Midterm periods of solar filaments

Abstract

AbstractOn the basis of the Carte Synoptique catalogue of solar filaments from March 1919 to December 1989, we measure power spectra of detrended full‐disk (FSFNs, latitudinal bands: 0∘–90∘), low‐latitude (LSFNs, latitudinal bands: <50∘), and high‐latitude (HSFNs, latitudinal bands: ≥50∘) solar filament numbers by Maximum Entropy Method (MEM) and Continuous wavelet transform to detect midterm periods. It is found as the following: (1) FSFNs and LSFNs have the same midterm periodicity, while HSFNs show a different midrange periodicity. Some periods frequently mentioned in other solar indices are also detected from the solar filament numbers, such as 2–3 year period (quasi‐biennial oscillation—QBO), ∼1.7 year, ∼1.3 year, ∼1 year, 150–157 day period (Rieger period), and 6.0–6.4 months (Rieger‐type period). These periods are intermittent during considered time span. Some of them are missing in some solar cycles. (2) QBO is detected from total data and most solar cycles of FSFNs, LSFNs, and HSFNs. It may be related to oscillation of magnetic field of solar surface. (3) Approximately 1.3 year period occasionally appears, but ∼1.7 year period is hardly seen. These two periods probably are seasonal effects. (4) Approximately 1 year period is detected from both total data and every solar cycle of FSFNs and LSFNs but hardly detected from HSFNs. It is perhaps connected with sunspot activity. (5) Rieger period of 5.0–5.2 months is detected in total data and even solar cycles of HSFNs. Rieger‐type period of 6.0–6.4 months is found in total data and most solar cycles, except cycle 18 of LSFNs and FSFNs. These periods seem to be subharmonics of ∼11 year period.