The Global Solar Magnetic Field Through a Full Sunspot Cycle: Observations and Model Results 

Karel Schrijver Submitted: 20080619 17:16
Based on 11 years of SOHO/MDI observations from the cycle minimum in 1997 to the next minimum around 2008, we compare observed and modeled axial dipole moments to better understand the largescale transport properties of magnetic flux in the solar photosphere. The absolute value of the axial dipole moment in 2008 is less than half that in the corresponding cycleminimum phase in early 1997, both as measured from synoptic maps and as computed from an assimilation model based only on magnetogram data equatorward of 60 degrees in latitude. This is incompatible with the statistical fluctuations expected from fluxdispersal modeling developed in earlier work at the level of 710 sigma. We show how this decreased axial dipole moment can result from an increased strength of the diverging meridional flow near the Equator, which more effectively separates the two hemispheres for dispersing magnetic flux. Based on the combination of this work with earlier longterm simulations of the solar surface field, we conclude that the fluxtransport properties across the solar cycle have changed from preceding cycles to the most recent one. A plausible candidate for such a change is an increase of the gradient of the meridionalflow pattern near the Equator so that the two hemispheres are more effectively separated. The required profile as a function of latitude is consistent with helioseismic and crosscorrelation measurements made over past decade.
Authors: C.J. Schrijver and Y. Liu
Projects: SoHOMDI

Publication Status: Solar Physics, in press.
Last Modified: 20080923 20:59


