Coalescence instability in chromospheric partially ionized 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 ionization in the development of fast reconnection through the study of the coalescence instability of plasmoids. Unlike the processes occurring in fully ionized coronal plasmas, relatively little is known about how fast reconnection develops in partially ionized plasmas (PIPs) 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 ionized 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 x 10 3, but are absent from the MHD case where S = 9.7 x 10 3: these are responsible for a more violent reconnection. Secondary plasmoids also form in linearly stable conditions as a consequence of the nonlinear dynamics of the neutrals in the inflow. In the light of these results, we can affirm that two-fluid effects play a major role in the processes occurring in the solar chromosphere.

Automated summary

This study investigates the role of partial ionization in the development of fast magnetic reconnection in the solar chromosphere. It shows that in partially ionized plasmas, the merging of plasmoids occurs faster and leads to the formation of secondary plasmoid dynamics, resulting in more violent reconnection compared to fully ionized plasmas. The results highlight the significant role of two-fluid effects in the processes occurring in the solar chromosphere.