Migration of solar polar crown filaments in the past 100 years

Abstract

Polar crown filaments (PCFs) are formed above the polarity inversion line, 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-pole" of PCFs represent the progress of unipolar polar fields from the previous solar cycle being canceled by the dispersed fields generated in the current cycle. Such 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 yr archive, covering cycles 16–24. 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 and show an obvious asymmetry in the northern and southern hemispheres. In addition, our results show that the PCFs usually reach their highest latitudes first in the northern hemisphere, except cycle 17. Similarly, previous studies show that the magnetic field reversed first at the north pole in six out of nine cycles. We also compare the temporal variations of PCF migration and the latitude gradient factor of the differential rotation, which shows a trend in the southern hemisphere. Moreover, the migration speed of PCFs does not seem to be well correlated with the maximum sunspot numbers.