NOAA 1875's solar backside adventure
posted: November 20, 2013

On 12 November, a big but symmetric sunspot rounded the eastern solar limb. Careful examination of white light images one solar rotation ago revealed that this new active region NOAA 1899 was in fact at the same location as NOAA 1875. This was a complex sunspot group that produced 12 medium and 2 extreme flares during its transit late October. Apparently, the main spot from NOAA 1875 had survived the backside transit on the Sun during the first 2 weeks of November, and was now giving an "encore performance". This movie shows the flaring activity during its backside transit.

Actually, the sunspot group did more than just surviving. Imagery of the STEREO spacecraft revealed that good old NOAA 1875 continued to be very active, producing numerous strong flares, energetic particle events and coronal mass ejections (CMEs). The STEREO spacecraft were just in the right location to record all this activity. The image underneath shows the locations of the satellites, the Earth and the sunspot group on 5 November.

Activity started on 29 October while the region had just crossed the west limb. Despite the group not being visible from Earth, the flare produced by NOAA 1875 was still bright enough to be recorded by the GOES-15 satellite as an X2.3 flare (eXtreme class). STEREO-A had a really good view on the event (not far from its eastern solar limb). The associated CME was mostly directed away from both the spacecraft and the Earth.

Among the numerous eruptive events, the flares on 2, 4 and 7 November clearly stood out. See images above, the bottom right image is from STEREO-B. They were all accompanied by transient coronal holes, post-flare coronal loops, and also strong radio bursts, as measured by STEREO's WAVES instrument. Such a burst is indicative for a CME, and indeed, the STEREO coronagraphs always showed a full halo CME, meaning that the CME was directed straight to the satellites. The halo CMEs can be seen in images underneath (bottom right image from STEREO-B). Note the 4 November halo CME was also seen as a halo event by SOHO, but here it obviously concerned a backside event, just as the two other November events.

The radio burst associated to the 7 November event was by far the most intense of the entire series. Though the CME itself was not as impressive as the other ones, its speed was estimated to be between 1400 and 2000 km/s. The fast plasma cloud apparently arrived already around noon the next day (only 26 hours after the flare), with wind speeds suddenly increasing from 375 to over 900 km/s, and densities from 1 to 25 particles per cm3. The cloudís magnetic field was oriented mainly northward though. Therefore, if at Earth, geomagnetic conditions would probably have been limited to moderate storming. After the 7 November event, NOAA 1875 significantly quieted down.

The movie starts by showing the X2-flare as seen by SDO/AIA 131. Then follows the view from STEREO-A thru its EUVI 195 telescope and coronagraph COR2 from 29 October till 4 November. The same type of instruments then shows the activity from 5 till 11 November, but this time as seen by STEREO-B. The movie concludes by showing the view thru SOHO's coronagraphs over the same time period (i.e. from 29 October till 11 November). NOAA 1875ís events can clearly be seen as (backside) partial or full halo CMEs.

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