PRECISE GROUND-BASED SOLAR PHOTOMETRY AND VARIATIONS OF TOTAL IRRADIANCE

Variations in the total solar irradiance measured by the active cavity radiometer irradiance monitor (ACRIM) on SMM have been correlated with measures of magnetic activity on the solar disk. Quantitative indices of magnetic activity were derived from ground-based, full-disk, photometric images of the Sun at red (6723 angstrom) and violet (3934-angstrom K line) wavelengths. The red images have been obtained on a daily basis at the San Fernando Observatory since 1985, and the K line images since 1988. Sunspot irradiance deficits are calculated directly from the red images while proxy measures of facular irradiance excesses are derived from the K line images. The images analyzed here were made during 21 days between June 20 and July 14, 1988, a period centered on the disk passage of a large sunspot group. The best two-parameter multiple correlation coefficient between the ACRIM data and the photometric data is R2 = 0.97 (21 data points, 18 degrees of freedom). The zero point S0 = 1367.27 W m-2 agrees well with the solar irradiance measured by ACRIM/SMM during the 1986 activity minimum; the residual standard deviation was 0.13 W m-2 (about 100 ppm). The multiple correlations were extended to include measures of the irradiance contribution of "network" magnetic fields, unassociated with active regions. NOAA 9 spacecraft observations of UV Mg 11 lines at 2800 angstrom gave R2 = 0.99 (17 degrees of freedom) with S0 = 1366.68 + 0.08 W m-2. The index of 10.7-cm microwave flux gave R2 = 0.98, with S0 = 1366.43 + 0.11 W m-2. We can thus model short-term irradiance changes to within 100 ppm relative precision from ground-based data.