Polar magnetic fields - Filaments and the zero-flux contour

Abstract

We investigate the reliability with which magnetograph observations of the large-scale polar fields establish the zero-flux contour by comparing magnetic maps from various sources with one another and with the locations of filament structures seen on the disk in Hα filtergrams. The daily MWO and NSOKP magnetograms smoothed over 120 arc sec provide consistent large-scale zero-flux contours which align with the filaments out to heliocentric angles of about 75°. Synoptic maps match in regions where the locations of the zero-flux contour and of the filaments are maintained for several days. Attention is drawn to regions at the tips of unipolar `plumes' and the polar crown gap where the contours are variable from day to day; these are tentatively identified as regions of active reorganization of large-scale flux.