Abstract. 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.

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Abstract. Electric field and energetic particle observations by the Fast Auroral Snapshot (FAST) satellite provide convincing evidence of particle acceleration by quasi-static, magnetic-fieldaligned (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.

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Abstract. 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.