Precise ground‐based solar photometry and variations of total irradiance

Abstract

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 Å) and violet (3934‐Å 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 II lines at 2800 Å 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.