The Sun on (extended) summer leave

All solar observers and space weather monitors have noticed it: Over the last few weeks, solar activity has dropped again to very low levels.
Indeed, as can be seen in the first part of this movie (SDO), sunspots have been really scarce for about 2 weeks starting 5 September. Between 8 and 12 September, a few tiny sunspot groups performed a perfect relay to avoid an otherwise spotless solar disk. It would have been the first spotless day since 14 August 2011! Images from 10 and 16 September underneath were made using the white light telescope of the Uccle Solar Equatorial Table (USET). In both cases, they show only one tiny sunspot group on the solar surface.

The graph below shows the evolution of the smoothed monthly sunspot number (blue) since 1976, and the lowest daily sunspot number recorded for these months (red; unsmoothed). Low daily sunspot numbers, similar to the ones recorded over the last few weeks, are very uncommon during the maximum years of solar cycles (SC) with a moderate to high amplitude such as SC21 to 23. In this respect, SC24 looks very similar to other low activity solar cycles that occurred e.g. around the 1900's.

The low solar activity was also noticeable in other solar indices, such as the x-ray flux measured by the GOES satellite which displayed a pronounced low between 7 and 17 September (see part 2 of the movie with images from GOES' Solar X-ray Imager). In fact, for this period of 11 consecutive days, not a single solar flare was recorded, not even in the low C-class! Since measurements began in the mid-1970's, there have been no such long periods without C-flares during the maximum years of the solar cycles. A similar story can be told for the medium flares ("M-class"), with the most recent M-flare dating back already from 17 August.

Hence, it will come as no surprise that also geomagnetically, we have experienced some very quiet weeks. Indeed, the last (minor) geomagnetic storm dates back from 27 August. However, the current stormless period of about a month is not so extraordinary compared to the previous three solar cycles. These cycles all had 2 or 3 periods with up to 6 weeks of geomagnetic calm during their years of maximum solar activity. SC24 does distinguish itself from the three others by having several 2-month periods with no geomagnetic storming. The longest stretch occurred from 25 November 2012 till 28 February 2013, or 96 days without strong geomagnetic disturbances.

It will be interesting to see if and when the Sun is going to wake up from this prolonged nap!

Credits - Data and imagery were taken from SDO/HMI, GOES/SXI, NOAA/SWPC, WDC for Geomagnetism/Kyoto, SIDC, and USET.



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