Driven waves in a two-fluid plasma

TL;DR

This paper analyzes wave propagation in a weakly ionized plasma, focusing on the formation of multifluid MHD shock waves, using analytical models and Green functions to describe driven wave behavior over different time scales.

Contribution

It introduces an analytical framework for driven MHD waves in a two-fluid plasma, highlighting the transition from decoupled to coupled fluid dynamics and modeling shock formation.

Findings

Driven MHD waves exhibit distinct short and long time scale behaviors.

Formation of magnetic precursors during cloud collisions.

Self-similar flow evolution after linear analysis validity ends.

Abstract

We study the physics of wave propagation in a weakly ionised plasma, as it applies to the formation of multifluid, MHD shock waves. We model the plasma as separate charged and neutral fluids which are coupled by ion-neutral friction. At times much less than the ion-neutral drag time, the fluids are decoupled and so evolve independently. At later times, the evolution is determined by the large inertial mismatch between the charged and neutral particles. The neutral flow continues to evolve independently; the charged flow is driven by and slaved to the neutral flow by friction. We calculate this driven flow analytically by considering the special but realistic case where the charged fluid obeys linearized equations of motion. We carry out an extensive analysis of linear, driven, MHD waves. The physics of driven MHD waves is embodied in certain Green functions which describe wave propagation on short time scales, ambipolar diffusion on long time scales, and transitional behavior at intermediate times. By way of illustration, we give an approximate solution for the formation of a multifluid shock during the collision of two identical interstellar clouds. The collision produces forward- and reverse J shocks in the neutral fluid and a transient in the charged fluid. The latter rapidly evolves into a pair of magnetic precursors on the J shocks, wherein the ions undergo force free motion and the magnetic field grows monotonically with time. The flow appears to be self similar at the time when linear analysis ceases to be valid.