Zonal harmonics of solar magnetic field for solar cycle forecast

TL;DR

This paper explores how zonal harmonic parameters of the solar magnetic field, like the $ ext{l}=3$ harmonic, can enhance solar cycle forecasting, especially in predicting the weakness of Cycle 25, by analyzing surface magnetic field data.

Contribution

It introduces the use of zonal harmonic parameters as additional predictors for solar cycle strength, supplementing traditional polar magnetic field proxies.

Findings

Zonal harmonic parameters correlate with solar cycle variations.

Parameters like the $ ext{l}=3$ harmonic can improve cycle prediction accuracy.

Differences in solar activity indices observed between Cycles 23 and 24.

Abstract

According to the scheme of action of the solar dynamo, the poloidal magnetic field can be considered a source of production of the toroidal magnetic field by the solar differential rotation. From the polar magnetic field proxies, it is natural to expect that solar Cycle 25 will be weak as recorded in sunspot data. We suggest that there are parameters of the zonal harmonics of the solar surface magnetic field, such as the magnitude of the $\ell$=3 harmonic or the effective multipole index, that can be used as a reasonable addition to the polar magnetic field proxies. We discuss also some specific features of solar activity indices in Cycles 23 and 24.