Nonequilibrium Ionization Modeling of Petschek-type Shocks in Reconnecting Current Sheets in Solar Eruptions

Nonequilibrium ionization (NEI) is essentially required for astrophysical plasma diagnostics once the plasma status departs from the assumption of ionization equilibrium. In this work, we perform fast NEI calculations combined with magnetohydrodynamic (MHD) simulations and analyze the ionization properties of a Petschek-type magnetic reconnection current sheet during solar eruptions. Our simulation reveals Petschek-type slow-mode shocks in the classical Spitzer thermal conduction models and conduction flux-limitation situations. The results show that under-ionized features can be commonly found in shocked reconnection outflows and thermal halo regions outside the shocks. The departure from equilibrium ionization strongly depends on plasma density. In addition, this departure is sensitive to the observable target temperature: the high-temperature iron ions are strongly affected by the effects of NEI. The under-ionization also affects the synthetic SDO/AIA intensities, which indicates that the reconstructed hot reconnection current sheet structure may be significantly underestimated either for temperature or apparent width. We also perform an MHD-NEI analysis on the reconnection current sheet in the classical solar flare geometry. Finally, we show the potential reversal between the under-ionized and over-ionized states at the lower tip of reconnection current sheets where the downward outflow collides with closed magnetic loops, which can strongly affect multiple SDO/AIA band ratios along the reconnection current sheet.

Automated summary

Nonequilibrium ionization (NEI) is crucial for astrophysical plasma diagnostics. The study combines fast NEI calculations with magnetohydrodynamic (MHD) simulations to analyze the ionization properties of a Petschek-type magnetic reconnection current sheet during solar eruptions. The results reveal under-ionized features in shocked reconnection outflows and thermal halo regions outside the shocks and demonstrate the strong dependence of departure from equilibrium ionization on plasma density. The study also shows the potential reversal between under-ionized and over-ionized states at the lower tip of reconnection current sheets, which can significantly affect multiple SDO/AIA band ratios along the reconnection current sheet.