Non-Maxwellian Analysis of the Transition-region Line Profiles Observed by the Interface Region Imaging Spectrograph

0478682 - ASÚ 2018 RIV US eng J - Journal Article
Dudík, Jaroslav - Polito, V. - Dzifčáková, Elena - Del Zanna, G. - Testa, P.
Non-Maxwellian Analysis of the Transition-region Line Profiles Observed by the Interface Region Imaging Spectrograph.
Astrophysical Journal. Roč. 842, č. 1 (2017), 1/1-1/18. ISSN 0004-637X. E-ISSN 1538-4357
R&D Projects: GA ČR(CZ) GA16-18495S; GA ČR(CZ) GA17-16447S
Institutional support: RVO:67985815
Keywords : line profiles * data analysis * radiation mechanisms
OECD category: Astronomy (including astrophysics,space science)
Impact factor: 5.551, year: 2017
Method of publishing: Open access

We investigate the nature of the spectral line profiles for transition-region (TR) ions observed with the Interface Region Imaging Spectrograph (IRIS). In this context, we analyzed an active-region observation performed by IRIS in its 1400 Angstrom spectral window. The TR lines are found to exhibit significant wings in their spectral profiles, which can be well fitted with a 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 angstrom. Typical values of kappa found are about 2, occurring in a majority of spatial pixels where the TR 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 full-width at half-maximum irrespective of whether the line is an allowed or an intercombination transition. A similar value of kappa is obtained for the electron distribution by the fitting of the line intensities relative to Si IV 1402.8 angstrom, if photospheric abundances are assumed. These 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, TR 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.
Permanent Link: http://hdl.handle.net/11104/0274742