL KILOMETRIC RADIATION SOURCES