Sunspots & Solar Magnetism: Physical Mechanisms

Sunspots often occur in magnetically linked groups with umbrae and penumbrae. The visible sunspot count rises and falls over an average cycle near 11 years, while magnetic polarity reversals make the full magnetic cycle about 22 years. The Sun also rotates differentially, with the equator rotating faster than higher latitudes.

The Sun's electrically conducting plasma and differential rotation stretch, shear, and regenerate magnetic fields through dynamo processes. Strong magnetic fields inhibit ordinary convective energy transport locally, so sunspot regions become cooler and darker than the surrounding photosphere. Magnetic field lines can store large amounts of energy in active regions.

Spectropolarimetric observations measure strong magnetic fields in sunspots, and the Zeeman effect splits spectral lines in proportion to field strength. Long-term sunspot records reveal the cycle, polarity measurements reveal magnetic reversal, and helioseismology plus surface tracking show differential rotation.

Strong magnetic fields suppress some convective heat transport, lowering photospheric temperature locally and making spots appear dark relative to surrounding granulation. Differential rotation and convective plasma motions contribute to the solar dynamo, which amplifies and reorganizes magnetic fields over the activity cycle.