Lovely curves
posted: January 22, 2015

During the morning hours of 13 January, a rather strong M-class flare took place in active region NOAA 2257, close to the northwest limb. This sunspot group seemed deceptively simple, but harboured some opposite magnetic polarity spots close to each other. That's a configuration that often results in a flare.

Closer examination of the GOES x-ray curve (image above) revealed that the event actually consisted of 2 M-class flares separated by only 34 minutes: An M5.6 flare peaking at 04:24UT, and an M4.9 flare peaking at 04:58UT. They both took place in the same region. Obviously, such closely time-separated M-class flares do not happen very often, but they are by no means exceptionnal. For example, on 22 December 2013, there were only 26 minutes between two M1 flares in NOAA 1928. And, though rare, this occasionally happens with X-class flares too, such as the X1-X6 flares on 18 August 1979 whose peaks were also separated by only 26 minutes.

The image above combines a white light image with an SDO/AIA1700 image of the Sun's lower atmosphere. Taken at 04:22UT, it shows the location of the flare ribbons relative to the sunspots. The flare seems to have been confined, i.e. no obvious coronal mass ejection seems to have been associated with this flare. A compact series of bright post-flare coronal loops ("arcade") quickly developed after the M-class event. This movie shows the loops in successively hotter temperatures as seen in SDO's filters AIA 304 (about 80.000 degrees), AIA 171 (about 700.000 degrees), and AIA 131 (several million degrees). The images underneath show the loops around 07:04UT in the three filters.

Starting after 12:00UT, some recurring jets (see images underneath) can be seen emanating from the leading portion of NOAA 2257. The jets consist of rather cold plasma (best seen in the "colder" AIA304 filter), recur about every hour, but at decreasing height and intensity. In previous, similar events, these features were thought to have their origin in a reconnection of the restructuring magnetic fields in the Sun's lower atmosphere. With NOAA 2257 so close to the solar limb, confirmation and further details require some more extensive investigation. None of these jets were associated with CMEs.

Credits - Data and imagery for the movie clips were taken from SDO, PROBA2, STAFF, and Helioviewer.