Synthetic IRIS spectra of the solar transition region: Effect of high-energy tails

¹ Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 Ondřejov, Czech Republic
e-mail: elena@asu.cas.cz
² Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: cvocks@aip.de

Received: 29 June 2016
Accepted: 29 March 2017

Abstract

Aims. The solar transition region satisfies the conditions for presence of non-Maxwellian electron energy distributions with high-energy tails at energies corresponding to the ionization potentials of many ions emitting in the extreme-ultraviolet and ultraviolet portions of the spectrum.

Methods. We calculate the synthetic Si iv, O iv, and S iv spectra in the far ultraviolet channel of the Interface Region Imaging Spectrograph (IRIS). Ionization, recombination, and excitation rates are obtained by integration of the cross-sections or their approximations over the model electron distributions considering particle propagation from the hotter corona.

Results. The ionization rates are significantly affected by the presence of high-energy tails. This leads to the peaks of the relative abundance of individual ions to be broadened with pronounced low-temperature shoulders. As a result, the contribution functions of individual lines observable by IRIS also exhibit low-temperature shoulders, or their peaks are shifted to temperatures an order of magnitude lower than for the Maxwellian distribution. The integrated emergent spectra can show enhancements of Si iv compared to O iv by more than a factor of two.

Conclusions. The high-energy particles can have significant impact on the emergent spectra and their presence needs to be considered even in situations without strong local acceleration.