Dynamics of narrow electron streams in magnetized plasmas

Abstract

In this analytical and computer simulation study we explore the dynamics of a narrow electron stream embedded in a magnetized plasma. The transverse dimension of the stream is envisioned to be on the order of the electron skin depth, as is appropriate to several problems of current interest (e.g., auroral beams, reconnection). Within the layer the drift velocity exceeds the thermal velocity, and thus the Buneman instability is excited. The method of matched asymptotic expansions is used to describe the linear stage of the instability for a nonuniform drift profile. It predicts a lowering of the growth rate and a rapid decrease in wave amplitude at the spatial location where the beam mode resonance is encountered. A particle-in-cell (PIC) simulation is used to verify the predictions of the analysis and to illustrate the important nonlinear behavior. It is found that the rapid flash of the Buneman instability excites a lower-hybrid wave, causes strong perpendicular ion acceleration, and results in a region of depleted density. The PIC study also considers a quasisteady situation that models the effect of a shear Alfvén wave of narrow transverse extent.