Negative flare in the He I 10830 \AA\ line in facula

TL;DR

This study reports on a rare negative flare observed in the He I 10830 Å line, revealing complex chromospheric and coronal dynamics and suggesting a photoionization-recombination process driven by UV radiation during a small-scale solar flare.

Contribution

The paper presents detailed spectral and imaging observations of a rare negative flare in the He I 10830 Å line, highlighting its morphology, dynamics, and underlying physical processes, which are not well documented in prior studies.

Findings

He I 10830 Å line showed a 25% increase in absorption during the flare.

Chromospheric and coronal lines reached peak intensities 5-6 minutes after the 171 Å line.

Observed perturbation propagation speed was approximately 70 km/s.

Abstract

A small-scale flare SOL2012-09-21T02:19 (B2) occurred in a spotless active region that we observed at a ground-based telescope equipped with a spectrograph. During the flare, we registered an increase in absorption in the He I 10830 \AA\ line by 25%, while other chromospheric and coronal spectral lines demonstrated increase in brightness at the same location. This phenomenon called negative flare had rarely been observed at the Sun before. In this paper, we describe the morphology of this flare and investigate its dynamics based on our spectral observations and space imaging data. The H$\alpha$ and He I 10830 \AA\ lines reach their extreme intensities 5 and 6 minutes after the 171 \AA\ line. The brightening first occurred in the 171 \AA\ and 193 \AA\ Solar Dynamics Observatory (SDO) channels followed by the 94 \AA, 304 \AA, and 1600 \AA\ signals $\sim$2 minutes after (for the maximum phases). However, the abrupt changes in line-of-sight (LOS) velocities in the chromospheric lines occur simultaneously with the intensity changes in the 304 \AA\ and 1600 \AA\ lines: we observed a downward motion that was followed by two upward motions. The measured horizontal speed of the perturbation propagation was close to 70 km s$^{-1}$ both in the chromospheric and coronal lines. We assume that we observed the photoionization-recombination process caused by UV radiation from the transition region during the coronal flare. With this, we point out the difficulties in interpreting the time lag between the emission maximum in the SDO UV channels and the second absorption maximum in the He I 10830 \AA\ line.