Multi-Passband Observations of A Solar Flare over the He I 10830 \AA\ line

TL;DR

This paper investigates a solar flare using multi-passband observations of the He I 10830 Å line, revealing complex spectral behaviors and suggesting non-thermal ionization effects as the cause of observed absorption features.

Contribution

It provides detailed spectral analysis of a solar flare in He I 10830 Å, demonstrating that enhanced absorption is due to non-thermal ionization rather than Doppler shifts.

Findings

Enhanced absorption at flare ribbon edges indicates non-thermal ionization.

Spectral shapes vary between convex and concave in different flare regions.

Absorption features correlate with Mg II reversal patterns.

Abstract

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) of GST was set to spectral imaging mode to scan five spectral positions at $\pm$ 0.8 \AA, $\pm$ 0.4 \AA and line center of He I 10830. At the flare ribbon's leading edge the line is observed to undergo enhanced absorption, while the rest of the ribbon is observed to be in emission. When in emission, the contrast compared to the pre-flare ranges from about $30~\%$ to nearly $100~\%$ at different spectral positions. Two types of spectra, "convex" shape with higher intensity at line core and "concave" shape with higher emission in the line wings, are found at the trailing and peak flaring areas, respectively. On the ribbon front, negative contrasts, or enhanced absorption, of about $\sim 10\% - 20\%$ appear in all five wavelengths. This observation strongly suggests that the negative flares observed in He I 10830 with mono-filtergram previously were not caused by pure Doppler shifts of this spectral line. Instead, the enhanced absorption appears to be a consequence of flare energy injection, namely non-thermal collisional ionization of helium caused by the precipitation of high energy electrons, as found in our recent numerical modeling results. In addition, though not strictly simultaneous, observations of Mg II from the IRIS spacecraft, show an obvious central reversal pattern at the locations where enhanced absorption of He I 10830 is seen, which is in consistent with previous observations.