The latitudinal migration of the sunspot groups

TL;DR

This study analyzes sunspot group migration patterns over multiple solar cycles, finding that exponential models fit the data well and revealing details about drift velocities, crossing points, and influences of cycle strength and sunspot area.

Contribution

It provides a detailed quantitative analysis of sunspot group latitudinal migration, including modeling of drift paths and velocities across several solar cycles, with insights into cycle strength and sunspot area effects.

Findings

Exponential functions fit sunspot drift data better than quadratic models.

Drift velocities decrease from about 0.30 to 0.07 degrees/month over the cycle.

Stronger cycles and larger sunspot areas influence migration paths slightly.

Abstract

We analyse the monthly sunspot group (SG) data for Solar Cycles C8-C23 and calculate the average latitude of the drift path for the northern and southern hemisphere of the Sun. We find that exponential function fits slightly better than the second-order polynomial to the average drift of the SGs. The drift velocities are are 0.30 and 0.27 degrees/month (meridional speed about 1.41 and 1.27 m/s) in the beginning and 0.071 and 0.069 degrees/month (about 0.33 and 0.32 m/s) at the end of the average cycle for northern and southern hemisphere, respectively. We do also bi-linear fits (first-order polynomial fits in two fractions) because they tell us the crossing point were the drift velocity changes the most. This crossing point for the Cycles C8-C23 is at 50 months and 15.4 degrees of latitude in the northern hemisphere and 54 months and -14.4 degrees for the southern hemisphere. The drift velocities calculated from the slopes of the bi-linear fits are 0.233 and 0.216 degrees/month (about 1.10 and 1.02 m/s) before the crossing point and 0.118 and 0.101 degrees/month (about 0.55 and 0.47 m/s) after the crossing point. We find a slight dependence of the migration on the strength of the cycle such that the drift path of the stronger cycles follow slightly higher latitudes than those of the weaker cycles. The areas of the sunspot groups affect also slightly to the drift path such that large area SGs have path at somewhat lower latitudes in the ascending phase and higher latitudes in the descending phase of the cycle than the smaller area SGs.