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

Polar crown filaments (PCFs) form above the magnetic polarity inversion line, which separates the unipolar polar fields and the nearest dispersed fields from the trailing part of active regions with opposite polarity. The statistical properties of PCFs are correlated with the solar cycle. Therefore, the study of PCFs plays an important role in understanding the variations of the solar cycle, especially the prolonged cycle 23 and the current "abnormal" solar cycle 24. In this Letter, we investigate PCFs using full-disk Hα data from 1973 to early 2018, recorded by Kanzelhöhe Solar Observatory and Big Bear Solar Observatory, in digital form from 1997 to 2018 and in 35 mm film (digitized) from 1973 to 1996. PCFs are identified manually because their segmented shape and close-to-limb location were not handled well by automatical detections in several previous studies. Our results show that the PCFs start to move poleward at the beginning of each solar cycle. When the PCFs approach to the maximum latitude, the polar field strength reduces to zero followed by a reversal. The migration rates are about 0.°4 to 0.°7 per Carrington rotation, with a clear N-S asymmetric pattern. In cycles 21 and 23, the PCFs in the northern hemisphere migrate faster than those in the southern hemisphere. However, in the "abnormal" cycle 24, the southern PCFs migrate faster, which is consistent with other observations of magnetic fields and radio emission. In addition, there are more days in cycle 23 and 24 without PCFs than in the previous cycles.