Abstract

Sunspots are areas in the Sun’s photosphere where the intense magnetic field can pass through from the solar interior, where it is generated, into the Sun’s atmosphere where it dominates the dynamics of the solar corona. Motions of sunspots cause the coronal magnetic field to become deformed with the effect that energy can be stored in the coronal magnetic field. This energy can be released through eruptive events such as solar flares. Sunspots are known to rotate about their umbral centres. This rotation contributes to the build-up of energy within an active region that can be released during an eruptive solar event, such as a solar flare. This thesis aims to investigate the relationship between rotational forms of sunspot dynamics by conducting a statistical survey of sunspot rotation and sunspot pair corotation.

To generate the statistical sample, a fully automatic sunspot identification and tracking method is developed. This method is tested on a previously analysed four-month dataset that was generated using a semi-automatic sunspot rotation tool. The new method has many advantages over the previous method, as the automatic nature enables more sunspots to be systematically identified, and these sunspots are identified earlier and tracked for longer. The new method is able to identify when sunspots are undergoing structural changes, such as splitting or mergers, and track these variations successfully.

The method is applied to a twelve-month series of observations covering the rise-phase and initial peak of solar cycle 25. Within the sample there are 560 sunspots across 189 active regions, and 71% of active regions generate sufficient energy due to sunspot rotation to account for the radiated flare energy, with 32% of active regions generating an excess of 10 times the amount of energy required for the flaring activity. The method is also applied to two case-study active regions that triggered intense space weather events in 2024, AR 13664 and AR 13842. These active regions were found to build-up rotation energy differently to each other, with AR 13664 requiring the contribution from all sunspots within the active region, and AR 13842 being dominated by the rotation profile of the largest sunspot.

A method to measure the angular variation between sunspot pairs is developed, and the method is applied to all active regions within the four-month and twelve-month samples, and the two case study active regions. It is found that 49% of active regions exhibit more than 180◦ of corotation, and the net active region corotation was found to have a positive correlation with the radiated flare energy.

The work in this thesis shows that sunspot rotation can generate enough energy within an active region to account for flaring activity, and suggests that the level of corotation present in an active region correlates with the volume of flaring activity.