The recent class X solar flare was the most energetic in several years in part because the Sun has been going through an unusually long minimum in the 11 year sunspot cycle. Solar astronomers have been struggling to understand this minimum. In a paper published in the March 3, 2011 issue of Nature, three solar physicists offer a possible explanation for the unusually long solar minimum.
Lead author, Dibyendu Nandy working with Andrés Muñoz-Jaramillo and Petrus C.H. Martens performed computer simulations that help them understand the large number of spotless days in the recent solar minimum during 2008 and 2009. Their work will also contribute to astronomer's understanding of the sunspot cycle in general. (Click here for a brief explanation of sunspots and the sunspot cycle.)
The computer simulations accounted for the Sun's changing magnetic field and the flow of solar material underneath the Sun's visible surface. Directly beneath the surface the Sun has what astronomers call meridional flow from the equator north or south to the polar regions. Near the poles the solar material sinks into the interior and flows back to the equator.
Nandy's group found that this meridional flow affects the sunspot cycle. Faster than normal meridional flow during the first half of sunspot cycle 23 (during the late 1990s) coupled with slower flow during the second half of the cycle (early 2000s) produced an unusually long solar minimum with virtually no sunspots in 2008 and 2009. During this minimum, the Sun's magnetic field was also weaker than normal.
This research was funded by NASA and the government of India. NASA's Solar Dynamics Observatory is expected to make observations to test the predictions of this theoretical model of the sunspot cycle.
This recent solar minimum was the longest in about a century, but pales compared to the Maunder minimum in the late 17th century, during which there were very few sunspots and Europe experienced a little ice age.
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