Sunspots have their origin in magnetic flux tubes rising from the solar interior and breaking through the solar surface. These magnetic disturbances create a local cooling compared to their surroundings. Due to the temperature difference, sunspots are visible in white light as dark specks on the solar surface. Seen in a magnetogram, i.e. a map of the solar disk showing the strength and direction of the Sun's magnetic field across its visible surface, sunspots look slightly different. Spots where the magnetic field (tube) comes out of the surface are often displayed as red or white. Spots where the magnetic field returns into the Sun are usually depicted in blue or black. Thus, typical sunspot groups are bipolar and have magnetic polarities that can easily be distinguished, i.e. their leading and trailing portion have opposite magnetic polarities (see sketch below). Unipolar regions are usually the result of the magnetic field being too weak to form sunspots. This often happens in the trailing portion of the active region, usually when the sunspot group is decaying. In white light, observers then see a single sunspot while the corresponding magnetogram shows an unipolar magnetic polarity.
NOAA 4334 (SIDC sunspot group 722) was the return of the decaying active region NOAA 4307. It consisted of a rather big sunspot with some small spots surrounding it, all of the same negative magnetic polarity (blue or black in the magnetogram). The region became visible on 2 January, and showed the typical behaviour of a shrinking, decaying sunspot during the next few days. On 6 January, the spot started to fall apart, again a typical step in the evolution of a decaying spot. The next phase would have been that these smaller spots would decay and dissolve further locally, but then something unexpected happened: one of the spot fragments started to separate from the main portion of the sunspot, as a naughty kitten that is straying too far from the mother cat. This can be seen in the SDO/HMI imagery underneath, with on the left the white light image and on the right the magnetogram. By 8 January, the separation between the two was about 3 degrees and growing. From then onwards, most observers considered it as a bipolar sunspot region (McIntosh 1990), with -in this case- both the leading and the trailing portion having the same magnetic polarity. This kind of configuration is rare, and has to do with the movement of the magnetic fields underlying the sunspot group.
Despite its all-in-all simple magnetic structure, NOAA 4334 produced more than a dozen of small C-class flares. These were in part triggered by interaction with some nearby, weak, opposite polarity magnetic fields, as well as with nearby filaments. Solar filaments are clouds of ionized gas above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the plasma underneath and their surroundings, they appear as dark lines when seen on the solar disk using special filters such as hydrogen-alpha in the red portion of the solar spectrum. A particularly long and curvy filament (about 1.5 times the Earth-Moon distance!) was just south of NOAA 4334, with small extensions to the east and north of this active region. Such long filaments are known to become unstable and erupt, and indeed: the filament erupted on 8 January with an associated long-duration C5 flare. The filament and the eruption can be seen in the H-alpha imagery (GONG) underneath. An earth-directed coronal mass ejection was associated with this event, and would eventually be the source of a moderate geomagnetic storm late on 10 January.
