NOAA 2781 showing off

NOAA 2781 rotated into view on 2 November, quickly increasing in size to become the largest sunspot group so far this solar cycle. At its maximum on 6 November, its area was the equivalent of nearly 3 times the surface area of the Earth (NOAA). The changes in the sunspot group were quite dramatic. Images underneath shows the group in white light (SDO/HMI) on 7 November, with the insets (zoom) early on 6 (top) and late on 8 (bottom) November highlighting the significant decay of the region, in particular in the middle and trailing portion. The movie runs from 5 till 9 November.

Despite its size and magnetic complexity, the group produced only C-class ("common") flares. The largest were a C7 flare early on 5 November and a C5 event on 8 November. The images in extreme ultraviolet (EUV) underneath show both flares. For the C7 flare, views in two filters are compared. AIA 171 (left) shows the flaring event at "moderate" temperatures of about 700.000 degrees, and AIA 131 (right) provides a high temperature view at several million degrees. In AIA 171, peak brightness is a few minutes after AIA 131, and it provides a finer view of the (few) post-flare coronal loops. The C5 flare is highly zoomed and is shown in the green component of AIA 171. No obvious coronal mass ejections were associated with these flares.

The size and flaring activity of NOAA 2781 drove the 10.7cm solar radio flux to the highest levels so far this solar cycle. The wavelengths around 10.7 cm (frequencies around 2800 MHz) are ideal for monitoring the solar activity, as they are very sensitive to conditions in the upper chromosphere and at the base of the corona. See this STCE newsitem for more information. The daily radio flux reached 93.8 sfu (solar flux units, with 1 sfu = 10-22 W m-2 Hz-1) well above the typical solar cycle minimum values around 70 sfu. Of course, these values are still low when compared to solar cycle maximum values, when 200-250 sfu can easily be reached.




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