Improvement of solar cycle prediction: Plateau of solar axial dipole moment

TL;DR

This study identifies a stable plateau in the solar axial dipole moment before solar minima, enabling improved cycle predictions by using the surface flux transport model without considering new magnetic flux emergence.

Contribution

It demonstrates that the axial dipole moment plateau can be used for more accurate solar cycle predictions without accounting for new magnetic flux emergence.

Findings

The axial dipole moment becomes approximately constant during the years before each solar minimum.

The observed axial dipole moment aligns well with the SFT model predictions without new flux emergence.

The predicted axial dipole moment for Cycle 24/25 minimum is 60-80% of the previous cycle, indicating a weaker Cycle 25.

Abstract

Aims. We report the small temporal variation of the axial dipole moment near the solar minimum and its application to the solar cycle prediction by the surface flux transport (SFT) model. Methods. We measure the axial dipole moment using the photospheric synoptic magnetogram observed by the Wilcox Solar Observatory (WSO), the ESA/NASA Solar and Heliospheric Observatory Michelson Doppler Imager (MDI), and the NASA Solar Dynamics Observatory Helioseismic and Magnetic Imager (HMI). We also use the surface flux transport model for the interpretation and prediction of the observed axial dipole moment. Results. We find that the observed axial dipole moment becomes approximately constant during the period of several years before each cycle minimum, which we call the axial dipole moment plateau. The cross-equatorial magnetic flux transport is found to be small during the period, although the significant number of sunspots are still emerging. The results indicates that the newly emerged magnetic flux does not contributes to the build up of the axial dipole moment near the end of each cycle. This is confirmed by showing that the time variation of the observed axial dipole moment agrees well with that predicted by the SFT model without introducing new emergence of magnetic flux. These results allows us to predict the axial dipole moment in Cycle 24/25 minimum using the SFT model without introducing new flux emergence. The predicted axial dipole moment of Cycle 24/25 minimum is 60--80 percent of Cycle 23/24 minimum, which suggests the amplitude of Cycle 25 even weaker than the current Cycle 24. Conclusions. The plateau of the solar axial dipole moment is an important feature for the longer prediction of the solar cycle based on the SFT model.