New data on sunspot decline

The present ultradeep solar minimum has seen few sunspots appearing on the solar surface. These areas of high magnetic field density emit slightly less light than surrounding areas and so appear black. They indicate magnetic disturbances, which undergo an eleven year cycle of activity. We’re at the minimum of the cycle, but even for a minimum it is quiet up there.

Asteroseismologists earlier this year measured subsurface flows on the Sun. These flows travel from the poles to the equator over the course of an activity cycle, normally setting off sunspots when they hit a certain latitude. This latitude is just below where they are, suggesting we’ll get more spots within a year or so.

More data has however come in from NASA researchers who have been measuring the strength of the magnetic field of sunspots. They believe the average field strength has declined by 50 gauss a year. If this is so, the average field strength should hit 1500 gauss in 2015 – this is the threshold beneath which spots no longer form. Their measurements use the Zeemen effect – strong magnetic fields split spectral lines and the wider the split, the stronger the magnetic field. There’s no doubt about the readings, but the conclusions remain controversial.

The data, taken by Matt Penn of the National Solar Observatory (NSO) in Tucson, Arizona, only goes back as far as 2002, when the previous sunspot maximum had already occurred and activity was in decline. Historical data of this type only goes back as far as seventeen years. Even Penn himself bets that sunspots will be back. Having said that, it wouldn’t be unprecedented for an ultradeep minimum as the Maunder minimum of the seventeenth century proved, lasting from 1645-1715, killing a nascent auroral science field that had sprung up not long before. Spot records of the time brought the numbers down from thousands to just dozens recorded per year. The aurora retreated to the poles where they continued to glow, but not undergo any great storms of the usual regularity.


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