Other
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sp31b02l&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SP31B-02 INVITED
Other
1650 Solar Variability, 7529 Photosphere, 7536 Solar Activity Cycle (2162), 7537 Solar And Stellar Variability, 7538 Solar Irradiance
Scientific paper
Radiometers on satellites have monitored the Sun's total irradiance since 1978. The measurements have been combined into a composite record after adjustments for absolute calibrations and in-flight instrumental drifts. Variability is evident from days to the 11-year cycle. Total irradiance was about 0.08% higher during solar maxima in 1980 and 1990 than in solar minimum. Irradiance levels in 2000, near present solar activity maximum, appear to be about 20% less than during the two previous maxima, although unresolved differences among measurements made since 1996 preclude a definitive conclusion. The observational record of only two solar cycles is too short, as yet, to determine longer-term trends. Total solar irradiance varies in response to the formation, evolution and rotation of magnetic features on the disk. Two primary sources of solar irradiance variability are dark sunspots, where bolometric radiation is locally depleted and bright faculae, where it is enhanced. Empirical models that parameterize the competing influences of these two effects account for more than 80% of the variance in the composite total irradiance record. The models utilize a sunspot darkening function calculated directly from visible images of sunspot areas, but the corresponding facular and network brightening function is not yet known to requisite accuracy from photometric imaging. This makes it necessary to use instead a proxy of this brightening contribution, based on the observed close correlation between photospheric and chromospheric bright structures in plages and network. The models determine the conversion of the proxy to bolometric facular brightening by regression against the observed irradiance adjusted for sunspot darkening. Some testing of this proxy-based approach has been possible. Combining plausible bolometric contrasts and center-to-limb functions with bright active region and network sources identified by enhanced Ca K emission permits independent calculations of the total irradiance brightness component that agree with the observed irradiance adjusted for sunspot darkening. This validation is consistent with facular and network brightening as the interpretation of the irradiance residuals that remain after spot darkening removal. Although the uncertainties in both the radiometry and photometry remain too large to rule out possible brightening contributions from other sources also proportional to facular and network area, the present data suggest that solar magnetism is the primary cause of total irradiance variability during the solar cycle.
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