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Polar Crown Filaments (PCFs) form above the magnetic polarity inversion line,which separates the unipolar polar fields and the nearest dispersed fields fromtrailing part of active regions with opposite polarity. The statisticalproperties of PCFs are correlated with the solar cycle. Therefore, study ofPCFs plays an important role in understanding the variation of solar cycle,especially the prolonged cycle 23 and the current "abnormal" solar cycle 24. Inthis study, we investigate PCFs using full disk H{\alpha} data from 1973 toearly 2018, recorded by Kanzelhohe Solar Observatory (KSO) and Big Bear SolarObservatory (BBSO), in digital form from 1997 to 2018 and in 35 mm film(digitized) from 1973 to 1996. PCFs are identified manually because theirsegmented shape and close-to-limb location were not handled well by automaticaldetections in several previous studies. Our results show that the PCFs start tomove poleward at the beginning of each solar cycle. When the PCFs approach tothe maximum latitude, the polar field strength reduces to zero followed by areversal. The migration rates are about 0.4 to 0.7 degree per Carringtonrotation, with clear N-S asymmetric pattern. In cycles 21 and 23, the PCFs inthe northern hemisphere migrate faster than those in the southern hemisphere.However, in the "abnormal" cycle 24, the southern PCFs migrate faster, which isconsistent with other observations of magnetic fields and radio emission. Inaddition, there are more days in cycle 23 and 24 without PCFs than in theprevious cycles.

Collective Study of Polar Crown Filaments in the Past Four Solar Cycles