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

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 Å, $\pm$ 0.4 Åand 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.

10 pages, 4 figures Accepted by ApJ Letters

References in corpus (4)

• The influence of coronal EUV irradiance on the emission in the He I 10830 A and D3 multiplets
• He I 10830 Å Dimming During Solar Flares, I. The Crucial Role of Non-Thermal Collisional Ionisations
• A Flare Observed in Coronal, Transition Region and Helium I 10830 Å Emissions
• Comparison of Enhanced Absorption in He I 10830 Å in Observations and Modeling During the Early Phase of a Solar Flare

Cited by in corpus (4)

• Solar Flare Ribbon Fronts I: Constraining flare energy deposition with IRIS spectroscopy
• On the center-to-limb variations of the He I 10 830 Å triplet
• Small-Scale and Transient EUV Kernels in Solar Flare Ribbons
• Machine Learning-based Separation of the He I 10830Å Chromospheric Signal: Quantitative Analysis of Chromosphere-Corona Intensity in the Quiet Sun