High resolution He I 10830 \AA\ narrow-band imaging of an M-class flare. I - analysis of sunspot dynamics during flaring

TL;DR

This study presents high-resolution He I 10830 Å imaging of an M1.8 flare, revealing detailed sunspot dynamics and oscillations, and suggests flare-induced changes in helium atom density affecting chromospheric features.

Contribution

First detailed analysis of sunspot dynamics during a flare using high-resolution He I 10830 Å imaging, revealing new insights into chromospheric responses.

Findings

Penumbra and umbral flashes appear as absorption features.

Flare causes broadening of dark lanes in sunspot regions.

Oscillating absorption strips are observed during flare ribbon sweeping.

Abstract

In this paper, we report our first-step results of high resolution He\,\textsc{i} 10830 \AA\ narrow-band imaging (bandpass: 0.5 {\AA}) of an M1.8 class two-ribbon flare on July 5, 2012. The flare was observed with the 1.6 meter aperture New Solar Telescope at Big Bear Solar Observatory. For this unique data set, sunspot dynamics during flaring were analyzed for the first time. By directly imaging the upper chromosphere, running penumbral waves are clearly seen as an outward extension of umbral flashes, both take the form of absorption in the 10830 \AA\ narrow-band images. From a space-time image made of a slit cutting across a flare ribbon and the sunspot, we find that the dark lanes for umbral flashes and penumbral waves are obviously broadened after the flare. The most prominent feature is the sudden appearance of an oscillating absorption strip inside the ribbon when it sweeps into the sunspot's penumbral and umbral regions. During each oscillation, outwardly propagating umbral flashes and subsequent penumbral waves rush out into the inwardly sweeping ribbon, followed by a returning of the absorption strip with similar speed. We tentatively explain the phenomena as the result of a sudden increase in the density of ortho-Helium atoms in the area of the sunspot being excited by the flare's EUV illumination. This explanation is based on the observation that 10830 \AA\ absorption around the sunspot area gets enhanced during the flare. Nevertheless, questions are still open and we need further well-devised observations to investigate the behavior of sunspot dynamics during flares.