On 11 September, a -at first sight- relatively large and complex sunspot rounded the Sun's east limb. Labeled NOAA 4216 (SIDC 621), it seemed to be the return of a small bipolar group (NOAA 4203) that had started its development 2 weeks earlier just before rounding the west limb. Nonetheless, that region was already the source of 3 M1 flares on 28-29 August. When space weather forecasters analyzed its magnetic structure upon its return on 11-12 September near the east limb, there was a problem: the main sunspots in NOAA 4216 all had the same magnetic polarity. This is contrary to a typical bipolar region where the leading and trailing portions have opposite magnetic polarity.
So, how to solve this mystery? Enter Solar Orbiter (SO), and more precisely one of its instruments: the Polarimetric and Helioseismic Imager (PHI; see MPS: https://www.mps.mpg.de/solar-physics/solar-orbiter-phi). Its main goal is to study the magnetic field at the Sun's "surface" (the photosphere). It has a High Resolution Telescope (HRT) and a Full Disk Telescope (FDT) that make images in white light as well as magnetograms, thus revealing the magnetic structure of the active regions. As seen from Earth, SO was located on the opposite side of the Sun, quickly moving towards the Sun's east limb. In the annotated imagery above (image credits: MPS), the left image was taken in white light by SO/PHI on 11 September. The dashed lines of 180° and 270° denote the meridian directly opposite of the Earth (the Sun's farside), and the Sun's east limb as seen from Earth. The respective location of SO (green dot), the Sun (yellow), and the Earth (blue dot) can also be seen on the grid in the upper right corner. The sunspot region indicated by the white ellipses shows NOAA 4216 as seen by SO/PHI (left image) and by SDO/HMI (right), the latter image taken around the same time as SO/PHI's. The similarity of the main, leading portion of NOAA 4216 can easily be recognized in both images.
The imagery underneath (clips in white light and magnetogram) show the evolution of NOAA 4203/4216 in white light and magnetograms while it was on the Sun's farside on 8 September, rounding the east limb on 11 September, and while it was facing Earth on 15 September. It becomes clear that over those 1-2 weeks, NOAA 4203 developed into a lengthy group with a big leading sunspot, but was already decaying by the time it reached the Sun's east limb. By that time (11 September), only a fragmented leading sunspot remained with almost no sunspots left in its trailing portion. Without SO's images of the farside evolution of this active region, one could easily confuse the leading spots of NOAA 4216 with the entire active region NOAA 4203, making it much more complex than it actually was. As NOAA 4216 rotated better into view of SDO/HMI, the long region continued its gradual decay as shown in the 11 and 15 September images by SDO.
Sunspot observers usually do not look at magnetograms or satellite imagery when they count sunspots and make sunspot classifications (see the STCE's space weather classification page ). The main reason is that they have to keep their observations comparable with sunspot observations from 100 or 200 years ago when observers did not have magnetograms or satellites at their disposal. In contrast, today's space weather forecasters have to use all data available in order to make as accurate forecasts of the flare activity as possible. Without knowing the history of NOAA 4216 as outlined above, forecasters could incorrectly attribute a two to three times higher probability on an M-class flare from NOAA 4216 than with the correct classifications at hand. In the end, aside a few C-class flares, NOAA 4216 would produce only a single M-class flare: an M1.5 flare on 19 September. Underneath white light (left) and extreme ultraviolet imagery (right) were taken by SDO around the time of the flare's peak (still).
