Spontaneous focusing of plasma flow in a weak perpendicular magnetic field

Abstract

Structure formation of high-beta plasma flow in a perpendicular magnetic field is investigated in the ion kinetic regime by a fully kinetic particle-in-cell simulation. We demonstrate that directional plasma flow is spontaneously focused to form a sharp density structure. The primary focusing process comes from field-aligned electron inflow associated with the whistler mode and plasma confinement due to a self-generated magnetic field. The resulting concave magnetic field lines modulate ion gyration to cause a secondary focusing process with significant plasma concentration. Required conditions for these processes are determined by a dimensionless parameter α=βi0(ΔW/ρi0), where βi0, ΔW, and ρi0 denote the plasma kinetic beta, window size, and ion gyration radius, respectively. The focusing process is apparent for small α, whereas diamagnetic expansion is dominant for large α. This condition describes a transition between diamagnetic cavity formation and the focusing process.