Reversed sunspot groups
posted: December 03, 2014

NOAA 2223 was a small sunspot group that appeared on 27 November and rounded the west limb on 02 December. It would have been just another number in the long list of sunspot groups, if it wasn't for its reversed polarity. Indeed, during this solar cycle 24 (SC24), the leading spots of a bipolar sunspot group in the northern hemisphere should have a black (negative) magnetic polarity, meaning the magnetic field lines return to the solar interior. The trailing spots should have a white (positive) magnetic polarity, meaning the magnetic field lines come out of the solar interior. On the southern hemisphere, this is just the reverse. When a new solar cycle starts (e.g. SC25), the spots appear at a high latitude with a polarity opposite to those groups of the old cycle, located near the solar equator. All this is perfectly in accordance with our current understanding of the solar dynamo.


NOAA 2223 had a polarity opposite to that of the other groups of the northern solar hemisphere. It is of course not correct to conclude from the above that this reversed polarity group is the first sign of the new solar cycle. Indeed, apart from its moderate latitude, it is well known that during a solar cycle, some sunspot groups with reversed magnetic polarity will appear, often after solar cycle maximum has taken place. In fact, statistics show that on the average 3% of the sunspot regions will show this opposite polarity. So far, and based on a quick count, SC24 seems to be a bit below this average, with only a dozen having appeared since January 2011. But then again, SC24 is only halfway, and still has plenty of years to catch up.


These reversed polarity regions have similar characteristics than the correctly oriented ones, perfectly able to become large and produce strong flares. Even small NOAA 2223 is suspected to have contributed to the solar activity. Indeed, it appeared just south of a long filament that had been transiting the solar disk already for a week. Then, hardly a day after NOAA 2223 appearance, it erupted and probably contributed to a complex series of non Earth-directed coronal mass ejections (CMEs) visible during the afternoon of 28 November and early on 29 November.


Credits - Imagery taken from the SDO-website.