The last C-class flare

With only a few small inactive sunspot regions and the recent series of spotless days, it comes as no surprise that no C-class or stronger flares have been observed for quite a while. In fact, the last C-class flare dates back from 5 October 2017 (NOAA 2683, C1.0 - see SDO white light and extreme ultraviolet imagery underneath), that is 103 days ago. This is no record, with the time-frame from 3 April till 3 November 2008 (213 consecutive days!) being the longest period without C-class flares since systematic GOES x-ray measurements started in 1976. Nonetheless, this is a quite unusually long period, as only 3 other stretches with more than 100 days between two C-class flares have been recorded.

Interestingly, there was a single M-class flare (M1.1) on 20 October 2017, which was produced by NOAA 2685 while it was still just beyond the east limb. As the region rotated onto the solar disk, it became clear this was only a small region, and it did not produce any other C- or M-class flares. The record consecutive days with no M-class flares is 664 days (25 March 2008 - 19 January 2010), for X-class flares this is no less than 1523 days (14 December 2006 - 15 February 2011). The recent period from 5 May 2015 till 6 September 2017 (854 days) ranks third. Including the 4 X-class flares from September 2017, the current solar cycle (SC24) counts 49 X-class flares so far (see the news items e.g.  here and here).

The graph underneath is an update from the 29 March 2017 news item and shows the daily number of C-, M- and X-class flares (Common, Medium and eXtreme flares, resp. in orange, red and black colors) for the period 2015 till mid-January 2018. The burst in activity early September last year was due to active region NOAA 2673, which produced 4 X-class flares in as many days, including the 2 strongest so far this solar cycle. The strong activity was extensively covered in the STCE news items, e.g. here and here. As can be seen from the graph, solar flare activity stalled shortly afterwards...

UPDATE: The drought in C-class flares ended on 4 February when NOAA 2699 produced a C-class event.

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