Why Is Non-thermal Line Broadening of Spectral Lines in The Lower Transition Region of the Sun Independent of Spatial Resolution?

TL;DR

This study investigates why non-thermal line broadening in the solar transition region remains unchanged despite higher spatial resolution, suggesting the broadening occurs along the line-of-sight or on very small spatial scales, influenced by shocks and magnetic structures.

Contribution

It demonstrates that increased spatial resolution does not reduce non-thermal broadening, indicating the process occurs on scales smaller than 250 km and involves complex magnetic and shock interactions.

Findings

Non-thermal broadening remains at ~20 km/s despite higher resolution.

Shocks and magnetic twist influence line broadening and its correlation with intensity.

Simulations support the role of shocks and non-equilibrium ionization in broadening.

Abstract

Spectral observations of the solar transition region (TR) and corona show broadening of spectral lines beyond what is expected from thermal and instrumental broadening. The remaining non-thermal broadening is significant (5-30 km/s) and correlated with the intensity. Here we study spectra of the TR Si IV 1403 Angstrom line obtained at high resolution with the Interface Region Imaging Spectrograph (IRIS). We find that the large improvement in spatial resolution (0.33 arcsec) of IRIS compared to previous spectrographs (2 arcsec) does not resolve the non-thermal line broadening which in most regions remains at pre-IRIS levels of about 20 km/s. This invariance to spatial resolution indicates that the processes behind the broadening occur along the line-of-sight (LOS) and/or on spatial scales (perpendicular to the LOS) smaller than 250 km. Both effects appear to play a role. Comparison with IRIS chromospheric observations shows that, in regions where the LOS is more parallel to the field, magneto-acoustic shocks driven from below impact the TR and can lead to significant non-thermal line broadening. This scenario is supported by MHD simulations. While these do not show enough non-thermal line broadening, they do reproduce the long-known puzzling correlation between non-thermal line broadening and intensity. This correlation is caused by the shocks, but only if non-equilibrium ionization is taken into account. In regions where the LOS is more perpendicular to the field, the prevalence of small-scale twist is likely to play a significant role in explaining the invariance and correlation with intensity.