Migration of Polar Crown Filaments in the Past 100 Years

Polar Crown Filaments (PCFs) are formed above the polarity inversion line (PIL), which separates unipolar polar fields and the nearest dispersed fields. They are important features in studying solar polar fields and their cyclical variations. Due to the relatively weak field strength and projection effects, measuring polar magnetic fields is more difficult than obtaining the field strengths concentrated in active regions at lower latitudes. ``Rush-to-the-Poles'' of PCFs represents the progress of unipolar polar fields from the previous solar cycle being cancelled by the dispersed fields generated in the current cycle. Such a progress is a good indicator of the polarity reversal in the polar areas and a precursor for the solar maximum. In this study, PCFs are identified from a 100-year archive, covering cycles 16 to 24. This archive consists of full disk Hα images obtained from the Kodaikanal solar Observatory of the Indian Institute of Astrophysics, Kanzelhöhe Solar Observatory and Big Bear Solar Observatory. The poleward migration speeds are measured, which show an obvious asymmetry in northern and southern hemispheres. In addition, our results show that the PCFs always reach their highest latitudes first in the northern hemisphere. This is consistent with previous studies showing that reversal of the magnetic field at the north pole always occurs before the reversal at the south pole. Moreover, we compare the temporal variations of PCF migration and the latitude gradient factor of the differential rotation, which shows a trend in the southern hemisphere.