On the linear mirror mode threshold in high-$\beta$ space plasma

TL;DR

This paper derives an exact analytical threshold condition for the mirror mode in high-beta space plasma, clarifies the nature of observed magnetic structures, and distinguishes between dips and peaks in the data.

Contribution

It provides a precise analytical threshold for mirror modes considering drift and finite Larmor radius effects, and clarifies the origin of observed magnetic dips and peaks.

Findings

Only magnetic dips are produced by the mirror instability.

Peaks are likely caused by nonlinear evolution of the fast mode.

A semi-experimental threshold condition is derived from observational data.

Abstract

The linear threshold condition of the mirror mode including weak drift and finite Larmor radius effects is analysed and shown that it can be given to arbitrary precision in closed analytical form. Combined with observations of mirror modes in the magnetosheath it is shown that only magnetic `dips' are produced by the mirror instability. A semi-experimental (non-linear) threshold condition is derived from the data. The occasionally observed so-called `peaks' cannot be attributed to mirror modes. They result probably from nonlinear evolution of the compressive fast mode.