  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.
