A NEW NUMERICAL CODE FOR SIMULATING CURRENT-DRIVEN INSTABILITIES ON AURORAL FIELD LINES

We have studied the microphysical processes that occur on auroral field lines carrying field-aligned currents by means of a newly developed two-dimensional electrostatic PIC (particle in cell) numerical code. We are, in particular, interested in the question of whether field-aligned currents carried by obliquely propagating shear Alfven waves can lead to the formation of quasi-stationary field-aligned electric fields and double layers in order to provide a detailed test of the shear Alfven wave model for generating small-scale field-aligned currents and auroral particle acceleration. Our model includes upward and downward propagating oblique shear Alfven waves in a phenomenological way through an impedance characterizing the source plasmas outside the simulation box. In this paper we concentrate on the description of the numerical scheme, the testing of it, and a few important results which are obtained from a number of runs. Some of the findings are as follows: the reflection of Alfven waves at regions containing plasma waves coupling the electron and ion motion causes a reduction of the field-aligned current and limits it to a critical value which is marginal for the excitation of electrostatic ion cyclotron waves at a height of 1-2 R(e). Electron heating due to current-driven plasma turbulence causes the overall plasma density in the system to decrease, because we do not require the number of particles to remain constant inside the system. This may be related to the formation of the auroral cavity. The plasma density in the region of upward current is enhanced relative to that in the region of downward current, and the current density in upward directed current channels (carried by precipitating electrons) can be larger than in downward directed current channels. We also observe a filamentation of the current in the upward directed current channel. We find upward propagating solitary potential structures in the upward directed current channel and intermittent double layers in both channels.