Coalescence instability in chromospheric partially ionized plasmas

Abstract

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 ¼ 7:8 Â 10 3 , but are absent from the MHD case where S ¼ 9:7 Â 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.

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

“…In this paper, we examine in detail the effects of collisional ionization, recombination, and optically thin radiative losses on the coalescence instability developing in partially ionized plasmas. This model improves upon our previous work 53 by including the contribution of these processes. We aim to understand how the new type of coupling between charges and neutral species influences the reconnection rate, compared to our previous research.…”

“…2,36,[70][71][72][73] The general behavior is that neighboring plasmoids inside a current sheet attract each other, a second current sheet forms in between them, and reconnection occurs in it, leading to the plasmoids complete merging. Extending the model to PIP (using only thermal collisions) results in a faster coalescence and the promotion of small-scale dynamics such as sub-critical secondary plasmoids formation and fractal coalescence (see our recent paper 53 ). Here, the model is further extended to account for ionization/recombination and ionization potential effects, which provide a more realistic description of the physical processes occurring in a partially ionized plasma.…”

“…52 Relatively little is known about how the coalescence instability is altered by the multi-fluid physics of a partially ionized plasma. In our preliminary study, 53 we found that, in an ion-neutral plasma with fluids coupled through elastic collisions, partial ionization speeds up both phases of coalescence and promotes non-linear effects during reconnection. In this paper, we examine in detail the effects of collisional ionization, recombination, and optically thin radiative losses on the coalescence instability developing in partially ionized plasmas.…”

“…Note that these initial conditions mimic our previous paper. 53 The Fadeev equilibrium is unstable to the coalescence instability. 2 An initial velocity perturbation in both plasma and neutral species is imposed to break the initial equilibrium, given by…”

Doppler-velocity Drifts Detected in a Solar Prominence

“…In a similar vein, Hillier (2019) demonstrated different dynamics and shapes of the structures formed in the ionized and neutral fluid during the KHI. Local reconnection events produce neutral and ionized outflows with very different speeds as well (Leake et al 2013;Murphy & Lukin 2015;Alvarez Laguna et al 2017;Ni et al 2018;Murtas et al 2021).…”