Auroral plasmas in the evening sector: Satellite observations and theoretical interpretations

Abstract

Observations and theoretical interpretations of auroral plasma distributions have led to a spectacular advance, in the latter part of the 1970's, in understanding the formation of auroral arcs and the role that the aurora plays in the coupling between the magnetosphere and ionosphere in the evening sector.The key to this understanding is the verification of the existence of electric field components parallel to the magnetic field. The parallel electric field accelerates electrons downwards to form the aurora. At the same time, it accelerates ionospheric ions upwards to provide the magnetosphere with a new source of hot plasma. The auroral plasma observations indicate that the hot auroral plasmas behave according to laws of adiabatic motion coexisting with a measure of plasma turbulence. Theoretical considerations of auroral arc formation are in accord with this plasma characteristic.

Mirror instability and the origin of morningside auroral structure

“…A major advance in the understanding of auroral arc formation processes in the evening sector has been achieved in the past few years by satellite observations and theoretical interpretations of auroral plasmas and fields [see, for example, Akasofu and Kan, 1981;Mozer et al, 1980;Fennell et a!., 1981;Chiu et al, 1982;and references therein]. In particular, the electrodynamic coupling between hot magnetospheric and cold ionospheric plasmas gives rise to a magnetic field-aligned electric potential drop which accelerates electrons downward to form the inverted-V and arc structures.…”

Evidence for Particle Acceleration During Magnetospheric Substorms

“…Subsequent investigations found that most of the incoming energy was in the form of a "near-monoenergetic beam" superposed on a broader Maxwellian spectrum (Evans 1968;Westerlund 1968). Low-altitude satellite observations of substorm auroral forms such as westward traveling surges have shown that these are also produced by precipitating electrons with the same general spectral features as in quiet arcs, although the energy of the monoenergetic peak in the spectrum tends to be greater than for quiet arcs (e.g., Meng et al 1978;Chiu et al 1983). Evans (1974) provided a model in which a field-aligned potential drop accelerates electrons, and Figure 4 presents an example of model results compared with observations.…”

Plasma Observations in the Auroral and Polar Cap Region

“…For example, Hellis and Winningham (1984) have reported a DE-2 observation of upward thermal ion fluxes as high as 101 0 cm -2 sec -1 at the edges of inverted-V events. The upwards field aligned currents were 100 P A m -2 with the upgoing ions contributing more than 10 v A m -2 and at times currents comparable to the observed field aligned current.…”