Multi-passband Observations Of A Negative Flare Near He I 10830 Å

This study presents a C3.0 flare observed by the BBSO/GST and IRIS, on 2018-May-28 around 17:10 UT. The Near Infrared Imaging Spectropolarimeter (NIRIS) was set to spectral imaging mode to scan five spectral positions at ±0.8 Å, ±0.4 Å and line center of He I 10830 Å. Negative contrasts of around 10%, appear in all of the five wavelengths, with a weak dependence of these wavelengths. This means that the line is undergoing enhanced absorption at these times. The observations confirm that the negative flares observed in He I 10830 Å with mono-filtergram previously were not caused by pure Doppler shifts of this spectral line. Instead, the enhanced absorption is a consequence of nonthermal ionization of helium following precipitation of high energy electrons, as found in recent numerical modeling results. In addition, though not strictly simultaneously, the IRIS observations show clear central reversals in Mg II lines and strong Doppler shifts in C II and Mg II lines at the locations where enhanced absorption in He I 10830 Å is occurring, consistent with previous observations and the modeling. In other locations the Mg II profiles appear as single peaked.