Besides sunspots, filaments are one of the most important features in studying solar magnetic fields. They are usually formed above the polarity inversion line (PIL), supported by either helically coiled or dipped magnetic fields (Gilbert et al. 2001). Therefore, filaments are representative signatures of discrete magnetic fields at large scales. Scientific observations of filaments started in the 1880s at the Arcetri Astrophysical Observatory in Florence, Italy (McIntosh et al. 2019). Thus, the long time span of the archive of filaments becomes crucial in studying the long-term variations of solar magnetic fields. Poleward and equatorward migrations of filaments have already been studied extensively, e.g., Cliver (2014), Xu et al. (2018), McIntosh et al. (2019), and Tlatova et al. (2020). Among different types of filaments (Hansen & Hansen 1975), polar crown filaments (PCFs) locate at the highest latitudes and usually reside near the polar crown cavities (Leroy et al. 1983). Their underlying PILs separate the predominate polar field of the previous cycle and the dispersed field of the current cycle, drifting poleward from the trailing polarities of sunspots or sunspot groups in lower latitudes (Tang 1987; Panesar et al. 2014). Also known as “rush-to-the-pole,” the poleward migration is an important property of PCFs and a good indicator of the polarity reversal of the solar magnetic field and the occurrence of the solar maximum (e.g., Ananthakrishnan 1952; Cliver 2014). Moreover, polar magnetic fields are difficult to measure because of the projection effect, and they are much weaker than the fields in low-latitude and active regions. Recently, Xu et al. (2018) studied the PCFs observed by the Kanzelhöhe Solar Observatory (KSO) and Big Bear Solar Observatory (BBSO) between 1973 and 2018 (cycles 21–24). Their results show the following. The poleward migration speeds of PCFs are between 0.4 and 0.7 per Carrington rotation (CR), which is about 5°–9° yr–1. This result is consistent with the speed found by Ulrich (2010) and Altrock (2014; for coronal Fe xiv emission). The tilt angles of the filaments were studied in Mazumder et al. (2018) and Diercke & Denker (2019). The authors found that the tilt angles of the filaments have opposite signs in the northern and southern solar hemispheres, as compared to the expected interactive region formation. Similar results are found in a much longer time span by Hao et al. (2015) and Tlatov et al. (2016). In addition, the asymmetric distribution of filaments in the northern and southern solar hemispheres is found to be correlated with the asymmetry of sunspot areas. More recently, McIntosh et al. (2019) studied the activity cycles near the solar equator, using a comprehensive data set going back to 1880. The authors showed that the timing of PCFs reaching the polar caps is correlated with cycle terminators, which are defined as the rapid intensity changes of extreme-UV brightenings occurring between cycles. They also suggested that PCFs are representative features in studying the polar magnetic reversal quantitatively. Tlatova et al. (2020) analyzed prominences observed in the Ca ii K line at the solar limb, starting as early as 1897 with hand drawings and digitized records after 1910. Their results show systematically higher migration speeds of PCFs than those found by Xu et al. (2018), Ulrich (2010), and Altrock (2014) but with similar variation trends. The discrepancy is likely due to several factors, such as the selection of prominences over filaments and the latitude range. Chatterjee et al. (2020) performed an automated detection of prominence latitudes from Ca ii K disk blocked images from Kodaikanal Solar Observatory (since 1906) and quantified the nonlinearity in poleward migration with its variation over 10 solar cycles combining data from KSO and Meudon Observatory in Meudon, France (Demarcq et al. 1985).
Citation: Panesar et al. 2014
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Citation: McIntosh et al. 2019
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Citation: Xu et al. (2018)
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Citation: Tlatova et al. (2020)
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Citation: Tlatova et al. (2020)
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