Other
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh51a05l&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SH51A-05 INVITED
Other
1650 Solar Variability, 7537 Solar And Stellar Variability, 7538 Solar Irradiance
Scientific paper
Solar irradiance varies in response to changes in solar magnetism. Enhanced emission in faculae is a primary source of total irradiance variability. Bright chromospheric plages and network overlying photospheric faculae are dominant sources of variations in UV irradiance that affect ozone, and in strong EUV emission lines that impact neutral and ionized densities in the upper atmosphere. Knowledge of the sources of long-term trends in solar irradiance is important for specifying the Sun's role in global change and space weather, in the past and in the future. Neither irradiance nor the chromospheric Ca and Mg indices that are sensitive indicators of bright emission sources, was monitored prior to 1976 so scenarios for their long-term variability must be developed indirectly. One approach utilizes the range of variability evident in Ca emission in Sun-like stars; another correlates measured solar fluxes with historical proxies of solar activity, such as the aa index. The amplitudes of long-term total solar irradiance variability estimated by these two independent approaches agree quite well. But irradiances reconstructed from smoothed group sunspot numbers increase primarily in the first part of the twentieth century and lack a significant long-term rising trend in their cycle minima in the past 50 years, such as is inferred from geomagnetic indices. The historical reconstructions, though speculative, provide a basis for assessing future solar irradiance trends. Progress in specifying the sources of long-term solar irradiance trends requires new understanding of how the closed flux associated with bright magnetic sources in the solar atmosphere relates to the open flux associated primarily with coronal holes. The closed flux produces irradiance variations whereas the open flux extends into the heliosphere, modulates the interplanetary magnetic field and produces geomagnetic activity. Initial results based on potential field extrapolations show that the emergence of new active regions together with flux transport processes continually alter the relative amounts of open and closed flux. Whether or not closed and open flux evolve differently or together over long time scales is presently unknown but is essential for determining the physical relevance of geomagnetic indices for inferring historical solar irradiance variations.
Lean Judith L.
Wang Yadong
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