Nov 6 2001
From The Space Library
A group of scientists from Stanford University and NASA’s GSFC published research providing the first clear evidence of the causes of sunspots. Sunspots are large dark areas on the surface of the Sun that can last for weeks or longer. The scientists had used the Solar and Heliospheric Observatory (SOHO)~a project of NASA and ESA~to conduct an acoustic tomography of a large sunspot on 18 June 1998. This technique, much like medical ultrasound diagnostics, had enabled the scientists to conduct subsurface observations of the sunspot, whereas previous research had relied only on surface observations. Researchers had already known that material flows out of sunspots and that the magnetic field below sunspots obstructs the upward flow of energy from the solar interior, thus making the sunspot cooler than its surroundings. However, the SOHO data revealed that material also flows into sunspots, because the cooler material at the sunspot’s top condenses and flows downward, pulling the surrounding plasma and magnetic field inward with it. This concentration of material leads to further cooling of plasma, which, in turn, flows downward and draws in more plasma in a self-perpetuating cycle. These findings also clarified the previously unexplained attraction of magnetic fields with like polarities in sunspots. This characteristic had puzzled scientists because such magnetic fields generally repel rather than draw towards each other. (Junwei Zhao, Alexander G. Kosovichev, and Thomas L. Duvall Jr., “Investigation of Mass Inflows Beneath a Sunspot by Time-Distance Helioseismology,” Astrophysical Journal 557, no. 1 (10 August 2001): 384–388; NASA, “The Sun’s Dark Secret: How Sunspots Pull Themselves Together,” news release 01-216, 6 November 2001.)
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