A Brief History of Astronomy
The Sun, a Representative Star
Sunspots also show a sunspot cycle where the number of sunspot occurrences varies between a high value at solar maximum and a small value at solar minimum, with an approximate 11-year periodicity. Other solar activity (flares, solar wind variations) follows this cycle between an active Sun and a quiet Sun. Theory suggests that the Sun generates its magnetic field by interior electrical currents in a solar dynamo. The differential rotation proceeds to wrap the magnetic field around the Sun and large-scale convection pulls magnetic field lines up and down through the photospheric layer, producing sunspots where compressed field lines move out or into the photospheric layer. After 11 years, more or less, field lines are so jumbled together that the magnetic field disappears, to be regenerated for the next cycle with an opposite polarity. This reversal is associated with a reversal of the polarity of sunspot pairs, hence two sunspot cycles actually are one manifestation of a longer 22-year solar magnetic cycle. The solar cycle is also associated with changes in the coronal structure, which appears round at sunspot maximum, but greatly distorted at sunspot minimum.
