Plasma Physics on Auroral Field Lines: The formation of Ion Conic Distributions

Abstract

Auroral field lines near 1 R_E altitude exhibit rich and complex plasma physics phenomena. These include the precipitation of energetic electrons, electrostatic shocks, potential double layers, electrostatic hydrogen cyclotron waves, and upward streaming ions (Mozer et al., 1980). Figure 1 is an attempt to illustrate schematically our present understanding of the phenomena of auroral field lines in the dusk sector. At high altitudes the observations are consistent with double layers and the associated parallel electric fields. These field aligned potential differences can produce beams of both downward electrons and upward ions with fluxes peaked in energy and enhanced in the direction parallel to the magnetic field. The visible discrete auroral arcs result from the precipitation of 1 -10 keV electrons which carry an upward field aligned current. In order to maintain charge neutrality the ionosphere must respond by sending an outflow of cold electrons which carry downward field aligned currents. Particle observations indicate that ion beams are not the only distribution which are observed. At altitudes ≈ 1500 km, measurements from the ISIS-1 and ISIS-2 satellites report the existence of ion distributions with enhanced particle fluxes at pitch angles between 90–130 degrees (Klumpar, 1979; Ungstrup et al., 1979). Such distributions are commonly termed ion conic distributions.