High $β$ magnetic reconnection at different altitudes in the cool low solar atmosphere

Abstract: We numerically studied magnetic reconnection in a high $\beta$ hydrogen-helium plasma at different altitudes from the photosphere to the upper chromosphere. The time dependent ionization degrees were included to get more realistic diffusivities and viscosity, and appropriate radiative cooling models were applied. Our numerical results indicate that the plasmoid instability always plays a vital role in speeding up magnetic reconnection at different atmospheric layers. In addition, both the strong radiative cooling and the magnetic diffusion caused by the electron-neutral collision ($\eta_{en}$) can significantly accelerate magnetic reconnection below the middle chromosphere. On the other hand, both the ambipolar diffusion and the viscosity result in higher temperature and plasma pressure in the reconnection region in the upper chromosphere, which then hinder the fast reconnection process from developing. The local compression heating triggered by turbulent reconnection mediated with plasmoids is the dominant heating mechanism in the unstable reconnection stage at different atmospheric layers, but the viscous heating and the ambipolar diffusion heating are equally important in the upper chromosphere. The Joule heating contributed by $\eta_{en}$ dominates during the early quasi-steady reconnection stage below the middle chromosphere, the strong radiative cooling also leads to much stronger compression heating and more generation of thermal energy in this region. Though the plasma $\beta$ is the same in all the simulation cases at different altitudes, the temperature increase is more significant in the upper chromosphere with much lower density and weaker radiative cooling.