Sunspot group tilt angles and the strength of the solar cycle

TL;DR

This study analyzes historical sunspot tilt angles and finds strong correlations with solar cycle strength, suggesting tilt angles can help predict future solar activity and improve understanding of the solar dynamo.

Contribution

It reveals a strong anti-correlation between cycle-averaged tilt angles and cycle strength, and links tilt angles to the prediction of subsequent solar cycle amplitudes.

Findings

Strong anti-correlation between tilt angles and cycle strength (r≈-0.95).

Product of tilt angle and cycle strength correlates with next cycle's strength.

Tilt angles are important for modeling the solar dynamo and predicting solar activity.

Abstract

It is known that the tilt angles of active regions increase with their latitude (Joy's law). It has never been checked before, however, whether the average tilt angles change from one cycle to another. Flux transport models show the importance of tilt angles for the reversal and build up of magnetic flux at the poles which is, in turn, correlated with the strength of the next cycle. Here we analyse time series of tilt angle measurements and look for a possible relationship of the tilt angles with other solar cycle parameters, in order to glean information on the solar dynamo and to estimate their potential for predictions of solar activity. We employ tilt angle data from Mount Wilson and Kodaikanal observatories covering solar cycles 15 to 21. We analyse the latitudinal distribution of the tilt angles (Joy's law), their variation from cycle to cycle and their relationship to other solar cycle parameters, such as the strength, amplitude and length. The two main results are: 1. An anti-correlation between the mean normalized tilt angle of a given cycle and the strength (or amplitude) of that cycle, with a correlation coefficient of r=-0.95 and r=-0.93 for Mount Wilson and Kodaikanal data, respectively. 2. The product of the cycle averaged tilt angle and the strength of the same cycle displays a significant correlation with the strength of the next cycle (r=0.65 and r=0.70 for Mount Wilson and Kodaikanal data, respectively). An even better correlation is obtained between the source term of the poloidal flux in Babcock-Leighton-type dynamos (which contains the tilt angle) and the amplitude of the next cycle. The results of this study indicate that in combination with the cycle strength, the active region tilt angles play an important role in building up the polar fields at cycle minimum.