Shocks and instabilities in the partially ionised solar atmosphere

TL;DR

This paper reviews how two-fluid models are essential for understanding shocks and instabilities in the partially ionized solar atmosphere, emphasizing multi-scale interactions between charged and neutral particles.

Contribution

It provides a detailed analysis of two-fluid physics in the solar atmosphere, focusing on shock-waves and instabilities, and discusses observational diagnostics and future directions.

Findings

Two-fluid effects significantly influence shock dynamics.

Instabilities in the solar atmosphere are affected by multi-scale interactions.

Observations can help diagnose two-fluid phenomena.

Abstract

The low solar atmosphere is composed of mostly neutral particles, but the importance of the magnetic field for understanding observed dynamics means that interactions between charged and neutral particles play a very important role in controlling the macroscopic fluid motions. As the exchange of momentum between fluids, essential for the neutral fluid to effectively feel the Lorentz force, is through collisional interactions, the relative timescale of these interactions to the dynamic timescale determines whether a single-fluid model or, when the dynamic frequency is higher, the more detailed two-fluid model is the more appropriate. However, as many MHD phenomena fundamentally contain multi-time-scale processes, even large-scale, long-timescale motions can have an important physical contribution from two-fluid processes. In this review we will focus on two-fluid models, looking in detail at two areas where the multi-time-scale nature of the solar atmosphere means that two-fluid physics can easily develop: shock-waves and instabilities. We then connect these ideas to observations attempting to diagnose two-fluid behaviour in the solar atmosphere, suggesting some ways forward to bring observations and simulations closer together.