Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh51a06l&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SH51A-06 INVITED
Physics
1560 Time Variations--Secular And Long Term, 1650 Solar Variability, 2134 Interplanetary Magnetic Fields, 7524 Magnetic Fields, 7538 Solar Irradiance
Scientific paper
By studying energy coupling between the solar wind and the magnetosphere, Lockwood et al. (Nature, 399, 437, 1999) obtained the highest reported correlation (0.97) between interplanetary conditions and geomagnetic activity. By inverting this theory and using the 27-day recurrence of geomagnetic activity to eliminate the effect of fast flow streams, these authors were able to compute the interplanetary magnetic field from annual means of the aa geomagnetic index. Because on annual time scales the IMF obeys Parker spiral theory and using Ulysses results on the 3-dimensional structure of the heliosphere, these authors were able to compute the total coronal source flux, the open flux leaving the corona and entering the heliosphere. Test with independent interplanetary data confirmed the validity of the technique. The aa index is a homogeneous series extending back to 1868 and the results showed that the coronal source flux drifted upward throughout the last century so that its solar cycle average was a factor of 2.4 larger by its end than in 1900. This rise is confirmed by studies of isotopes deposited in ice sheets, tree rings and meteorites by the action of cosmic ray bombardment, and regression analysis with, for example 10Be abundances in ice sheets reveals that the open solar flux fell to about 25 percent of present-day values by the end of the Maunder minimum. Recent theoretic work by Solanki et al. (Nature, 408, 445, 2000) has explained this variation extremely well, in terms of the length of the solar cycle and the rate at which flux emerges through the photosphere. This therefore relates the open flux variation to magnetic phenomena in the photosphere (sunspots and faculae) that are known to modulate the total solar irradiance.
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