Can plasmoid-mediated reconnection occur in collisionless systems?

TL;DR

This study investigates whether plasmoid-mediated magnetic reconnection can occur in collisionless plasma systems, finding that secondary plasmoids can form under certain conditions but do not significantly enhance reconnection rates in realistic scenarios.

Contribution

It demonstrates through simulations that secondary plasmoids can form in collisionless systems but are suppressed at realistic ion-electron mass ratios, challenging the idea that plasmoid-mediated reconnection explains fast reconnection in such systems.

Findings

Secondary plasmoids can form in collisionless reconnection.

Mass ratio strongly affects plasmoid formation.

No significant reconnection rate enhancement observed.

Abstract

Magnetic reconnection is a process that converts magnetic energy into plasma energy by changing the magnetic field line topology. The outstanding question is why the reconnection rate is $\mathcal{O}(0.01 - 0.1)$ in many astrophysical phenomena, for example solar flares and terrestrial substorms. Previous studies have shown two ideas of Hall reconnection and plasmoid instability. However, there is no consensus on which process is the reason for the fast reconnection. In this paper, we discuss the formation of secondary plasmoids in \rewrite{2D antiparallel collisionless reconnection} using 2.5-dimensional particle-in-cell simulations and discuss whether plasmoid-mediated reconnection occur in collisionless systems by comparing with plasmoid instability in resistive MHD simulations. We find that in collisionless systems secondary plasmoids can indeed form. However, the mass ratio has a strong effect on the formation of secondary plasmoids, and it indicates that secondary plasmoids do not emerge using realistic ion-electron mass ratio ($m_i/m_e = 1836$). Furthermore, we find that there is no enhancement of the reconnection rate due to the secondary plasmoid in the collisionless system, as discussed in the plasmoid-mediated reconnection. Although our simulation $\mathcal{O}(100\lambda_i)$ box is not large enough to discuss astrophysical phenomena such as solar flares, it can reflect a relatively small plasma system such as the Earth's magnetotail.