Magnetic Reconnection Process in Partially Ionized Fluids: Insights from the Solar Chromosphere

Abstract

Magnetic reconnection converts stored magnetic energy into kinetic energy, heat, and radiation. While extensively studied in fully ionized plasmas, observations show that ionization and recombination also play a role by modifying the local plasma resistivity and enabling additional heating channels. This study examines magnetic reconnection in the partially ionized solar chromosphere, analyzing its morphology and energy releases with attention to elastic collisions, ionization, and recombination. The MAGNUS code, originally built for ohmic resistivity and heat transfer, was modified to handle elastic and inelastic collisions. The simulations are 2.5D resistive MHD with two-fluid effects (charged + neutrals), adapted to handle interactions via these collision terms. A mixed explicit-implicit scheme was implemented to manage the stiffness of these terms. Simulations were carried out for three magnetic field strengths: 100~G, 110~G, and 120~G. These values correspond to the low- to mid-chromosphere under quiet-Sun conditions. We found that reconnection heats the plasma components by 18\% to 110\%. Although plasma beta rises only slightly, the energy release grows far more, indicating that charged--neutral collisions, not just beta or available magnetic energy, drive the enhanced reconnection. Furthermore, ionization and recombination become most significant in regions of peak temperature, particularly where particles are accelerated. Maximum reconnection rates from temporal analysis are 0.226, 0.253, and 0.279, respectively. Finally, in terms of energy, our results show that in a chromospheric volume of $0.4 \times 0.01 \times 0.4$~Mm$^{3}$, the energy released ranges from $10^{22}$ to $10^{23}$.