Plasma Processes Driven by Current Sheets and Their Relevance to the Auroral Plasma

Abstract

Satellite and rocket observations have revealed a host of auroral plasma processes, including large dc perpendicular electric fields (E⊥) associated with electrostatic shocks, relatively weak parallel electric fields (E║) associated with double layers, upflowing ions in the form of beams and conics, downflowing and upflowing accelerated electron beams, several wave modes such as the electrostatic ion-cyclotron (EIC), lower hybrid (LH), very low frequency (VLF), extremely low frequency (ELF), and high-frequency waves and associated nonlinear phenomena. Recently, we have attempted to simulate the various processes using a two-dimensional particle-in-cell code in which the plasma is driven by current sheets of a finite thickness. Striking similarities between the observed auroral plasma processes and those seen in the simulations are found. In this paper we give a review of the plasma processes dealing with dc and ac electric fields, formation of ion beams and conics, and electron acceleration. Electrostatic shock-type electric fields (E┴e) occur near the current sheet edges. Such fields arise because of the contact between the high-and low-density plasmas inside and outside the sheet, respectively. Double layers having upward electric fields form inside the sheet and they are distinguishable from the large perpendicular electric fields (E⊥e) only in wide sheets with thicknesses l >> ρi, the ion Larmor radius. Double layers with a reverse polarity form outside the sheet where downward currents flow. The most energetic ions are found to have pitch angles near 90°, implying a large perpendicular acceleration of the ions.