A Virial Theorem Investigation of Magnetic Variations in the Sun

The magnetic virial theorem is applied here to a long-standing astrophysical problem, namely, the sign of the Sun's radius change during the solar activity cycle. The solar radius is theoretically found to decrease around the time of maximum magnetic field strength, in agreement with the best available observational evidence. This theoretical prediction, although simply based, instills some confidence by explicitly satisfying the conservation of total energy.