Published: 2016-06-05 22:59:01
Updated: 2016-06-05 22:59:01
Posted June 5
By Tony Rice
Raleigh, N.C. — Friday June 3 was first day of 2016 where temperatures exceeded 90º at the Raleigh-Durham International Airport. It was also the first time in nearly 4.5 years that the international Sunspot Number dropped to 0. The latest images from NASA’s Solar Dynamics Observatory showed a sunspot free disk through the weekend.
The number has ranged from the current 0 to over 500 during the solar maximum period of 1870. It is calculated by adding the number of individual spots observed to 10 times the number of groups of spots. A scaling factor is applied to each observation to account for differences in equipment and conditions, a handicap of sorts. Observations worldwide are averaged together for the day’s number to be recorded.
Observations of the sun and these dark spots date back to the invention of the telescope itself. Galileo used them to estimate the rotational period of the sun, about once every 25 days. Sunspot records are among the most complete in astronomy, Detailed observations by the Royal Greenwich Observatory go back to 1874 and continue today at sites like NOAA’s Space Environment Center.
Sunspots are relatively cool spots at the surface which are formed as the sun’s magnetic fields are twisted and snapped like a rubber band. Each spot is about 6400º F, capping off intense energy below them, compared to the surrounding photosphere at nearly 10,000ºF.
So you might be thinking a spot free sun might deliver more energy. This weekend’s heat has a much more Earthly, seasonal explanation however. Many of these spots are as big as the Earth, but overall their effect on our temperatures is less than negligible. Even at the highest sunspot numbers, they are tiny compared to the overall size of the sun. Areas immediately around the spots are even hotter than the rest of the photosphere as energy jetting out from below escapes around the edges of the spot.
The recent lack of sunspots is probably very temporary. While it’s clear that sunspot activity has been decreasing rapidly, forecasters don't expect a solar minimum for another 3-4 years. There will likely be more and longer spotless periods like this weekend between now and then.
The North Carolina Museum of Natural Sciences is participating in International SunDay on June 26. There will be talks on the anatomy of the sun and the NASA heliophysics missions studying our star. Visitors will also have the opportunity to safely view the sun through solar telescopes. There will likely be some fresh new sunspots to observe by then.