A particle simulation of current sheet instabilities under finite guide field

The instability of a Harris current sheet under a broad range of finite guide field (B-G) is investigated using a linearized (delta f) gyrokinetic electron and fully kinetic ion particle simulation code. The simulation is carried out in the two-dimensional plane containing the guide field along y and the current sheet normal along z. In this particle model, the rapid electron cyclotron motion is removed, while the realistic mass ratio m(i)/m(e), finite electron Larmor radii, and wave-particle interactions are kept. It is found that for a finite B-G/B-x0 <= 1, where B-x0 is the asymptotic antiparallel component of magnetic field, three unstable modes, i.e., modes A, B, and C, can be excited in the current sheet. Modes A and C, appearing to be quasielectrostatic modified two-stream instability/whistler mode, are located mainly on the edge of the current sheet. Mode B, on the other hand, is confined in the current sheet center and carries a compressional magnetic field (delta B-y) perturbation along the direction of electron drift velocity. Our new finding suggests that mode B may contribute directly to the electron anomalous resistivity in magnetic reconnection. In the cases with extremely large B-G/B-x0 >> 1, the wave modes evolve to a globally propagating instability. The simulation shows that the presence of finite B-G modifies the physics of the current sheet significantly. (C) 2008 American Institute of Physics.