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On the Weakening of the Polar Magnetic Fields during Solar Cycle 23 View all abstracts by submitter

Eva Robbrecht   Submitted: 2009-12-08 12:39

The Sun's polar fields are currently 40% weaker than they were during the previous three sunspot minima. This weakening has been accompanied by a corresponding decrease in the interplanetary magnetic field (IMF) strength, by a ∼20% shrinkage in the polar coronal-hole areas, and by a reduction in the solar-wind mass flux over the poles. It has also been reflected in coronal streamer structure and the heliospheric current sheet, which only showed the expected flattening into the equatorial plane after sunspot numbers fell to unusually low values in mid-2008. From latitude?time plots of the photospheric field, it has long been apparent that the polar fields are formed through the transport of trailing-polarity flux from the sunspot latitudes to the poles. To address the question of why the polar fields are now so weak, we simulate the evolution of the photospheric field and radial IMF strength from 1965 to the present, employing a surface transport model that includes the effects of active region emergence, differential rotation, supergranular convection, and a poleward bulk flow. We find that the observed evolution can be reproduced if the amplitude of the surface meridional flow is varied by as little as 15% (between 14.5 and 17 m s-1), with the higher average speeds being required during the long cycles 20 and 23.

Authors: Wang, Y.-M., Robbrecht, E., Sheeley, N. R. Jr.
Projects: None

Publication Status: published in ApJ 707 (2009), 1372
Last Modified: 2009-12-09 08:07
Go to main E-Print page  A Statistical Study of Spectral Hardening in Solar Flares and Related Solar Energetic Particle Events  Nonlinear force-free coronal magnetic field modelling and preprocessing of vector magnetograms in spherical geometry  Edit Entry  Download Preprint  Delete Entry 

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