Coalescence Instability in Chromospheric Partially Ionised Plasmas

Fast magnetic reconnection plays a fundamental role in driving explosive dynamics and heating in the solar chromosphere. The reconnection time scale of traditional models is shortened at the onset of the coalescence instability, which forms a turbulent reconnecting current sheet through plasmoid interaction. In this work we aim to investigate the role of partial ionisation on the development of fast reconnection through the study of the coalescence instability of plasmoids. Unlike the processes occurring in fully ionised coronal plasmas, relatively little is known about how fast reconnection develops in partially ionised plasmas of the chromosphere. We present 2.5D numerical simulations of coalescing plasmoids in a single fluid magnetohydrodynamic (MHD) model, and a two-fluid model of a partially ionised plasma (PIP). We find that in the PIP model, which has the same total density as the MHD model but an initial plasma density two orders of magnitude smaller, plasmoid coalescence is faster than the MHD case, following the faster thinning of the current sheet and secondary plasmoid dynamics. Secondary plasmoids form in the PIP model where the effective Lundquist number $S = 7.8 \cdot 10^3$, but are absent from the MHD case where $S = 9.7 \cdot 10^3$: these are responsible for a more violent reconnection. Secondary plasmoids also form in linearly stable conditions as a consequence of the non-linear dynamics of the neutrals in the inflow. In the light of these results we can affirm that two-fluid effects play a major role on the processes occurring in the solar chromosphere.