NOAA 2146 bursts its bubble...
posted: September 3, 2014

This movie shows the development of a magnetic delta near the initially simple sunspot group NOAA 2146, starting on 23 August. One can indeed see how a white patch (positive polarity, field lines coming out of the solar surface) gradually shaped itself south of the main spot (black, negative polarity). Then, late on 24 August, the penumbra of both spots merged and gave NOAA 2146 a delta configuration.

Magnetic delta's are prone to strong flares, so it was no surprize when on 25 August, this sunspot group produced 2 M-class solar flares along the magnetic inversion line, i.e the line separating the areas of opposite polarity. This is shown in the zoom, where extreme ultraviolet (EUV) images from SDO (AIA 171) are overlaid on the magnetogram.

The first M-class flare was an M2.0 peaking at 15:11UT, and the second was an M3.9 peaking at 20:21UT, meaning there were hardly 5 hours between the two events. The movie shows the phenomena in EUV imagery of successively higher temperatures (AIA 304, 171 and 193 filters, or resp. about 80.000 degrees, 700.000 degrees and 1.3 million degrees). Both flares were associated to coronal dimmings and showed post-flare coronal loops. The image underneath compares the region near NOAA 2146 about one hour before and 1 hour after the peak of the M2.0 flare. The coronal dimming is clearly visible to the west (right) and south of the flaring area, and indicative of a coronal mass ejection.

The EUV combination movie at the end shows how the canopy of coronal loops -still expanding after the first M-flare- apparently got torn apart by the expanding flux rope (twisted bundle of magnetic fields) associated to the second event. It seems that this flux rope was at least only partially ejected during the first reconnection event (see EUV-imagery underneath). An alternative explanation might be that the erupting flux rope actually pushed aside one part of the overarching loops and took with it the remaining part of the coronal loops. This will require some further study. All this activity happened before the M3.9 flare reached its peak (2 minutes after the last image). Then, new post-flare coronal loops got formed as the reconnection moved further upwards in the corona.

Credits - Data and imagery for the movie were taken from the SDO and (J)Helioviewer.