One year with no X-class flares
posted: May 5, 2016

X-class flares are "eXtreme" x-ray solar events, 10 times more powerful than a "medium" M-class flare, and 100 times more powerful than the "common" C-class flares. Today (5 May), it is precisely one year ago since the most recent X-class flare was recorded. It concerned an impulsive X2.7 flare produced by NOAA 2339 (see this press item). Underneath imagery shows the flare in extreme ultraviolet light (left; SDO/AIA 131) near the time of its maximum intensity, and the source sunspot region in white light (right; SDO/HMI).

So far this solar cycle (SC24), there have been 45 X-class events recorded by the GOES spacecraft. That's quite a bit less than during previous solar cycles. For example, solar cycle 23 (1996-2008) counted a total of 126 extreme events. Of course, the current SC24 is not over yet. Indeed, averaging per year the total number of X-class flares over the last 3 cycles, the bar-diagram underneath shows that there is a second peak of "eXtreme" flares about 2-3 years after the year with maximum sunspot number (mind the large deviations from these averages...). A typical example are the Halloween flares (October 2003) that occurred about 3 years after SC23 sunspot maximum. So, though we have not observed that many X-class flares so far this solar cycle, "statistically" we may still have a good chance on X-class flare activity until late 2017. Note also that, even during the years of solar cycle minimum, an isolated X-class flare may occur (e.g. July 1996!).

A period of one year without X-class events is not exceptional, with the current one far from being the longest. Since the GOES observations started in 1976, the current period (May 2015-May 2016) would rank only on the 7th place. The longest period was from 14 December 2006 till 15 February 2011 (1524 days!), during the latest solar cycle minimum. Another very long period occurred during the declining phase of SC22, when no X-class flare was observed between 3 November 1992 and 9 July 1996. This can be seen in the graph underneath where on the curve with the smoothed evolution of the sunspot number (blue), months with 1 or 2 X-class flares are indicated by green dots, 3 to 9 extreme events by yellow dots, and 10 or more X-class flares by red dots. So far, no month with 10 or more X-class flares has been observed during SC24 (no red dots).