Long-term Evolution of Sunspot Magnetic Fields

Abstract: Independent of the normal solar cycle, a decrease in the sunspot magnetic field strength has been observed using the Zeeman-split 1564.8nm Fe I spectral line at the NSO Kitt Peak McMath-Pierce telescope. Corresponding changes in sunspot brightness and the strength of molecular absorption lines were also seen. This trend was seen to continue in observations of the first sunspots of the new solar Cycle 24, and extrapolating a linear fit to this trend would lead to only half the number of spots in Cycle 24 compared to Cycle 23, and imply virtually no sunspots in Cycle 25.

We examined synoptic observations from the NSO Kitt Peak Vacuum Telescope and initially (with 4000 spots) found a change in sunspot brightness which roughly agreed with the infrared observations. A more detailed examination (with 13,000 spots) of both spot brightness and line-of-sight magnetic flux reveals that the relationship of the sunspot magnetic fields with spot brightness and size remain constant during the solar cycle. There are only small temporal variations in the spot brightness, size, and line-of-sight flux seen in this larger sample. Because of the apparent disagreement between the two data sets, we discuss how the infrared spectral line provides a uniquely direct measurement of the magnetic fields in sunspots.

The last time we had solar cycles with “virtually no sunspots” during a solar cycle was during the Maunder Minimum. Solar Cycle 24 should peak some time in 2013 and the next solar minimum should start to kick in around 2015. If this pattern of weakened sunspot activity and weakened magnetic flux continues, we may well experience some pretty severe winters.  Unless I miss my projection of climate change, we had better prepare for really cold winter temperatures and cool summers.

Read more at GOP Exiles