Pre-flare coronal dimmings
Qingmin Zhang Submitted: 2016-11-27 18:45
Coronal dimmings are regions of decreased extreme-ultravoilet (EUV) and/or X-ray (originally Skylab, then Yohkoh/SXT) intensities,
which are often associated with flares and coronal mass ejections (CMEs). The large-scale, impulsive dimmings have substantially been
observed and investigated. The pre-flare dimmings prior to the flare impulsive phase, however, have rarely been studied in detail.
In this paper, we focus on the pre-flare coronal dimmings. We report our multiwavelength observations of the GOES X1.6 solar flare and the
accompanying halo CME produced by the eruption of a sigmoidal magnetic flux rope (MFR) in NOAA active region (AR) 12158 on 2014 September 10.
The eruption was observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamic Observatory (SDO). The photospheric line-of-sight
magnetograms were observed by the Helioseismic and Magnetic Imager (HMI) aboard SDO. The soft X-ray (SXR) fluxes were recorded by the GOES
spacecraft. The halo CME was observed by the white light coronagraphs of the Large Angle Spectroscopic Coronagraph (LASCO) aboard SOHO.
About 96 minutes before the onset of flare/CME, narrow pre-flare coronal dimmings appeared at the two ends of the twisted MFR. They extended very
slowly with their intensities decreasing with time, while their apparent widths (8-9 Mm) nearly kept constant. During the impulsive and decay phases
of flare, typical fanlike twin dimmings appeared and expanded with much larger extent and lower intensities than the pre-flare dimmings.
The percentage of 171 Å intensity decrease reaches 40%. The pre-flare dimmings are most striking in 171, 193, and 211 Å with
formation temperatures of 0.6-2.5 MK. The northern part of the pre-flare dimmings could also be recognized in 131 and 335 Å.
To our knowledge, this is the first detailed study of pre-flare coronal dimmings, which can be explained by the density depletion as a result of the
gradual expansion of the coronal loop system surrounding the MFR during the slow rise of the MFR.
Authors: Q. M. Zhang, Y. N. Su, and H. S. Ji
Publication Status: to be accepted for publication by A&A
Last Modified: 2016-11-30 12:19