Recipe for a sunspot group
posted: June 12, 2013

Non-existing on 4 June, a sunspot group rapidly developed during the subsequent days. During the growing process of this group, the main spot offered a substantially different outlook even over the course of just a few hours. This was noticed by several solar observers, who gladly imaged the photogenic sunspot group NOAA 1765. Pictures underneath were taken by Geert Verbanck, member of the Belgian Solar Section, on 7 June with a 13 cm refractor and a Herschel wedge. They show indeed that major changes were taking place, in particular in the leading spot.


SDO/AIA 1700 observes the Sun just above the photosphere (the visible solar "surface"), at a height where the temperature reaches its minimum before quickly rising to million degrees in the corona. Being in the upper photosphere, it provides still a good view on the sunspots, but e.g. also on the interaction of the emerging plasma (charged particles) with that present in the Sunís lower atmosphere.


The AIA 1700 image above was taken at noon on 7 June. One has a clear view on the sunspots and on the various magnetic elements. During the early phase of the sunspot group development, somewhat brighter dots near the middle of the sunspot group can be seen. These are possibly Ellerman bombs (see sketch underneath for one of the possible mechanisms). One of the explanations is that as the magnetic flux tube rises through the solar surface, its magnetic elements collide with the pre-existent small scale magnetic field elements. As such, short-lived magnetic reconnections ("short circuits") occur between small emerging flux tubes of opposite polarity, resulting in the observed small flashes ("microflares"). Further research at higher resolution is required to fully understand this phenomenon.


The other (less) bright dots are concentrations of magnetic flux, floating with the bubbly dynamics of the Sun's upper atmosphere. The image underneath shows NOAA 1765 as seen in the light of the AIA 1700 filter, overlaid by a magnetogram: Purple is positive polarity, with magnetic field lines coming out, while yellow is negative polarity, with magnetic field lines returning into the Sun. The bright points clearly correspond to magnetic flux elements. As the main flux tube rises through the photosphere, the main spots are gradually getting composed, which explains the different outlook of the spots that was observed. This movie shows the evolution of the sunspot group from 4 June (noon) till 9 June (noon), first in white light, then in AIA 1700, finally a combination of AIA 1700 with a colorized magnetogram.