Mathematics – Logic
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sp31b04f&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SP31B-04 INVITED
Mathematics
Logic
7538 Solar Irradiance
Scientific paper
A variety of diagnostics has been developed to discriminate among competing physical explanations of solar luminosity variation, and to use them for new insight into magneto-convection on the sun and similar stars. The shape of spot -induced irradiance dips correlates well with changes in spot projected area, but not with growth or decay rate. This argues against storage of the spot's missing heat flux in the spot magnetic field.Efficient heat flow blocking and storage in the HCZ seems to provide the simplest mechanism.The weakness of sunspot bright rings and thermal shadows is consistent with eddy thermal diffusivities calculated from models of the HCZ. The contribution of faculae and network is still too uncertain to decide whether they account for all of the remaining variance in luminosity, after spot dimming is removed.But the correlation of irradiance variations is much higher with the difference of compensating spot and facular contributions than with total magnetic flux. This argues against "magnetic stirring" as an important factor in luminosity variation. The darkness of small flux tubes in continuum near disc center, especially near 1.63 microns,and their center - to - limb contrast variation, seems to favor their interpretation as photospheric heat leaks.This is supported by the lowered facular temperature gradient measured using two -color photometric imaging. Photometric searches for large - scale photospheric temperature inhomogeneities have yielded useful upper limits.Possible global variations in effective temperature, studied through monitoring of photospheric limb-darkening, and of temperature - sensitive Fraunhofer lines, have not revealed any convincing variations.But the percentage sensitivity to solar irradiance change is limited to about 0.2 per year, and might be improved with helioseismological techniques. The decrease in facular-to-spot area ratio observed at high solar activity levels suggests a simple explanation for the increased photometric variability of younger sun - like stars in terms of photospheric magnetism. The apparent absence of detectable solar luminosity variations outside modulation due to photospheric magnetism poses an interesting new constraint on stellar convection theory. Ongoing advances in cryogenic radiometry, thermal imaging,and helioseismology are all likely to contribute to the search for possible more subtle luminosity variations below the threshold of present measurements.
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