Prediction of the amplitude of solar cycle 25 using polar faculae observations

Jan Janssens

doi:10.1051/swsc/2020081

Abstract

Based on the monthly number of polar faculae, a forecast of the amplitude of solar cycle 25 (SC25) is provided, as well as a prediction of the number of solar flares. Faculae near both solar poles have been visually observed using a commercial off-the-shelf 20 cm Schmidt-Cassegrain telescope since 1995. The monthly averages were corrected for varying seeing conditions and the heliographic latitude of the center of the solar diskB0. From the deduced relationship between the smoothed number of monthly polar faculae during the solar cycle minimum, and the subsequent maximum of the monthly sunspot number, a prediction has been made for the amplitude of the next solar cycle. The methodology used can be considered as a precursor technique. The expected number of M- and X-class flares was calculated based on a statistical approach. The maximum of SC25 is predicted to be 118 ± 29, of similar strength than the previous SC24. Also the number of M5 or stronger flares is expected to be comparable to that of the previous solar cycle.

Highlights

Polar faculae (PF) usually appear as isolated bright pointlike features near the solar poles, at latitudes of about 60° and higher
PF are between 3% and 10% brighter than the Sun’s photosphere (Makarov & Makarova, 1996), making them dimmer than the main zone faculae which are about 9%–16% brighter than the photosphere (Hirayama & Moriyama, 1979)
Shape, lifetime, and location, the main difference between polar and main zone faculae is that most PF are observed around solar cycle (SC) minimum, contrary to the main zone faculae that show a peak around the time of the sunspot maximum (e.g. Sheeley, 1964)

Summary

Introduction

Polar faculae (PF) usually appear as isolated bright pointlike features near the solar poles, at latitudes of about 60° and higher. Sheeley (1991) pointed out the excellent correlation existing between the number of polar faculae as recorded by the Mount Wilson Observatory (MWO), and the polar magnetic field strengths as measured by the Wilcox Solar Observatory (WSO). By WSO (since 1976), solar cycle forecasts based on the polar field method (direct or proxies) proved to be consistently quite accurate for solar cycles 21–24 (Pesnell, 2016; Petrovay, 2020). This has been confirmed for other data series, such as from the National Astronomical Observatory of Japan during solar cycles 19–23 (Li et al, 2002; Tlatov, 2009).

Observations and data

Data analysis and methodology

Results and discussion

Flaring activity

Conclusions