Negative flare in the He i 10830 Å line in facula

Abstract

A small-scale flare SOL2012-09-21T02:19 (B2) occurred in a spotless active region that we observed the Horizontal Solar Telescope which is equipped with a spectrograph. During the flare, we registered an increase in absorption in the He i 10830 Å line by 25%, while other chromospheric and coronal spectral lines demonstrated increases in brightness at the same location. This phenomenon, called a negative flare, has rarely been observed on 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 brightening first occurred in the 171 Å Solar Dynamics Observatory (SDO) channel followed by the 94 Å and 304 Å signals ∼2 min after (for the maximum phases). The Hα and He i 10830 Å lines reach their extreme intensities 5 and 6 min after the 171 Å line. However, the abrupt changes in line-of-sight (LOS) velocities in the chromospheric lines occur simultaneously with the intensity changes in the 304 Å and 1600 Å lines: we observed a downward motion that was followed by two upward motions. The measured apparent horizontal speed of the perturbation propagation was close to 70 km s−1 both in the chromospheric and coronal lines.We suggest that we observed the photoionization-recombination process caused by UV radiation from the transition region and corona 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 Å line.