Simulations of auroral plasma processes: Electric fields, waves and particles

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 elctrostatic ion-cyclotron (EIC), lower hybrid (LH), VLH, ELF and high frequency waves and associated non-linear phenomena. We have attempted to simulate these various processes using a two-dimensional particle-in-cell code. In these simulations the plasma is driven by current sheets (carrying upward currents) of a finite thicknesses. Striking similarities between the observed auroral plasma processes and those seen in the simulations are found. Large perpendicular electric fields ( E_{⊥ e}) develop near the edges of the current sheets, which have features similar to electrostatic shocks. Double layer formation occurs inside the current sheets and also outside the sheet where downward current flows. The polarities of the parallel electric fields of the double layers inside and outside the current sheet are found to be mainly upward and downward, respectively. The double layer electric field strength ( E_∥) is found to be highly variable, depending on the plasma parameters. However, it is generally found that E_{⊥ m} ≫ and E_{∥ m} where, E_{⊥ m} and E_{∥ m} are respectively, the maximum strengths of E_⊥ and E_∥ seen in the simulations irrespective of their relative locations. Normally, the electric field strength of double layers in larger than the parallel electric fields occurring at the locations where E_{⊥ e} maximizes. The non-linear phenomenon associated with the waves near the electron plasma frequency is found to cause a low frequency relaxation-type of oscillition in the potential structure. The low frequency waves seen in the simulations range from below the ion-cyclotron frequency to above the lower hybrid frequency. In some simulations, spiky ion-cyclotron waves are seen. Ion beam formation with a positive slope in the parallel velocity distribution function is seen only in a narrow currentsheet with a thickness l ∼ p_i, where p_i is an appropriate ion Larmor radius. In wide sheets l ≫ p_i; the upflowing ions are found to be considerably heated. The most energetic ions seen in the simulations have pitch angles near 90°, implying perpendicular ion acceleration, which is the main mechanism for the generation of ion conics. The inverted-V feature in the horizontal distribution of the downflowing accelerated electrons is seen. However, the accelerated electrons are found to be neither monoenergetic nor perfectly field-aligned. Double layers in the current sheets are found to be the triggering mechanism for the plasma processes which shape the distribution function of the accelerated electrons. Counterstreaming accelerated electrons are also seen. Upward electron beams in the downward current regions are found to be nearly a permanent feature of the simulations with wide sheets. On the other hand, in simulations with narrow sheets such beams are transient.