A new broadening technique of numerically unresolved solar transition region and its effect on the spectroscopic synthesis using coronal approximation

The transition region is a thin layer of the solar atmosphere that controls the energy loss from the solar corona. Large numbers of grid points are required to resolve this thin transition region fully in numerical modeling. In this study, we propose a new numerical treatment, called LTRAC, which can be easily extended to the multi-dimensional domains. We have tested the proposed method using a one-dimensional hydrodynamic model of a coronal loop in an active region. The LTRAC method enables modeling of the transition region with the numerical grid size of 50--100 km, which is about 1000 times larger than the physically required value. We used the velocity differential emission measure to evaluate the possible effects on the optically thin emission. Lower temperature emissions were better reproduced by the LTRAC method than by previous methods. Doppler shift and non-thermal width of the synthesized line emission agree with those from a high-resolution reference simulation within an error of several km/s above the formation temperature of $10^5$ K.
Comment: 17 pages, 10 figures, accepted for publication in ApJ