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Session 33 - Dynamics of Solar Magnetic Fields.
Oral session, Tuesday, June 11
Observations demonstrate that the large-scale magnetic fields vary in a systematic and consistent way during a solar cycle. This paper will discuss the evolution and development of the magnetic field patterns based on 23 years of full-disk, high-resolution NSO/KP magnetograms. An analysis of the properties of active regions and the variation of the magnetic field distribution with time, the global pattern of magnetic flux over the cycle can be described as follows: During the minimum phase of a cycle, most of the surface magnetic fields form a pattern of mixed polarities, i.e., the distribution of the magnetic network elements that, on a spatial scale of at least a supergranule, is a mixture of both polarities. The polar regions, however, are covered by predominately unipolar fields that are of opposite polarity in each hemisphere and extend to latitudes of around 50\deg. With increasing activity levels, the mixed-polarity fields at lower latitudes are replaced by active regions and at higher latitude by the large-scale patterns of unipolar magnetic flux that develop as the active region magnetic fields decay and disperse. These large scale magnetic flux patterns appear to be transported systematically toward the poles by a random-walk dispersal mechanism, merdidional motions, differential rotation. The net effect of this process in the rise of the cycle and into the maximum phase is a succession of large-scale patterns of magnetic flux of both polarities extending from the activity belts to higher latitudes. Those of the same polarity augment the polar fields, while those of opposite polarity cancel with and eventually reverse the polar fields during the sunspot cycle maximum. As cycle activity declines, the unipolar fields in the polar regions increase in area and strength, while the large-scale unipolar fields are replaced by the time of sunspot minimum with patterns of mixed-polarity fields.
Program listing for Tuesday