Asymmetry of solar polar fields and the southward shift of HCS observed by Ulysses

Abstract

We study the hemispheric asymmetry of high-latitude unipolar fields and the latitudinal shift of the heliospheric current sheet (HCS) using Ulysses magnetic field observations during the perihelion passes in 1994-1995, 2001-2002, and 2007. Using the cumulative flux density and the best fit lines to its high-latitude observations in the two hemispheres, we find that the absolute value of the high-latitude radial field of the Southern Hemisphere is larger than in the north during both minimum time scans in 1994-1995 and 2007. The hemispheric difference is about 0.2 nT during both scans, suggesting that the northern field area is some 5%-10% (5%-15%) larger than the southern area and that the HCS is shifted southward by about -2° during both scans. The results resolve the discrepancy between earlier results in 1994-1995 and clarify similar observations in 2007 in the ecliptic. They also verify the southward shift of the HCS during the exceptional solar cycle 23. We also study the detailed structure of the equatorial region and find that generally the Ulysses observations compare favorably with simultaneous heliospheric magnetic field predictions given by Wilcox Solar Observatory synoptic maps. Using a simple HCS model, we find that even in case of southward shifted HCS, the highest peak of the cumulative flux density could be located above the equator. Thus, Ulysses observations around the heliographic equator cannot alone give an unambiguous information of the HCS shift, which emphasizes the importance of studying the high-latitude sections of Ulysses orbit.