It's always impressive to see sunspots develop from scratch, and cluster into an ominously looking sunspot group. Early on Friday 19 April, magnetic fields pushed through the solar surface and did just that in less than 24 hours. NOAA 1726 was born, and it was a quite large group: In the image underneath, there is room to easily fit 10 Earths between the outer edges of the two main spots!
This movie covers the region's evolution from 19 April at midnight till 20 April at noon. The SDO/HMI magnetograms clearly show the development of at least 2 magnetic bipolar regions very close to each other. The trailing part of the leading region (right) is bumping into the leading part of the trailing region (left). Because these parts have opposite magnetic polarity, space weather forecasters kept a close eye on this region, as major flares may be produced in such configurations. For example, this was the case with NOAA 1263 on 9 August 2011 producing the strongest X-class solar flare so far this solar cycle.
In white light, it is much harder to distinguish the two regions. Indeed, it is common practice to consider sunspot groups as one group if they are less than 10 degrees in longitude apart from each other. This was certainly the case here, hence NOAA 1726 is visually considered as one group. Note that similar guidelines exist for groups differing less than 5 degrees in latitude. These guidelines are applied with the necessary flexibility though, taking into account the evolution of the region and some other white light properties of sunspot groups, but without the use of magnetograms. That way, the number of sunspot groups remains comparable to those from the beginning of sunspot observations. Indeed, prior to the 20th century, no magnetograms existed and observers had only visual means to distinguish sunspot groups.
The clip based on SDO/AIA 171 images show a very dynamic region, with continuous magnetic restructuring and the gradual development of bundles of coronal loops between the principal spots of the group. Very quickly, small C-class flares are being produced, mainly near the "collision area" between regions of opposite magnetic polarity. As can be seen in the SDO images underneath (22 April, 07:45UT), a small opposite polarity spot is contained within the penumbra of the main sunspot. The red and blue areas are nearly touching each other, providing the ideal situation for at least medium class solar flares.
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