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NOAA yesterday posted its monthly update of the solar cycle, covering sunspot activity for May 2018. As I do every month, I have annotated the graph and posted it below.
The graph above has been modified to show the predictions of the solar science community. The green curves show the community’s two original predictions from April 2007, with half the scientists predicting a very strong maximum and half predicting a weak one. The red curve is their revised May 2009 prediction. The yellow line compares the present activity with the activity during solar minimum in 2008 and 2009.
In a sense, the increased activity in May was less a return to an active Sun and more a reflection of the Sun’s 27-day rotation period. I have noted this previously but it bears repeating. We count sunspots by the number seen daily on the visible hemisphere of the Sun. Since it is not unusual for sunspot activity to be unevenly distributed across the entire solar globe, it often happens that, during the time when sunspot activity is weak but still occurring, one hemisphere will be blank while the other has sunspots. As the Sun rotates we therefore go through a two week period with no sunspots followed by a two week stretch with increased activity.
This happened in May, as shown by the SILSO graph on the right. While a majority of the Sun’s surface had some sunspots in May, there was a week-long period mid-month when no sunspots were visible. It was at this time that the hemisphere that happened to be less active was facing us.
That quiet period ended on May 20th. Based on the Sun’s 27-day rotation period, you could then predict that a quiet period would next return around two weeks later, around June 1st. And lo and behold, if you look at the graph on the right you will see that on June 3rd the Sun returned to a blank state once again.
Obviously, this type of prediction is very superficial and explains nothing. Moreover, it is reliable only for short periods, as sunspot activity on the surface eventually shifts about so that no specific hemisphere of the Sun remains blank for long. Nonetheless, the fact that about half the Sun’s surface is now routinely blank is another indication that we are heading toward solar minimum, and it looks like we shall reach it, based on the first graph above, sometime late this year or early in 2019. This will make this solar cycle a short ten-year-long cycle. It will also be a weak cycle. This is unprecedented, as in the past short cycles were always more active, not less.
The Sun continues to baffle, and exhibit behavior that we have not seen in more than three hundred years, since the last Grand Minimum.