Collisional ionization and recombination effects on coalescence instability in chromospheric partially ionized plasmas

Abstract

Plasmoid-mediated fast magnetic reconnection plays a fundamental role in driving explosive dynamics and heating, but relatively little is known about how it develops in partially ionised plasmas (PIP) of the solar chromosphere. Partial ionisation might largely alter the dynamics of the coalescence instability, which promotes fast reconnection and forms a turbulent reconnecting current sheet through plasmoid interaction, but it is still unclear to what extent PIP effects influence this process. We investigate the role of collisional ionisation and recombination in the development of plasmoid coalescence in PIP through 2.5D simulations of a two-fluid model. The aim is to understand whether these two-fluid coupling processes play a role in accelerating reconnection. We find that in general ionisation-recombination process slow down the coalescence. Unlike the previous models in G. Murtas, A. Hillier \& B. Snow, Physics of Plasmas 28, 032901 (2021) that included thermal collisions only, ionisation and recombination stabilise current sheets and suppress non-linear dynamics, with turbulent reconnection occurring in limited cases: bursts of ionisation lead to the formation of thicker current sheets, even when radiative losses are included to cool the system. Therefore, the coalescence time scale is very sensitive to ionisation-recombination processes. However, reconnection in PIP is still faster than in a fully ionised plasma environment having the same bulk density: the PIP reconnection rate ($M_{_{\operatorname{IRIP}}} = 0.057$) increases by a factor of $\sim 1.2$ with respect to the MHD reconnection rate ($M_{_{\operatorname{MHD}}} = 0.047$).