Modeling the Solar Transition Region: Effects of Spatial Resolution on the Atmospheric Structure, Emission and Non-equilibrium Ionization

Abstract The solar transition region (TR) is a narrow interface between the chromosphere and corona, where emitted radiation contains critical information pertinent to coronal heating processes. We conducted two-dimensional radiation magnetohydrodynamics simulations using adaptive mesh refinement to spatially resolve the fine structure of the TR while simultaneously capturing the larger-scale dynamics originating from surface convection. The time evolution of ionization fractions for oxygen ions is computed alongside the simulations. A minimum grid size of 1.25 km is achieved in the TR, enabling adequate resolution of the upper TR (log 10 T ≳ 5), although the lower TR (log 10 T ≲ 5) remains under-resolved. Doppler shifts and nonthermal widths synthesized from TR lines exhibit convergence with grid sizes as coarse as 40 km, though some discrepancies persist between our results and observed TR line properties. A notable enhancement in emission from O vi lines, converging at a grid size of 2.5 km, shows an intensity 1.2 times that expected under ionization equilibrium, attributable to shock interactions with the TR. While model refinements are still required, our ability to resolve the TR offers critical insights into TR line characteristics arising from non-equilibrium ionization states, advancing our understanding of the coronal heating problem.