NOAA 2321: one, two or three groups?
posted: April 15, 2015

At the end of a rather dull week, a cluster of sunspots appeared from behind the northeast limb. The active region was numbered NOAA 2321, had a sunspot area of about 4 times the surface area of the Earth, and produced an M1 flare on 12 April.

Solar observers had a hard time deciding if this concerned one sunspot group, or two or three closely packed sunspot regions. NOAA, ASSA and Mount Wilson classified this as one group, USET as two groups, and Catania, Kanzelhöhe and STAR as three groups. This becomes clear in a comparison of drawings made on 14 April by Kanzelhöhe, Catania, USET and Mount Wilson (see Note 1).

A senior observer from SILSO commented that he was also in favor of splitting in three groups: "... I would separate the spot in the northeast, with a doubt about separating the double big spot in the southeast. ... This big clustering is an example of the few complex groupings appearing during each solar cycle. ... Such tricky cases are always associated with a tight clustering of groups sitting next to each other, i.e. when the separation between spots of different groups is equal or less than the dimension of each group. ..." A comparison of an SDO "white light" image with a magnetogram (14 April, 05:12UT) reveals the magnetically very complex situation (see Note 2).

SILSO also comments that "... As splitting can make a large difference in the sunspot number, the choice of splitting can considerably raise the level of the sunspot index. This is reflected by the steep rise of the estimated international sunspot number (EISN) over the past 2 days, as can be seen in the graph underneath. However, as this involves only a few cases per cycle, which have an influence over less than 13 days, the impact is limited with respect to the overall envelope of the solar cycle."

Note 1 - Never mind the annotations on the drawings. These concern sunspot group classifications, number of sunspots, specific group numbers, location of the sunspot group, and specific magnetic information on the sunspots.

Note 2 - Solar observers are not allowed to use magnetograms when they are splitting or counting sunspot groups. That way, their obtained sunspot numbers remain comparable to those from let's say prior to 1900, when no such magnetic observations were available. Of course, magnetograms can be used when it comes to magnetic classifications or solar flare predictions.

Credits - Data and imagery were taken from NOAA, ASSA, USET, Catania, Kanzelhöhe, STAR, Mount Wilson, Debrecen, SDO and SILSO.