Effect of Ionization and Recombination on the Evolution of the Harris-type Current Sheet in Partially Ionized Plasmas

Abstract

Abstract Two-dimensional magnetohydrodynamics (MHD) simulations, treating plasma and neutral populations (hereafter, neutrals) as two separate components of the magneto-fluid, are performed in order to investigate the effect of ionization and recombination (or I/R) on the time evolution of the Harris-type current sheet in partially ionized plasmas. Our MHD simulations, including the effect of ambipolar diffusion (arising due to ion-neutral interactions) along with the I/R, show that the current sheet thinning occurs due to the diffusion of neutral particles from the current sheet. In addition to ambipolar diffusion, frictional heating also appears and affects the evolution of the current sheet. In a current sheet that is formed in a partially ionized plasma, the neutral population tries to spread outward and the plasma population tries to converge toward the center of the current sheet, and the overall process is influenced by the I/R. One of the important feature that is captured in our 2D simulations is that the escape of neutrals from the current sheet is sometimes suppressed due to the increase in ionization rate at the center of the current sheet, for the case of collisional I/R. As long as the ionization degree is kept low inside the current sheet, the current sheet thinning and elongation takes place and the current sheet becomes unstable due to the tearing-mode and plasmoid formation. The ion-neutral interactions coupled with I/R and the dynamics of the magnetic reconnection play an important role in plasmoid-mediated reconnection, therefore, the present study on the current sheet thinning and plasmoid formation could serve as a key for understanding bursty and intermittent plasma ejections observed in the solar chromosphere.