Measurements of Solar Irradiance and Effective Temperature as a Probe of Solar Interior Magnetic Fields

We argue that a variety of solar data suggest that the activity-cycle timescale variability of the total irradiance is produced by structural adjustments of the solar interior. Assuming these adjustments are induced by variations of internal magnetic fields, we use measurements of the total irradiance and effective temperature over the period from 1978 to 1992 to infer the magnitude and location of the magnetic field. Using an updated stellar evolution model, which includes magnetic fields, we find that the observations can be explained by fields with peak values ranging from 120 to 2.3 kG, located in the convection zone between 0.959 and 0.997 R_solar, respectively. The corresponding maximal radius changes are 17 km when the magnetic field is located at 0.959 R_solar and 3 km when it is located at 0.997 R_solar. At these depths, the W-parameter (defined by ΔlnR/ΔlnL, where R and L are the radius and luminosity) ranges from 0.02 to 0.006. All these predictions are consistent with helioseismology and recent measurements carried out by the Michelson Doppler Imager experiment on the Solar and Heliospheric Observatory.