# Intermediate shock substructures within a slow-mode shock occurring in partially ionised plasma

## TL;DR

This study uses high-resolution simulations to reveal the formation and persistence of intermediate shocks within slow-mode shocks in partially ionised plasmas, relevant to solar and astrophysical phenomena.

## Contribution

It demonstrates the formation of long-lived intermediate (Alfven) shocks within slow-mode shocks in a partially ionised plasma using a two-fluid numerical approach.

## Key findings

- Intermediate shocks form within slow-mode shocks in partially ionised plasma.
- Neutral fluid heating causes a Sedov-Taylor-like expansion and velocity overshoots.
- The intermediate shock persists across various physical parameters and initial conditions.

## Abstract

Slow-mode shocks are important in understanding fast magnetic reconnection, jet formation and heating in the solar atmosphere, and other astrophysical systems. The atmospheric conditions in the solar chromosphere allow both ionised and neutral particles to exist and interact. Under such conditions, fine substructures exist within slow-mode shocks due to the decoupling and recoupling of the plasma and neutral species. We study numerically the fine substructure within slow-mode shocks in a partially ionised plasma, in particular, analysing the formation of an intermediate transition within the slow-mode shock. High-resolution 1D numerical simulations are performed using the (P\underline{I}P) code using a two-fluid approach. We discover that long-lived intermediate (Alfven) shocks can form within the slow-mode shock, where there is a shock transition from above to below the Alfven speed and a reversal of the magnetic field across the shock front. The collisional coupling provides frictional heating to the neutral fluid, resulting in a Sedov-Taylor-like expansion with overshoots in the neutral velocity and neutral density. The increase in density results in a decrease of the Alfven speed and with this the plasma inflow is accelerated to above the Alfven speed within the finite width of the shock leading to the intermediate transition. This process occurs for a wide range of physical parameters and an intermediate shock is present for all investigated values of plasma-$\beta$, neutral fraction, and magnetic angle. As time advances the magnitude of the magnetic field reversal decreases since the neutral pressure cannot balance the Lorentz force. The intermediate shock is long-lived enough to be considered a physical structure, independent of the initial conditions.