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The solar transition region satisfies the conditions for presence ofnon-Maxwellian electron energy distributions with high-energy tails at energiescorresponding to the ionization potentials of many ions emitting in the EUV andUV portions of the spectrum. We calculate the synthetic Si IV, O IV, and S IVspectra in the far ultra-violet (FUV) channel of the Interface Region ImagingSpectrograph (IRIS). Ionization, recombination, and excitation rates areobtained by integration of the cross-sections or their approximations over themodel electron distributions considering particle propagation from the hottercorona. The ionization rates are significantly affected by the presence ofhigh-energy tails. This leads to the peaks of the relative abundance ofindividual ions to be broadened with pronounced low-temperature shoulders. As aresult, the contribution functions of individual lines observable by IRIS alsoexhibit low-temperature shoulders, or their peaks are shifted to temperaturesan order of magnitude lower than for the Maxwellian distribution. Theintegrated emergent spectra can show enhancements of Si IV compared toO IV bymore than a factor of two. The high-energy particles can have significantimpact on the emergent spectra and their presence needs to be considered evenin situations without strong local acceleration.

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