Sources of Solar Total Irradiance Variations

Abstract

The daily images and magnetograms acquired by MDI are a rich source of information about the contributions of different types of solar regions to variations in the total solar irradiance (TSI). These data have been used to determine the temporal variation of the MDI irradiance, the mean intensity of the solar disk in the continuum at 676.8 nm. The short-term (days to weeks) variations of the MDI irradiance and TSI are in excellent agreement with rms differences of 0.011%. This indicates that MDI irradiance is an excellent proxy for short-term variations of TSI from the competing irradiance contributions of regions causing irradiance increases, such as plages and bright network, and regions causing irradiance decreases, such as sunspots. However, the long-term or solar cycle variation of the MDI proxy and TSI differ over the 11-year period studied. The results indicate that the primary sources of the long-term (several months or more) variations in TSI are regions with magnetic fields between about 80 and 600 G. The results also suggest that the difference in the long-term variations of the MDI proxy and TSI is due to a component of TSI associated with sectors of the solar spectrum where the contrast in intensity between plages and the quiet Sun is enhanced (e.g., the UV) compared to the MDI proxy. This is evidence that the long-term variation of TSI is due primarily to solar cycle variations of the irradiance from these portions of solar spectrum, a finding consistent with modeling calculations indicating that approximately 60% of the change in TSI between solar minimum and maximum is produced by the UV part of the spectrum shortward of 400 nm (Solanki and Krivova, Space Sci. Rev. 125, 53, 2006).