Rotating with the sunspot cycle.

Rotating with the sunspot cycle 

Tracking the progress of sunspots across the sun's face provides a handy way of studying how rapidly the sun rotates on its axis. Early studies showed that equatorial regions of the sun rotate somewhat faster than regions near its poles. More recent work has indicated that the rate of rotation also seems to depend on the size, shape and relative position of sunspots and other markers of solar activity. Now, David H. Hathaway and Robert M. Wilson of the NASA Marshall Space Flight Center in Huntsville, Ala., present new evidence that the sun appears to rotate more rapidly when fewer sunspots mark its surface.

Hathaway and Wilson carefully analyzed published sunspot rotation rotates for the period from 1921 to 1982. They discovered that over the course of each solar cycle, the most rapid rotation usually occurred at the sunspot minimum. Moreover, when the sun's southern hemisphere had fewer spots than the northern hemisphere, it rotated more rapidly than the northern hemisphere. And, averaged over each solar cycle, the sun as a whole appeared to rotate more rapidly during cycles with fewer sunspots and smaller sunspot area.

"Rapid rotation is observed when and where the sunspot area is small," the researchers conclude in the July 1 ASTRO-PHYSICAL JOURNAL. The size of the effect suggests that the sun may rotate about 0.k percent faster during periods when few sunspots appear, such as the 70-year Maunder minimum.

"The source of this effect remains uncertain," Hathaway and Wilson say. "An explanation for the decrease in rotation with increasing sunspot area will require a better understanding of the dynamics of the solar interior and the formation of sunspot magnetic fields."