# Non-Maxwellian analysis of the transition-region line profiles observed by the Interface Region Imaging Spectrograph

## TL;DR

This study analyzes IRIS transition-region line profiles, finding they are better fitted by non-Maxwellian kappa-distributions than Gaussian models, revealing insights into plasma conditions and non-thermal broadening in the solar atmosphere.

## Contribution

It demonstrates that transition-region line profiles are well modeled by kappa-distributions, providing a new perspective on plasma diagnostics and non-thermal processes in the solar transition region.

## Key findings

- Kappa-distributions fit line wings better than Gaussians.
- Typical kappa values are around 2 for most pixels.
- Non-thermal broadening increases with kappa-distributions.

## Abstract

We investigate the nature of the spectral line profiles for transition region ions observed with the Interface Region Imaging Spectrograph (IRIS). In this context, we have analyzed an active-region observation performed by IRIS in its 1400 A spectral window. The transition-region lines are found to exhibit significant wings in their spectral profiles, which can be well-fitted with non-Maxwellian kappa-distribution. The fit with a kappa-distribution can perform better than a double Gaussian fit, especially for the strongest line, Si IV 1402.8 A. Typical values of $\kappa$ found are about 2, occurring in a majority of spatial pixels where the transition region lines are symmetric, i.e., the fit can be performed. Furthermore, all five spectral lines studied (from Si IV, O IV and S IV) appear to have the same FWHM irrespective of whether the line is an allowed or an intercombination transition. A similar value of kappa is obtained for the electron distribution by fitting of the line intensities relative to Si IV 1402.8 A, if photospheric abundances are assumed. The kappa-distributions however do not remove the presence of non-thermal broadening. Instead, they actually increase the non-thermal width. This is because for kappa-distributions the transition-region ions are formed at lower temperatures. The large observed non-thermal width lowers the opacity of the Si IV line sufficiently enough for this line to become optically thin.