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

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.