Ion mixing in the plasma sheet boundary layer by drift instabilities

Abstract

The linear stability properties of collisionless drift instabilities are analyzed in a Harris equilibrium model B0x tanh (z/L) of the plasma sheet boundary layer (PSBL). The streaming ions with drift type instabilities driven by υ0∥(z) ≃ υ(z) in the PSBL are considered. The fluid approximation leads to growth from ∂T/∂z and ∂υ0∥/∂z but predicts that the mode width Δz approaches the gyroradius ρi of the energetic ions. Thus an integral equation theory for the modes is developed taking into account that in the PSBL the curvature drift is weak compared with the grad‐B drift υD B. The exact wave particle resonance ω = kx υx + ky υD B (υ⊥²) is kept in the nonlocal response functions. Plasma density, temperature, and magnetic gradient drift motions are taken into account. The drift modes driven by ∂υx/∂z produce an anomalous cross‐field momentum transport mixing the PSBL ions on the time scale of tens of seconds. A nonlinear simulation is performed which shows the coalescence of the small scale, fast growing modes into large‐scale vortices. The relation between these collective modes and plasma sheet transport phenomena is discussed including the comparison with the competing plasma mixing from single‐particle stochasticity.