  The theory of stellar structure can be used to identify the most plausible
mechanisms for the irradiance variations associated with the solar cycle.
Changes in surface emissivity, i.e. the reduced cooling in spots and enhanced
emission by small scale magnetic fields, are the most effective mechanisms and
account for most of the observed variation. Helioseismology will soon be able
to test the consequences of changes in surface emissivity, and distinguish them
from other scenarios for irradiance variability.
