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J. L. Lean, Y. -M. Wang (Naval Research Laboratory)
Following its emergence in the sunspot latitudes, magnetic flux is dispersed across the Sun’s surface as a result of supergranular diffusion, meridional flow and differential rotation. Much of this flux is annihilated when regions of opposite polarity come into contact, but there is a net transport of flux to high solar latitudes which eventually reverses the polarity of the polar field, and continues the cyclic dynamo action. The accumulation of magnetic flux over many solar cycles may provide a plausible mechanism for the generation of a long-term secular trend in the open flux that originates in polar regions, and possibly in the closed flux as well. Open flux extends into interplanetary space where it modulates the levels of cosmogenic and geomagnetic proxies of long-term solar activity, which do display long-term secular trends in the past century. Secular trends in the Sun’s irradiance, which is associated with closed, rather than open, flux, are speculated to track these proxies. We investigate whether the surface transport processes applicable to the present-day Sun permit an accumulation of flux when persisting over multiple solar cycles that have amplitudes of increasing strength. For this purpose we employ a database of magnetic source regions acquired for solar cycle 21 and a flux transport model developed at NRL (Wang and Sheeley, Ap. J., 375, 761, 1991). We adjust the strengths of the sources in each of the past 10 solar cycles such that their total flux tracks the sunspot number. We simulate the evolution of the total, closed and open flux and investigate the accumulation of open and closed flux, and its sensitivity to the strengths of the various transport processes. NASA's LWS program supported this work.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.