The formation of IRIS diagnostics III. NUV Spectra and Images

The Mg II h&k lines are the prime chromospheric diagnostics of NASA’s Interface Region Imaging Spectrograph (IRIS). In the previous papers of this series we used a realistic 3D radiation-MHD model to calculate the h&k lines in detail and investigated how their spectral features relate to the underlying atmosphere. In this work we employ the same approach to investigate how the h&k diagnostics fare when taking into account the finite resolution of IRIS and different noise levels. In addition, we investigate the diagnostic potential of several other photospheric lines and near continuum regions present in the NUV window of IRIS, and study the formation of the NUV slit-jaw images. We find that the instrumental resolution of IRIS has a small effect on the quality of the h&k diagnostics; the relations between the spectral features and atmospheric properties are mostly unchanged. The peak separation is the most affected diagnostic, but mainly due to limitations of the simulation. The effects of noise start to be noticeable at a SNR of 20, but we show that with noise filtering one can obtain reliable diagnostics at least down to a SNR of 5. The many photospheric lines present in the NUV window provide velocity information for at least eight distinct photospheric heights, and using line-free regions in the h&k far wings we derive good estimates of photospheric temperature for at least three heights. Both of these diagnostics, in particular the latter, can be obtained even at SNRs as low as 5.

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Date added: Wed, 9 Oct 13