posted: October 31, 2013

No, Beta-Gamma-Delta (BGD) is not some high-school prom reunion, but one of the possible magnetic classifications of a sunspot group. These classifications play a major role in determining whether or not a sunspot group is up to significant flare activity.

Sunspots have their origin in magnetic flux tubes breaking through the solar surface. The magnetic disturbances create local cooling (compared to the surroundings), hence sunspots are visible as dark specks on the solar surface. Seen in a magnetogram, spots where the magnetic field (tube) comes out of the surface are often displayed as blue or white. Spots where the field returns into the Sun are usually depicted in red or black.

Solar flares on the other hand find their origin in the magnetic reconnection between magnetic fields of opposite polarity that are very close together. The latter specification is required, because otherwise every bipolar sunspot group would produce many and strong flares, which is not what we observe!

Thus, normal sunspot groups are bipolar and have magnetic polarities that can easily be distinguished (see sketch above right). However, some groups may become quite complex with many sunspots of opposite polarities. At some point, it may become impossible to distinguish it as a bipolar group. When that happens, it gets the "Gamma" classification. This kind of group is quite rare, and usually we get a Beta-Gamma (BG) classification: There's an indication of bipolarity, but no continuous line can be drawn separating spots of opposite magnetic polarities.

The specification "Delta" is given to sunspot groups that contain spots of opposite polarity within the same penumbra and within 2.5 degrees from each other. A few examples have been indicated by blue arrows on the magnetograms of last week's 3 important flaring regions: NOAA 1875 (top), NOAA 1877 (middle), and NOAA 1882 (bottom). NOAA 1875 and 1882 had a BGD classification for most of the time, with the Delta's indicating an increased likelihood on magnetic reconnection between the opposite polarity sunspots, and thus on (strong) solar flares. NOAA 1877 was mostly BG, with an occasional delta.

True to the predictions, these sunspot groups produced several strong flares. An overview of the main events is given in this movie. During the evening hours of 22 October, NOAA 1875 produced a M4 solar flare ("medium class"). The eruption showed some interesting features in H-alpha (chromosphere), such as the ejection of dark ("cold" and dense) material which are probably "surges"; the jury is still out on this one. There's also a shock wave visible precipitating through the Sun's lower atmosphere. This is called a "Moreton wave" and can be seen as a whitish line moving fast (500 km/s) to the bottom right of NOAA 1875.

On 24 October, a M9 flare took place in NOAA 1877. Aside the very nice eruption, one could also observe magnetic interactions with NOAA 1879 (to the east; "left" of the flaring region) and material movement across the solar equator towards the trailing part of NOAA 1875.

On 25 October, NOAA 1882 claimed all attention by unleashing 2 X-class solar flares in a single day. These flares belong in the eXtreme class of solar eruptions, and -so far- the ongoing solar cycle had produced only 19 of these strong flares (see this STCE Newsitem for an overview).

The X1.7 flare peaked at 08:01UT in the trailing part of NOAA 1882, where a strong delta was present. Interestingly, almost immediately following the flare, a filament erupted many 100.000's km further away on the southern hemisphere. This was similar to an earlier eruption in NOAA 1882 that day, when a medium flare around 03:00UT coincided with an ongoing filament eruption on the northern hemisphere. Since the "global" solar eruption event in August 2010, scientists are well aware that magnetic connections between far away regions can exist on the Sun, which can lead to a domino effect of flares and filament eruptions (see this STCE Newsitem).

The X2.1 flare peaked at 15:03UT in the same area as the X1.7 event. It produced an EIT wave and was a typical example of a solar flare: a reconnecting magnetic loop with material being ejected, the bright flare itself, and the post-flare coronal loops (see image underneath). Not always are all these features so well visible, courtesy SDO!

All this extra x-ray radiation energized the ionosphere, a layer in the Earth's atmosphere that influences radio propagation to distant places on the Earth. Meteor observers using radio equipment were not very happy with last week's activity. Indeed, the extra radiation disturbed the ionosphere at the frequencies observed, and so no reflected signal from meteors could be recorded. See this link starting at 03:20 for more info on meteor detection using radio-equipment. In the case of the X2 flare, the disturbed ionosphere did not permit meteor observations for more than 20 minutes. See the annotated radiospectrogram made by Belgian radio astronomer Felix Verbelen.

Credits - Data and imagery were taken from SDO, the GONG H-alpha Network, PROBA2/SWAP, and (J)Helioviewer.
More info on the magnetic classification of sunspot groups can be found at NOAA/SWPC and STCE.
More info on the Belgian RAdio Meteor Stations (BRAMS) can be found at their website