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Frequent Spotless Days Signal Start of Lengthy, Deep Solar Minimum Phase:

from The ARRL Letter on April 12, 2018
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More-Frequent Spotless Days Signal Start of Lengthy, Deep Solar Minimum Phase:

The sunspot numbers have not been good, and it does not seem they'll be getting much better anytime soon. According to the April 12 "K7RA Solar Update," no sunspots were seen over the April 5 - 11 reporting week. Solar flux has been sitting in the 60s lately and is forecast to remain at that level for the next 45 days. We're not at solar minimum yet, though; better days lie ahead -- eventually.

Top contester Frank Donovan, W3LPL, asserts that solar precursors like spotless days, which correlate physical solar phenomena with future solar activity levels, are much better indicators of progress toward a solar minimum -- a broad phase of the solar cycle -- than of a specific event, such as the bottom of a broad solar minimum. Not only that, these and other solar precursors are useful in predicting the likely strength of Solar Cycle 25, although accurate predictions of the strength and timeframe of the Cycle 25 solar maximum aren't possible until the new sunspot cycle is under way.

"We entered the Cycle 24 solar minimum with the onset of 8 spotless days in June 2016," Donovan pointed out. "We had only 32 spotless days in 2016. We had 15 spotless days in a row in March 2017, followed by relatively infrequent spotless days for the next 7 months."

The frequency of spotless days accelerated in early November 2017, with 13 spotless days in a row. That was followed by many more spotless days over the next 5 months. In all, 2017 experienced 104 spotless days.

"The frequency of spotless days accelerated again last month, when we had 25 spotless days," Donovan noted. "We've already had 54 spotless days during the first quarter of 2018. This data and comparisons to frequent spotless days during the last solar minimum suggests that we may have just begun a period of very frequent spotless days for approximately the next 2 years, similar to the period of September 2007 through January 2009."

Donovan said the next solar precursor is frequent extended periods of spotless days. For example, 2016 saw 14 spotless days in a row, March 2017 had 15, November 2017 saw 13, and early March experienced 14.

This chart shows the number of spotless days -- in red -- since the 10th spotless day of the onset of solar minimum in July 2016. In blue are the average numbers of spotless days during sunspot minimums following weak sunspot cycles similar to Solar Cycle 24.

"Long periods of spotless days will become even more frequent as we go deeper into solar minimum," Donovan predicted. Once extended periods of spotless days become more frequent, another solar precursor will become important, he said.

"New high-latitude, opposite-polarity Cycle 25 sunspots will then begin to appear more frequently, perhaps by late next year," Donovan said. "More frequent Cycle 25 sunspots will signal that we're approaching the bottom of the [Cycle 24] solar minimum phase."

An ongoing solar precursor that could herald the future strength of Cycle 25 is the intensity of the solar polar magnetic fields prior to the Cycle 24 solar minimum, Donovan said. "The good news is that the solar polar magnetic field strength is already slightly stronger than it was prior to the last solar minimum," he said. "This suggests that Cycle 25 will be somewhat stronger than Cycle 24."

Donovan said if the long periods of spotless days end in about 1 year, that will be a precursor of a stronger Cycle 25. If they take more than 2 years to end, however, it will portend a weaker Cycle 25. Read more


The ARRL Letter

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