Analyzing deviations from optically thin emission in flare ribbon plasma using IRIS observations of Si IV resonance lines
Abstract
It is well-established that the Si IV resonance line pair at 1394 and 1403 Å observed by the Interface Region Imaging Spectrograph (IRIS) exhibits a 2:1 intensity ratio in optically thin plasma. Deviations from this ratio may arise from optical thickness (Mathioudakis et al. 1999) and/or geometric effects (Kerr et al. 2005) in the emitting plasma. These effects are expected to be particularly prevalent in the dense chromospheric plasma that forms the ribbons during a flare, and the Si IV resonance line ratio therefore provides a diagnostic of the plasma conditions at the flare loop footpoints. Recently, Kerr et al. (2019) used RADYN simulations to demonstrate that a significant fraction of the Si IV flare ribbon emission may form at cooler temperatures, even for relatively small flares. Their results showed changes to both line shape and intensity in flare plasma, and they recommended caution when interpreting Si IV emission in flare ribbons. Additionally, they noted that only a handful of studies using IRIS observations have reported the line ratio in flares. In this work, we report on our work to create a catalog of the line ratio for additional flare ribbon events in the IRIS observation database. We select IRIS observations of flares that record both Si IV resonance lines with an 8-step or fewer raster, and identify 28 candidate observations for which the flare ribbon is covered by the spectrograph slit. We establish criteria for identifying spectra that capture flare ribbon emission and for eliminating spectra that contain defects (e.g. saturated pixels). We calculate line intensities (less background contribution) and construct the line ratio for all selected spectra in each observation, and report the mean, median, and deviation of the line ratio for each flare ribbon. Based on our results, we briefly discuss physical implications for interpreting Si IV emission in flare ribbons.
- Publication:
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American Astronomical Society Meeting Abstracts #234
- Pub Date:
- June 2019
- Bibcode:
- 2019AAS...23420406B