NOAA 2415 started out as a rather quiet and dull sunspot group, until during the afternoon of 14 September a bipolar magnetic flux emerged right in front (to the southwest) of NOAA 2415's main leading spot. During the next few hours and days, the newly bipolar region developed further, with the positive (white) polarity spots drifting away to the west. However, its negative (black) trailing spots remained very close to the original main spot (white), and started to interact with each other. This resulted in increasingly stronger flares on 16 and 17 September, culminating in a C9 and an M1 flare on 17 September (peaking resp. at 03:03UT and 09:40UT).
The images underneath show NOAA 2415 in white light and the corresponding magnetogram resp. on 14 September around 12:00UT, on 14 September around 23:30UT (just before a long image gap), on 16 September around 18:00UT and on 17 September around 02:00UT (just before the start of the C9 and M1 flares). White means positive polarity (magnetic field coming out of the solar surface), black means negative polarity (magnetic field returning into the solar surface).
The animation underneath shows a colorized version of the magnetogram overlaid on the white light image from 16 September (noon) till 17 September (noon). A small delta can be seen to the southwest of the main spot (lower right). The C9 and M1 flares take place during the second half of the movie.
The image underneath shows the M1 flare near its maximum as seen through SDO's AIA 1600 filter. The AIA 1600 filter pictures the Sun in the transition zone between the Sun's lower atmosphere and the corona (hot outer atmosphere) at temperatures of around 100.000 degrees. At the same time, it is also very sensitive for emission from the upper photosphere ("solar surface") near 5.000 degrees, which is the reason why one sees the sunspots very well too. A movie of the colorized magnetogram (30 minutes cadence) and AIA 1600 (5 minutes cadence) can be found here.
This kind of "collision" flares happen quite often and can be much stronger than the 17 September events. In fact, the strongest flare so far this solar cycle (SC24), i.e. the X6.9 flare from 09 August 2011, was also the result of two sunspot groups bumping into each other. In that particular case, the newly bipolar flux emerged behind the existing NOAA 1263 on 06 August, with the leading portion of the new flux colliding with the already existing opposite polarity trailing spots. One sometimes wonder how those sunspot groups get their driver's license!
Credits - Imagery for the movie clips were taken from SDO and (J)Helioviewer.
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