Collisional ionisation and recombination effects in partially ionised plasma

Collisional ionisation and recombination effects in partially ionised plasma

Project dp050 PI: A. Hillier, Research by G. Murtas

Plasmoid-mediated fast magnetic reconnection plays a fundamental role in driving explosive dynamics such as chromospheric jets (Shibata et al. 2007, Singh et al. 2012) and heating in the solar chromosphere, but relatively little is known about how it develops in chromospheric partially ionised plasmas (PIP). Partial ionisation can greatly alter the dynamics of the coalescence instability (Murtas et al. 2021), 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.

Figure 1: Comparison of Jz between three PIP cases with different two-fluid coupling processes: elastic collisions only
(NIR, left), inclusion of ionisation and recombination (IR, centre) and inclusion of ionisation potential (IRIP, right). The
frames identify different steps of the coalescence instability. Panels (a), (b) and (c) show the initiation of the reconnection
process. In panels (d), (e) and (f) the evolution of coalescence is displayed at later stages. The final stage of coalescence
is shown in panels (g), (h), and (i) with the formation of the resulting plasmoid.

Using COSMA 7, we have investigated the development of plasmoid coalescence in PIP through 2.5D simulations of a two-fluid, ion-neutral hydrogen plasma. We have examined three different models for two-fluid interactions, where the two fluids are coupled by different processes: elastic collisions only (NIR), ionisation and recombination (IR), and ionisation, recombination and radiative losses (IRIP). The aim of this research is to understand to what extent the two-fluid coupling processes contribute in accelerating reconnection.

Figure 1 shows the time evolution of the coalescence through the current density for three fiducial cases corresponding to the three models for PIP. We have found that in general ionisation-recombination process slow down the coalescence. Unlike in the NIR model, 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, who dominate the reconnection dynamics.

Our study demonstrates that ionisation and recombination rates are large enough to suppress the onset of fractal coalescence and small-scale dynamics, as they act on current sheets properties. However, multi-fluid physics is still capable to promote fast reconnection, hence explaining the short time scales of chromospheric explosive events. Further results were collected in a paper that has been recently submitted to Physics of Plasmas.