Suprathermal Electron Bursts Research Articles

E‐POP Observations of Suprathermal Electron Bursts in the Ionospheric Alfvén Resonator

AbstractUtilizing the suprathermal electron imager onboard the e‐POP satellite, we performed a statistical study of groups of repetitive suprathermal electron bursts (STEBs) with 52 events (each with ≥3 STEBs) during 21 e‐POP orbits. The STEBs are observed on both the day side and night side, and within a wide range of geomagnetic latitudes, from 64° to 78°. We interpret these data as temporally varying signatures; we suggest the repetition is due to the ionospheric Alfvén resonator (IAR), since the statistical time separation of adjacent STEBs in the satellite frame is 0.5–1.5 s, which is consistent with the characteristic period of the IAR. Furthermore, the STEBs are associated with low‐frequency magnetic field perturbations. The statistical occurrence of repetitive STEBs shows a preference toward postnoon and midnight sectors at high magnetic latitudes. We also present one example event of four successive STEBs associated with auroral rays in conjugate imagery. The repetition period of STEBs is similar to the oscillation period of magnetic fields observed by Enhanced Polar Outflow Probe (e‐POP). One of the STEBs in an example event exhibits “normal” dispersion (high energy particles with field‐aligned pitch angle arriving first) and the adjacent STEB exhibits “inverse” dispersion (low energy particles with broad pitch angles arriving first). With a test particle simulation, we propose one possible explanation of inverse dispersion in terms of a suprathermal electron population and a downward‐propagating parallel electric field.

Suprathermal Electron Acceleration Perpendicular to the Magnetic Field in the Topside Ionosphere

AbstractWe present the first direct observations of suprathermal (tens to hundreds of eV) electron acceleration perpendicular to the magnetic field in the topside (900–1,500 km) ionosphere. Based on measurements from the suprathermal electron imager (SEI) onboard the Enhanced Polar Outflow Probe satellite over several months, we identify 30 events (28 in the dayside cusp) of enhanced suprathermal electron fluxes peaking at 90° pitch angles, with energies spanning from tens of eV to 325 eV, the upper limit of the SEI measurement. These events take place with a time duration of the order of 0.1 s, corresponding to a spatial scale of less than 1 km across the magnetic field. They are associated with parallel suprathermal electron bursts and upward currents. The correlation between perpendicular and parallel suprathermal electrons suggests a scenario in which downward bursts and the associated Alfvén waves that drive them provide a free energy source to destabilize plasma waves at electron frequencies, which in turn heat electrons in the perpendicular direction. This is a new energy dissipation process for cusp suprathermal electron precipitation in the topside ionosphere.

Measurement and modelling of suprathermal electron bursts generated in front of a lower hybrid antenna

Measurements in the tokamak scrape-off layer show the existence of fast electrons as far as a few centimeters radially in front of a lower hybrid antenna grill, which is one order of magnitude further than earlier predictions of interaction zones not exceeding 5 mm in width (Rantamäki et al 2000 Nucl. Fusion 40 1477). The present contribution aims at understanding these experimental results on the basis of electron Landau damping in a turbulent scrape-off layer. A crucial guiding element from the experiments is the observation of temporal intermittency of the fast electron signal at positions radially away from the grill mouth at a rate comparable with the detachment rate of relatively hot and dense field-aligned plasma filaments, or ‘blobs,’ from the last closed flux surface. When the dynamic character of the scrape-off layer is taken into account, transient low density and low temperature states occur during which the Landau damping can be weak enough to allow the radiated spectrum to penetrate radially deep inward. When the lower hybrid wave intercepts a dense and hot ‘blob’ a few cm in front of the antenna, then strong damping of the high wave number components can occur. This is the scenario we propose for explaining the generation of fast electrons far away from the grill mouth.

Further sounding rocket observations of structured whistler mode auroral emissions

Two recent sounding rockets were launched into active auroral substorms from Poker Flat, Alaska: CHARM on 27 February 2007 and SIERRA on 14 January 2002. Both payloads included HF wave receivers. As previously reported, the SIERRA HF receiver recorded short‐lived, periodic, time‐dispersed features at 200–600 kHz (“stripes”), in which high‐frequency components precede low frequencies, leading to a nearly linear frequency‐time signature. These features coincided with a region identified as Alfvénic and dominated by suprathermal electron bursts and 10–110 eV downward going ions poleward of the main inverted V auroral arc. The CHARM experiment, using different electric field sensors, confirmed the occurrence of the stripe features with characteristics similar to those observed with SIERRA. The most promising mechanism to explain stripes is a cyclotron resonance between downgoing Z mode waves and upgoing electron conics in the topside ionosphere at altitudes of 3000–5000 km. These electron conics resonate with successively lower‐frequency waves as they ascend, leading to emissions in which lower frequencies are delayed relative to higher frequencies; detailed modeling reproduces even fairly subtle nonlinear features of the stripes’ frequency‐time signature. The resulting structured Z mode waves then convert to whistler mode waves, which propagate to rocket altitudes. Previous simulations have shown that electron conics can be generated by Alfvénic acceleration and retain the periodicity of the driving Alfvén wave. If this acceleration could be driven by Alfvén waves at the frequency of the periodicity of the stripes, it could explain this periodicity and would be consistent with their correlation with an Alfvénic region.

Particle energization by oblique inertial Alfvén waves in the auroral region

Two‐and three‐dimensional electromagnetic particle‐in‐cell simulations are performed to study the particle energization by oblique inertial Alfvén waves in the auroral region. It is found that most of the wave energy is transferred to the parallel acceleration of electrons and transverse acceleration of ions (TAI) as the result of nonlinear evolution of an initial wave. The wave‐particle interactions between the Alfvén wave and the electrons form a field‐aligned electron beam, which is consistent with the time‐dispersive electron signature and the suprathermal electron bursts observed by spacecraft. The steepening and subsequent breaking evolution of the Alfvén wave generates a combination of ion cyclotron and ion acoustic waves which, in turn, heats the ions transversely. The parallel‐accelerated electron beam could result in ion cyclotron and ion acoustic instability, but it is generally too weak to be the wave generation mechanism. Three‐dimensional simulations suggest that the sheared field‐aligned current sheet embedded in a transversely finite Alfvén wave is subject to current shear‐driven (CSD) instabilities. It is very possible that the ion cyclotron and ion acoustic waves generated by the wave breaking process, as well as the low‐frequency electromagnetic fluctuations generated by CSD instabilities, are present in observed broadband ELF fluctuations and contribute to the associated TAI.

Energy dependence of electron pitch angle distribution widths in solar bursts

Solar activity regularly produces suprathermal electron bursts at energies below 1.4 keV. At 1 AU, these bursts have been detected at energies as low as 73 eV. The characteristics of such bursts vary considerably from event to event due in part to the physical processes involved in their propagation to Earth. In 2002, Advanced Composition Explorer/Solar Wind Electron Proton Alpha Monitor (ACE/SWEPAM) observed 101 solar electron bursts at energies below 1.4 keV, 40 of which exhibited broader pitch angle distributions than the preburst strahl. In general, the width of the pitch angle distribution from 73 to 1370 eV is a function of energy. In ∼65% of the events the pitch angle distribution width increased with energy during the burst, in ∼10% of the events it decreased with energy, and ∼25% of the cases demonstrated no clear energy dependence. The delay time between the onset of the burst and the onset of pitch angle distribution broadening also showed energy dependence. At 987 eV broadening almost always occurred after onset of the burst; however, at energies less than 519 eV broadening of the pitch angle distribution sometimes preceded the burst onset. Although energy dispersion was observed in the burst onset times, several events revealed nearly simultaneous broadening over an extended range of energies, in one case from 197 to 1370 eV. Broadening of the pitch angle distribution function during solar electron bursts almost certainly results from wave‐particle scattering. Scattering, in turn, should depend on solar wind plasma and magnetic field conditions. Nevertheless, we were not able to find any general statistical correlations between ACE plasma and magnetic field parameters and the burst pitch angle distribution width. Because ACE measurements are inherently local, whereas scattering can occur anywhere between the Sun and Earth, we also examined five bursts that we infer underwent relatively local scattering. These bursts exhibited maximum pitch angle distribution widths greater than 75°. Even these special cases failed to manifest any correlation with local plasma or magnetic field measurements.

Self‐consistent electron acceleration due to inertial Alfvén wave pulses

We present self‐consistent kinetic simulations of the electron response to finite duration shear Alfvén wave pulses in a magnetized plasma. In Earth's magnetosphere, the evidence suggests that parallel electric fields in inertial scale shear Alfvén waves can accelerate electrons in the geomagnetic field‐aligned direction. Here, we study large‐amplitude wave forms as they travel through ambient plasma at phase velocities which are consistent with resonant electron acceleration predicted by linear kinetic theory. Our self‐consistent simulations reveal that the wave potential evolves nonlinearly as shear Alfvén wave pulses travel through the simulation domain. The evolution of the wave pulse from a symmetrical to a nonsymmetrical potential structure, and the large perturbation in the distribution function required to carry the parallel current of the pulse, leads to nonresonant acceleration of electrons (i.e., acceleration of electrons which are not traveling at the same velocity as the wave). We compare the signature of resonant and nonresonant electron acceleration with data from a low‐altitude spacecraft and suggest an explanation for features often referred to as field‐aligned suprathermal electron bursts. Finally, we discuss how resonant and nonresonant acceleration of electrons is affected by the perpendicular wavelength and amplitude of shear Alfvén wave pulses.

SIERRA observations of Alfvénic processes in the topside auroral ionosphere

The NASA SIERRA rocket mission was a multiple‐payload, auroral plasma physics experiment that flew into a 100 nT auroral substorm at altitudes up to 735 km over the Poker Flat Research Range in Alaska on 14 January 2002. The flight environment was composed of an equatorward region of inverted‐V electron precipitation followed by a poleward region of mostly field‐aligned suprathermal electron bursts. In the inverted‐V region, the measured δE/δB implied a stationary field‐aligned current configuration. In the poleward region, the average electric field was much larger, 50 mV/m compared with 20 mV/m, included time‐varying components that exceeded 100 mV/m with periods of 0.5–5 s, and δE/δB was Alfvénic. Our multiple‐payload observations show that the perpendicular scale size of most of these waves is on the order of a kilometer or less. At smaller scales, broadband, extremely low‐frequency (BBELF) fluctuations associated with observed Alfvén waves had spectral ratios (δE/δB) appropriate for inertial Alfvén waves larger and smaller than the electron inertial length (λe = c/ωpe), but the phase angle differences between δE and δB indicate mostly propagating Alfvén waves for spatial scales much larger than λe and a transition to spatial perturbations, embedded in the plasma, for scales near λe. The data are consistent with the model of kilometer‐scale oblique Alfvénic arcs that are unstable to the current shear‐driven instabilities discussed in previous theoretical investigations, lending support to the conclusion that the observed BBELF fluctuations are likely driven by the wave‐associated field‐aligned current shear.

Correlation between suprathermal electron bursts, broadband extremely low frequency waves, and local ion heating in the midaltitude cleft/low-latitude boundary layer observed by Cluster

The Cluster spacecraft often cross the midaltitude cleft/cusp and observe a very well defined “electron‐only cleft” region consisting of electron injections without magnetosheath ions. This region contains soft suprathermal (<500 eV) electron bursts in antifield and/or field‐aligned directions. We present an example of such observations which shows that the O+ ion outflow at midaltitudes appears just poleward of the open‐closed boundary simultaneously with electron injections and was observed in the cleft, cusp, and mantle in the form of narrow energy beam. Inside the “electron‐only” cleft the suprathermal electron bursts are strongly correlated with strong O+ and H+ ion heating and with localized extra low frequency (ELF) (1–10 Hz) magnetic field wave power with broadband spectra. Our study shows that strong ion heating was observed only in the region with electron field‐aligned anisotropy more than 2. In addition, comparison of particle data from two spacecraft, which crossed the heating region with a time difference of 4 min, shows the correlation between ion outflow fluxes and fluxes of the injected electrons. Whereas ELF electromagnetic waves are localized inside the ion heating region, ELF electrostatic waves are detected throughout the cleft/cusp/mantle regions, where strong ion heating was not observed, suggesting that electromagnetic ELF waves heat ions in the cleft region. Owing to the absence of magnetosheath ions and strong field‐aligned currents, we suppose that inside “electron‐only” cleft region the suprathermal electron bursts are most likely an energy source for the wave destabilization. We suggest that the location of the heating region and the level of the outflow ion fluxes could be related to electron injection in the cleft in such events.

A statistical study of ion energization at 1700 km in the auroral region

Abstract. We present a comprehensive overview of several potentially relevant causes for the oxygen energization in the auroral region. Data from the Freja satellite near 1700 km altitude are used for an unconditional statistical investigation. The data are obtained in the Northern Hemisphere during 21 months in the declining phase of the solar cycle. The importance of various wave types for the ion energization is statistically studied. We also investigate the correlation of ion heating with precipitating protons, accelerated auroral electrons, suprathermal electron bursts, the electron density variations, Kp index and solar illumination of the nearest conjugate ionosphere. We find that sufficiently strong broad-band ELF waves, electromagnetic ion cyclotron waves, and waves around the lower hybrid frequency are foremost associated with the ion heating. However, magnetosonic waves, with a sharp, lower frequency cutoff just below the proton gyrofrequency, are not found to contribute to the ion heating. In the absence of the first three wave emissions, transversely energized ions are rare. These wave types are approximately equally efficient in heating the ions, but we find that the main source for the heating is broadband ELF waves, since they are most common in the auroral region. We have also observed that the conditions for ion heating are more favourable for smaller ratios of the spectral densities SE /SB of the broadband ELF waves at the oxygen gyrofrequency.Key words. Ionosphere (auroral ionosphere; wave propogation) Magnetospheric physics (electric fields)

Generation of ELF electromagnetic waves in the ionosphere by localized transverse dc electric fields: Subcyclotron frequency regime

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, Preja, 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.

Electron beam formation by small‐scale oblique inertial Alfvén waves

Particle‐in‐cell simulations reveal characteristics of small‐scale nearly perpendicular nonlinear inertial Alfvén waves in the electrostatic limit, during wave steepening and electron beam generation. These electrostatic inertial Alfvén waves (ESIAW) propagate with only weak electron Landau damping for Ti/Te ≫ 1. Nonlinear amplitude initial conditions convectively steepen and evolve higher wavenumber structures similar in morphology and magnitude to previous results from a fluid‐kinetic model. Shorter wavelength acoustic waves emergent during steepened ESIAW wave evolution are found to be instrumental in exciting cold electron beams possessing kinetic energies a few hundred times that of the thermal electron population. This mechanism is one possible explanation for suprathermal electron bursts observed in and near auroral inverted‐V events.

Correlation between core ion energization, suprathermal electron bursts, and broadband ELF plasma waves

Observations of the lowest energy or core ions provide a particularly sensitive measure of the early stages of auroral ion energization. Freja satellite observations of 0–20 eV core ions in the topside auroral ionosphere and cusp/cleft show signs of heating within both regions of VLF hiss and broadband ELF plasma waves. However, heating to several eV or more is associated predominantly with the ELF waves. A correlation analysis of wave and core ion data formed from orbital segments shows that, on average, correlations are highest for wave frequencies below several hundred Hz, and less at VLF hiss frequencies. A similar analysis shows a higher correlation between electron precipitation and ion heating for electron energies below several hundred eV (i.e., the energies associated with suprathermal electron bursts) and a lower correlation above the 1 keV energies associated with auroral inverted‐V's. Signs of core ion heating begin to appear when wave power at the O+ gyrofrequency exceeds about 10−3 (mV m−1)2/Hz, and when the integrated field‐aligned electron flux exceeds a few times 107 cm−2 s−1 sr−1. This electron energy flux threshold is at least an order of magnitude lower than previously inferred from earlier studies comparing suprathermal electron fluxes and energetic ions. Almost all observed heating events occur during enhanced or active geomagnetic conditions; i.e., Kp ≥ 4. While the most intense core ion heating is correlated with broadband ELF waves, we also present one example of weak ion heating of a few eV in a region of VLF auroral hiss.

Broadband ELF plasma emission during auroral energization: 1. Slow ion acoustic waves

High‐resolution measurements by the Freja spacecraft of broadband extremely low frequency (BB‐ELF) emission from dc up to the lower hybrid frequency (a few kHz) are reported from regions of transverse ion acceleration (TAI) and broad‐energy suprathermal electron bursts (STEB) occuring in the topside ionospheric auroral regions. A gradual transition of the broadband emission occurs near the local O+ cyclotron frequency (ƒO+ ≈ 25 Hz) from predominantly electromagnetic below this frequency to mostly electrostatic above this frequency. The emission below 200 Hz often reach amplitudes up to several hundred mV/m and density perturbations (δn/n) of tens of %. An improved analysis technique is presented, based on the quantity |δE/(δn/n)| versus frequency and applied to the Freja plasma wave measurements. The method can be used to infer the dispersion relation for the measured emission as well as give estimates of the thermal plasma temperatures. The BB‐ELF emission is found to consist partly of plasma waves with an ion Boltzmann response, which is interpreted as originating from the so‐called slow ion acoustic wave mode (SIA). This emission is associated with large bulk ion (O+) temperatures of up to 30 eV and low electron temperatures (1–2 eV) and therefore occurs during conditions when Te/Ti ≪ 1. The BB‐ELF emissions also contain other wave mode components, which are not equally easy to identify, even though it is reasonably certain that ion acoustic/cyclotron waves are measured. The ion Boltzmann component is characterized by a dominantly perpendicular polarization with respect to the Earth's magnetic field direction and a small magnetic component with amplitudes around 0.1–1 nT. The ion Boltzmann component dominates the lower‐frequency part (30–400 Hz) of the BB‐ELF emissions. The BB‐ELF emission have often an enhanced spectral power when certain waveform signatures, interpreted as solitary kinetic Alfvén waves (SKAW), or when large‐amplitude electric fields, possibly related to black aurora, are encountered in regions often associated with large‐scale auroral density depletions. A scenario where the SKAW provides the original free energy and via the BB‐ELF emission causes intense transverse ion heating (TAI) is suggested.

Characteristics of Suprathermal Electron Bursts Observed by the DMSP-F6/F7 Satellites in the Diffuse Aurora Region.

The particle data of the DMSP-F6 and -F7 satellites show the presence of suprathermal electron bursts in the diffuse aurora region. These bursts are characterized by intense (∼108-109 (cm2sr sec)-1), low energy (∼100-500 eV), and localized (∼20-30 km at satellite altitude h ≅ 830 km) electron precipitation. Simultaneous magnetic field data from the DMSP-F7 satellite show that these bursts are often, but not always, accompanied with intense (few μAm-2), small-scale (only few tens of km at satellite altitude) upward field-aligned currents. The bursts are observed from 19 h to 11 h MLT through midnight during the maximum to recovery phase of substorm. These bursts are less likely to be observed during geomagnetic storms than during times of isolated substorms. These results suggest that suprathermal electron bursts are generated by localized heating of ionospheric electrons which flow out from the ionosphere toward the magnetosphere during substorm.

On the auroral current‐voltage relationship

A simple model of collisionless plasma kinetic theory [Knight, 1973] has been used to predict the relationship between the upward parallel current and the parallel potential drop. The DE 1/DE 2 pair offers a unique opportunity to test this relationship because the DE 1 spacecraft can measure the high altitude plasma parameters without contamination from auroral heating. The field‐aligned current density from Knight's formula is almost linear in the ratio of j∥/eΦ∥ for values of eΦ∥/kT between 1 and 10. We find, using measured values of J∥ (mapped to the surface) and Φ∥ (inferred from measurements either at high altitudes or low altitudes), the ratio of J∥/eΦ∥ varies considerably but with a mean value about 0.5∼2.2×10−9 mho/m². Using either the full Knight's formula or the linear simplification, we can predict the parallel potential drop (given the high‐altitude density, temperature, current, and magnetic mirror factor) to test against the measured potential drop, or we can predict the current given the potential drop and the high‐altitude plasma parameters. The agreement is generally quite good and the linear simplification is reasonable; however, the Knight current, since it does not include upward ionospheric electrons, is not valid in the downward return current region. We also show that suprathermal electron bursts are observed in the diffuse aurora at the same invariant latitudes, both at high and at low altitudes. Thus we suggest that these “bursts” are more properly described as a spatial rather than temporal phenomenon.

Plasma and field properties of suprathermal electron bursts

Electron spectrometer measurements from polar‐orbiting satellites frequently show narrow regions of enhanced fluxes of electrons in the energy range from a few electron volts to less than 1 keV. These suprathermal electron bursts are observed throughout the auroral zone even under quiet conditions. They are typically distributed in latitude for hundreds of kilometers on either side of an upward current region which sometimes also contains an inverted V event. Whereas in inverted Vs the electron spectrum shows a peak at an energy of a few keV, suprathermal electron bursts are characterized by enhanced fluxes at all energies below a cutoff energy that is usually less than 1 keV. We present observational evidence to show that suprathermal electron bursts are correlated with enhancements in the ac electric field at frequencies below 1 kHz. The wave spectrum above 1 kHz does not differ significantly within and outside a burst. These observations suggest that the electrons in suprathermal bursts are heated through interaction with waves, most likely ion cyclotron waves. The phenomenology of suprathermal bursts indicates that the waves are not generated on the same field line at which the heating is observed. The evidence suggests that the waves are generated in regions of upward field‐aligned currents and propagate obliquely away from the source region. Electrons with parallel velocities equal to the parallel phase velocity of the waves can be heated through Landau resonance. This accounts for the fact that the cutoff energy in the bursts is never higher than the peak energy of the associated inverted V precipitation, even though the inverted V precipitation may be latitudinally displaced by several hundred kilometers from the burst.

Search for auroral belt E∥ fields with high‐velocity barium ion injections

Four high‐velocity shaped charge Ba+ injections were conducted from two Black Brant‐10 rockets at collision‐free altitudes (770‐975 km) over northern Alaska (L = 7.4–10.6) in April 1984 under active auroral and magnetic disturbance (Kp 4+ and 5) conditions. The motions of the Ba+ “pencil” beams from these injections were accurately triangulated to altitudes ranging from 9000 to 14000 km from multistation image observations. Scanning photometer observations from five sites provided checks on the triangulations as well as auxiliary data, including observations that illustrate the lack of any anomalous ionization that might be expected from critical velocity effects. Well‐defined initial conditions and improved software for predicting the unperturbed, E∥ = 0, trajectories in the presence of convection, E⊥, fields permitted an accurate detection of changes in the motion which could be attributed to E∥ fields. Large (>1 keV) potential changes that might be anticipated from double‐layer or V‐, U‐, and S‐shaped potential structures were not encountered even though the Ba+ rays were clearly located on auroral arc flux tubes on at least several occasions and were at various times in close proximity to auroral flux tubes for many minutes. Abnormally intense E⊥ fields that might also indicate that the above potential structures were also not observed. Transient accelerations and/or decelerations involving magnetic field‐aligned energy changes ≤375 eV were, however, encountered by each of the seven principal Ba+ rays tracked to high altitudes. Acceleration events were only slightly more frequent than deceleration events. Interpretation, taking into account limits on the duration of the events and simultaneous auroral conditions, favors explanation in terms of propagating waves, soliton trains, or other pulse forms provided that the propagation is primarily field‐aligned. The similarity of energy and intermittent occurrence characteristics suggests that these E∥ perturbations are likely to be associated with suprathermal electron bursts, both upward and downward. One release, which uniquely took place in a region of intense (200 V/km) rotational shear in the E⊥ field, was followed by the appearance of a small population of suprafast Ba+ ions traveling well ahead of the main high‐velocity Ba+ rays. A second set of faint streaks, representing suprafast Ba+ ions from the low‐velocity debris cloud, simultaneously appeared above the debris rays. These suprafast ions are attributed to transverse energizations of small fractions of both the initial Ba+ jet and Ba+ debris immediately following the release. Transverse energizations for the highest velocity ions extended to 5879 eV with maximum flux near 952 eV. Tip motions for the fastest Ba+ debris ions were not obtained, but triangulation of one streak showed a maximum energization of 131 eV. Wave‐particle stochastic heating models may be relevant to explaining both the high energies and the dependence of the energy on initial velocities.

Field-aligned flows of ionospheric plasma in the magnetosphere

Data are presented from occasions when one of the detectors of the Suprathermal Plasma Analyser on GEOS-2 observed along the magnetic field direction. Electrons were seen with larger fluxes along the field-aligned direction than at 80° to it. One particular class of these events was identified on the night-side when electrons in the energy range 10's to 100's eV were seen. These electrons of ionospheric origin may be associated with suprathermal electron bursts or the permanent power law population of the diffuse auroral zone.

Current carriers for the field-aligned current system

Recently the Dynamics Explorer satellites have returned a large body of data containing high resolution magnetometer measurements and distributions of charged particles of all but thermal electrons. From these data a systematic study has begun of the relations of the field-aligned currents to particle precipitation structures and the identification of the charge carriers. The data have been separated into three levels of magnetic activity and three local time sectors. Results of this study include the following: 1. During very quiet periods, field-aligned currents exist primarily as fine structure. 2. During onset of substorms, Region 1 and Region 2 become clearly evident but contain significant structure. 3. As magnetic activity subsides, current regions become less distinct, and structure becomes more dominant. 4. The distribution of the upward currents derived from magnetometer data and calculated from suprathermal electron data agree remarkably well in shape but not necessarily in magnitude. 5. At all local times, >5 eV electrons seldom carry most of the upward current. 6. Except for the accelerated Inverted-V electrons, the dominant upward current carriers which are measured are below 500 eV and are distributed in energy. 7. Dusk upward currents (Region 1) are associated with the Boundary Plasma Sheet (BPS). 8. Suprathermal electron bursts are important current carrying structures.