C. C. Chaston

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.

SMALL SCALE ALFVÉNIC STRUCTURE IN THE AURORA

This paper presents a comprehensive review of dispersive Alfvén waves in space and laboratory plasmas. We start with linear properties of Alfvén waves and show how the inclusion of ion gyroradius, parallel electron inertia, and finite frequency effects modify the Alfvén wave properties. Detailed discussions of inertial and kinetic Alfvén waves and their polarizations as well as their relations to drift Alfvén waves are presented. Up to date observations of waves and field parameters deduced from the measurements by Freja, Fast, and other spacecraft are summarized. We also present laboratory measurements of dispersive Alfvén waves, that are of most interest to auroral physics. Electron acceleration by Alfvén waves and possible connections of dispersive Alfvén waves with ionospheric-magnetospheric resonator and global field-line resonances are also reviewed. Theoretical efforts are directed on studies of Alfvén resonance cones, generation of dispersive Alfvén waves, as well their nonlinear interactions with the background plasma and self-interaction. Such topics as the dispersive Alfvén wave ponderomotive force, density cavitation, wave modulation/filamentation, and Alfvén wave self-focusing are reviewed. The nonlinear dispersive Alfvén wave studies also include the formation of vortices and their dynamics as well as chaos in Alfvén wave turbulence. Finally, we present a rigorous evaluation of theoretical and experimental investigations and point out applications and future perspectives of auroral Alfvén wave physics.

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.

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.

IDENTIFICATION OF KINETIC ALFVÉN WAVE TURBULENCE IN THE SOLAR WIND

The nature of small-scale turbulent fluctuations in the solar wind is investigated using a comparison of Cluster magnetic and electric field measurements to predictions arising from models consisting of either kinetic Alfven waves or whistler waves. The electric and magnetic field properties of these waves from linear theory are used to construct spacecraft-frame frequency spectra of (|δE|/|δB|) s/c and (|δB ∥|/|δB|) s/c , allowing for a direct comparison to spacecraft data. The measured properties of the small-scale turbulent fluctuations, found to be inconsistent with the whistler wave model, agree well with the prediction of a spectrum of kinetic Alfven waves with nearly perpendicular wavevectors.

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.

FAST observations of electron distributions within AKR source regions

Observations of high-time resolution 3-D electron distributions within the source regions of Auroral Kilometric Radiation (AKR) are reported. In general, the electron data display a broad plateau over a wide range of pitch angles indicating that these distributions have been rapidly stabilized by AKR wave growth. The source of the electron instability appears to come from several features in the distribution, including an isotropic beam feature and its mirroring components, occasional electrons in the “trapped” region, as well as steep gradients present in the atmospheric loss cone. Taken together these features may provide a nearly continuous region of ∂f/∂v⟂ which could contribute to the relativistic cyclotron maser instability. Computer simulations of the evolution of the electron distribution which assume plasma conditions similar to the parameters measured by FAST show similar results to the observed electron distributions. The FAST observations also show that relativistic corrections to the AKR dispersion relation may enable a small k|| mode with a resonance condition that is able to take maximum advantage of the initial instability in the mono-energetic electron distributions within the auroral acceleration regions.

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.

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.

Aircraft observations conjugate to FAST: Auroral are thicknesses

Optical observations conjugate to the FAST satellite show good agreement between the widths of auroral structures observed optically and those inferred from the measured electron energy flux. The implication is that these structures are imposed by processes at or above the -4000 km altitude of FAST. A variety of widths down to about 2 km were observed, but there were no examples of finer scale structures. A pre-breakup weak discrete arc at the poleward edge of the diffuse aurora showed electron produced optical structures located on either side of upward going ion beams. The optical emission in the equatorward part of the diffuse aurora was caused almost exclusively by precipitating ions. The optical observations were made over northern Alaska between Jan 31 and Feb 16, 1997, from a jet aircraft carrying an all-sky and three narrow-field TV cameras.