NOAA "Boom! Boom!" 2192
posted: October 21, 2014

On 19 October, an X1.1 flare was observed. Starting at 04:17UT, this long duration event (LDE) ended only at 05:48UT. Remarkably, the peak x-ray flux stayed at the X1.1-level for a full 6 minutes, i.e. from 05:01UT till 05:06UT. This flare was the 36th solar flare of ongoing solar cycle 24, but due to its low-level strength, it is ranked at the bottom of the X-class event list (see the Top-10).


The source of this "eXtreme" eruption was NOAA 2192, a big and complex sunspot region still close to the southeast border. Its position on the solar disk is very similar to the sunspot complex NOAA 2172/2173, which rounded the west limb just 2 weeks ago with a bang (see here for the M7-flare on 2 October). NOAA 2192 is by far the largest group so far this solar cycle, with a surface area about 15 times that of the Earth! Using safe solar eclipse glasses, several observers from the Belgian Solar Section reported it already as a naked eye object on Sunday morning, 19 October. The image underneath is a photo of the colossus taken by Jeffrey Carels just a day earlier.


Most interestingly, no coronal mass ejection (CME) seems to have been associated to this X-class flare, differing quite significantly from another flaring event from the same active region just a few days earlier. Indeed, on 14 October, an M2.2-flare peaked at 21:21UT and was associated to an impressive (backsided) asymmetric halo CME. This solid CME was extremely bright (image underneath), as a large area of the LASCO C2 coronagraph onboard the SOHO spacecraft even got saturated! Interestingly, imagery from SDO's AIA 304 (imaging at cooler temperatures of about 80.000 degrees) did not show any large prominence eruption that could have accounted for most of the bright material seen by LASCO. Another curious thing is that the CME was not very fast (only about 920 km/s), probably due to its very high mass (very bright!).


This M2-flare was also an LDE, starting at 19:07UT and ending only at 00:19UT the next day. It was preceded by an M1.1 flare of normal duration, peaking at 18:37UT. Scientists suspect the entire event was probably a single flare, out of which we could see only the beginning and the end. Indeed, NOAA 2192 was still about 2 (two!) days behind the east limb, and part of the rising phase was most probably occulted by the solar limb. Moreover, because of this occultation, the true strength of the flare was probably somewhat higher, and may perhaps have been an X-class event. The x-ray flux never fell back to its pre-M-flare level (B4), instead leveling off nearly a full day later (!) at the C1-level. In AIA images, there was a large, long staying volume of hot material above the post-flare loops that -scientists suspect- may probably explain that duration.


The M2-event finished with an arcade, which is the technical term for a series of post-flare coronal loops. Interestingly, these post-flare loops continued to grow, first reaching the limit of AIA's Field-Of-View (FOV) on 15 October around 17:00UT, then continuing to grow even beyond AIA's FOV. Fortunately, PROBA2's wider-field SWAP telescope came to the rescue and was able to monitor this arcade in its full glory till its disappearance around noon on 17 October. So, the loops of this long duration arcade were visible for about 2.5 days (60 hours!), and at their maximum height, they were towering at least 340.000 km above the solar surface. That's not far from the average Earth-Moon distance!


Despite the few data, STEREO-A recorded also energetic protons related to this flaring event. This was no surprise, as the flare was just beyond the east limb (seen from Earth) and both STEREO spacecraft were magnetically connected to this region. Given the short time between the flare and the event in STEREO-A, it is currently assumed that the energetic particles are coming from the eruption site, and not from a later acceleration by the CME shock.


Movies of the 2 flaring events can be found here.

Credits - Data and imagery for the movie clips were taken from SDO, STEREO, SOHO/LASCO, and PROBA2.