A 3D Nonequilibrium Ionization Model of a Shock Wave in the Low Corona. I. Extreme-ultraviolet Emission and Inefficient Electron Heating

Shocks in the low corona are best observed in the extreme ultraviolet (EUV); however, time-dependent ionization in the postshock region is generally incompatible with the otherwise straightforward plasma diagnostics typically applied to EUV imaging data. As a result, these rich data are largely underutilized. In this work, we present our approach to modeling the evolving EUV emission from coronal shocks, including nonequilibrium ionization (NEI) effects in the shocked plasma. Our framework combines (1) a 3D reconstruction of the shock geometry and kinematics derived from Solar Dynamics Observatory Atmospheric Imaging Assembly and STEREO-A/EUVI imaging, with (2) a simulated MHD snapshot of the pre-eruption corona built from magnetogram data, to solve Rankine-Hugoniot jump conditions. We then (3) track the density, temperature, and ionization history of the expanding 3D downstream region from which we generate synthetic postshock EUV light curves. We model the 2010 June 13 coronal mass ejection and shock wave using this framework, where we find that NEI is necessary to replicate the observed time-dependent EUV signal. This is especially true for the observed AIA 193 Å and 211 Å shock emission given that the predicted postshock temperatures would result in little Fe XII and Fe XIV. Moreover, the EUV model is highly sensitive to the postshock electron-to-ion temperature ratio (T_e/T_p). We find that the data are well described by the model and broadly prefer lower T_e/T_p < 1, suggesting inefficient electron heating throughout most of the shock structure.