Determining the North-South Displacement of the Heliospheric Current Sheet from Coronal Streamer Observations

Abstract

Inferences based on interplanetary field measurements have suggested a statistical tendency for the heliospheric current sheet (HCS) to be displaced southward of the heliographic equator during the past four solar cycles. Here, we use synoptic maps of white-light streamer structures to determine more directly the longitudinally averaged latitude of the HCS, after separating out the contribution of streamers without magnetic polarity reversals ("pseudostreamers"). We find a strong tendency for the HCS to be shifted southward by a few degrees during 2007-2011, but no significant shift during the 1996-1997 sunspot minimum. Fluctuations in the magnitude and direction of the north-south shifts often occur on timescales as short as one or two Carrington rotations, as a result of changes in the streamer structures due to active region emergence. The largest shifts occurred during 2010-2011 and were on the order of -6°. Such southward displacements are consistent with the overwhelming dominance of northern-hemisphere sunspot activity during the rising phase of the current solar cycle 24, resulting in a strong axisymmetric quadrupole component whose sign at the equator matched that of the north polar field; the symmetry-breaking effect of the quadrupole was further enhanced by the weakness of the polar fields.