R. F. Pfaff

FAST satellite observations of large‐amplitude solitary structures

We report observations of “fast solitary waves” that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ∼100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

FAST observations in the downward auroral current region: Energetic upgoing electron beams, parallel potential drops, and ion heating

Observations of plasma particles and fields by the FAST satellite find evidence of acceleration of intense upgoing electron beams by quasi-static parallel electric fields. The beam characteristics include a broad energy spectrum with peak energies between 100 eV and 5 keV, perpendicular temperatures less than 1 eV, and fluxes greater than 109/cm²sec. Diverging electrostatic shocks associated with the beams have integrated potentials that match the beam energy. These beams are found in regions of downward Birkeland current and account for the total field-aligned current when they are present. The most energetic ion conics in the auroral zone are found coincident with these beams, in agreement with the model for “trapped” conics. The measured particle densities of the electron beams and associated ion conics are approximately equal and typically range from 1 to 10 cm−3, with no evidence for additional cold density. The beams are seen frequently at altitudes between 2000 and 4000 km in the winter auroral zone. Their probability of occurrence has a strong dependence on season and altitude and is similar to that for upgoing ion beams in the adjacent upward current regions. This similarity suggests that the density and scale height of ionospheric ions play an important role in the formation of both types of beams.

FAST satellite observations of electric field structures in the auroral zone

Electric field and energetic particle observations by the Fast Auroral Snapshot (FAST) satellite provide convincing evidence of particle acceleration by quasi-static, magnetic-field-aligned (parallel) electric fields in both the upward and downward current regions of the auroral zone. We demonstrate this by comparing the inferred parallel potentials of electrostatic shocks with particle energies. We also report nonlinear electric field structures which may play a role in supporting parallel electric fields. These structures include large-amplitude ion cyclotron waves in the upward current region, and intense, spiky electric fields in the downward current region. The observed structures had substantial parallel components and correlative electron flux modulations. Observations of parallel electric fields in two distinct plasmas suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.

The Fast Auroral SnapshoT (FAST) Mission

The FAST satellite mission investigates plasma processes occurring in the low altitude auroral acceleration region, where magnetic field-aligned currents couple global magnetospheric current systems to the high latitude ionosphere. In the transition region between the hot tenuous magnetospheric plasma and the cold, dense ionosphere, these currents give rise to parallel electric fields, particle beams, plasma heating, and a host of wave-particle interactions. FAST instruments provide observations of plasma particles and fields in this region, with excellent temporal and spatial resolution combined with high quantitative accuracy. The spacecraft data system performs on-board 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.

FAST satellite wave observations in the AKR source region

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 (nhot> ncold). Electron distributions were depleted at low-energies and up-going ion beams were always present. Source region amplitudes were far greater than previously reported, reaching 2×10−4 (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 |E|/|B| ratio was constant as a function of frequency and rarely displayed any features that would identify a cold plasma cutoff or resonance.

Comparisons of Polar satellite observations of solitary wave velocities in the plasma sheet boundary and the high altitude cusp to those in the auroral zone

Characteristics of solitary waves observed by Polar in the high altitude cusp, polar cap and plasma sheet boundary are reported and compared to observations in the auroral zone. The study presented herein shows that, at high altitudes, the solitary waves are positive potential structures (electron holes), with scale sizes of the order of 10s of Debye lengths, which usually propagate with velocities of a few thousand km/s. At the plasma sheet boundary, the direction of propagation can be either upward or downward; whereas at the leading edge of high altitude cusp energetic particle injections, it is downward. For these high altitude events, explanations based on ion modes and on electron modes are both examined, and the electron mode interpretation is shown to be more consistent with observations.

FAST observations of VLF waves in the auroral zone: Evidence of very low plasma densities

The Fast Auroral SnapshoT (FAST) explorer frequently observes the auroral density cavity, which is the source region for Auroral Kilometric Radiation (AKR). An important factor in the generation of AKR is the relative abundance of hot and cold electrons within the cavity, since hot electrons introduce relativistic modifications to the wave dispersion. VLF wave-form data acquired by FAST within the auroral density cavity show clear signatures of whistler-mode waves propagating on the resonance cone. This allows us to obtain the electron plasma frequency, and the cavity often has densities <1 cm−3. Moreover, the hot electrons can be the dominant electron species, enabling AKR to be generated below the cold electron gyro-frequency.

The association of electrostatic ion cyclotron waves, ion and electron beams and field‐aligned currents: FAST observations of an auroral zone crossing near midnight

FAST particle and wave data for a single nightside auroral zone crossing are utilized to examine the free energy source for electrostatic ion cyclotron (EIC) waves. Comparisons of the unstable wave modes, obtained by an electrostatic linear dispersion relation solver, to the observed waves for two intervals with upflowing ion beams and two with upflowing electron beams are consistent with the conclusion that the observed waves near the cyclotron frequencies are EIC which are driven by the electron drift both in the upgoing ion beam regions and in the upgoing electron regions. A limitation is that the drifting bi-Maxwellian model used in the dispersion relation is not a good match to the observed upflowing electron distributions. The observed ion beams do not drive EIC waves; however, the relative drift of the various ion species comprising the ion beam can drive low frequency (<∼50 Hz) waves unstable. The electron drift, during some intervals, also destabilizes electron acoustic waves.

Spatial structure and gradients of ion beams observed by FAST

High time resolution measurements of ion distributions by the FAST satellite have revealed kilometer scale spatial structure in the low altitude auroral acceleration region. The low altitude edge of the acceleration region appears to contain fingers of potential that extend hundreds of kilometers along B but are only a few to tens of kilometers wide. These fingers of potential do not appear to be strongly correlated with the local current or total potential drop. Gradients in the ion beam energy are found to be consistent with the electric field signatures expected in the quasi-static potential drop model of auroral acceleration. Typical ion beams show gradients of 0.5–1.0 keV/km, with some events as large a 3 keV/km. Integrations of the electric field along the space-craft velocity are used to calculate parallel potential below FAST and are found to agree well with the ion beam energy for most events. One event is shown where an apparent temporal change in the auroral configuration occurs at the edge of the ion beam producing a disagreement between the beam energy and inferred potential.

Electron modulation and ion cyclotron waves observed by FAST

New observations from the FAST satellite demonstrate strong wave-particle interactions between energetic electrons and H+ EMIC waves in inverted-V arcs. The intense waves are shown to occur in strong upward current regions which contain intense downgoing field-aligned electron fluxes. Electrons near the inverted-V spectral peak have large, factor of 2 to 10, coherent flux modulations at or near the wave frequency. The electron modulations are typically centered at about f CH+ /2, where f CH+ is the local H+ cyclotron frequency. The EMIC waves are broadbanded, extending from about 0.3f CH+ to 0.7f CH+ . These waves also accelerate cold secondary electrons, forming counterstreaming field-aligned electrons at energies up to about 300 eV. In addition, electron modulations at f CH+ are observed in the density cavities associated with upgoing ion beams. Intense waves at f CH+ are simultaneously detected and shown to have a magnetic component similar to the EMIC waves.