Giulia Murtas

Magnetic reconnection is responsible for explosive events in astrophysical, space and laboratory plasmas: in this process oppositely directed magnetic field lines reconnect through a narrow diffusion region, enabling the conversion of stored magnetic energy into kinetic and thermal energy and particle acceleration. The presence of localised, transient outflows in the solar chromosphere such as the chromospheric jets is believed to be the result of the onset of magnetic reconnection. Traditional steady-state reconnection is described by the Sweet-Parker (SP) model, and it depends on the Lundquist number S. The SP reconnection rate is however too slow to explain the time scale of many phenomena observed in solar plasmas: a way to trigger fast reconnection is provided by the plasmoid coalescence instability. Plasmoids, found in direct imaging observations of solar flares, are commonly present in reconnecting systems and play a major role in speeding up reconnection. Under the formation of plasmoids at sufficiently high S, the current sheet breaks into fragments allowing turbulent reconnection.

In this work we aim to investigate the role of partial ionisation on the development of fast reconnection through the study of the plasmoid coalescence instability in the solar chromosphere. We present 2.5D numerical simulations in both a single fluid magnetohydrodynamic (MHD) fully ionised plasma, and a two-fluid model of a partially ionised plasma (PIP). The simulations are performed with the (PIP) code (Hillier et al. 2016), and the initial setup is provided by a force-free modified 2D Fadeev equilibrium, with the addition of a Bz component. The system is studied in a box with upper and lower symmetric boundaries and lateral periodic boundaries.

The coalescence instability can be divided in two phases. An initial sinusoidal velocity perturbation removes the plasmoids from their equilibrium, which attract each other and move closer. In the second phase a current sheet forms between the plasmoids and reconnection takes place, leading to the merging of the two plasmoids. This phase is shown in Figure (a) for both MHD and PIP cases, where the plasmoids are identified by positive peaks of the current density. We find that in PIP the coalescence timescale is shorter than the MHD case and the reconnection rate is sped up. Secondary plasmoids identified by a negative current are absent in the MHD case, which shows a SP-like reconnection, but they form in the central current sheet of the PIP model, making the reconnection more explosive. Besides the promoted production of secondary plasmoids, there is presence of other partial ionisation effects such as the formation of an extended neutral jet that have no counterparts in the MHD case. The jet, shown in Figure (b), forms during plasmoid coalescence and is subject to the Kelvin-Helmholtz instability. The extended jet appears in the neutral flow only and does not have a counterpart in the plasma, where there is a much smaller plasma reconnection jet developing along the y-axis at the current sheet extremities.

The collisional coupling between charged and neutral species affects the timescale of coalescence (which is faster the lower the collisional frequency is), and the stability of the central current sheet. The coalescence rate increases at the decrease of collisional frequency: this is shown in Figure (c) by the variation of the current in time at the centre of the current sheet, where the MHD case is taken as the limit of an infinite collisional frequency. All the PIP simulations display dramatic fluctuations in the current density: these are related to the production and expulsion from the current sheet of secondary plasmoids.