Towards a Long-Term Catalogue of Sunspot-Group Positions and Rotation Velocities from Kanzelhöhe Observatory: Differential Rotation and Possible Torsional Oscillation Signatures During Solar Cycle 25

This study presents Solar Cycle (SC) 25 (2020–2025) results from the homogeneous Kanzelhöhe Solar Observatory (KSO) catalogue project and completes the analysis of SCs 19–25. Only SC18 remains to be processed before the catalogue is finalized. Using KSO white-light observations, we determined sidereal rotation velocities and solar differential rotation (DR) parameters A and B from day-to-day positional changes in sunspot-groups and from robust linear fits to their position–time measurements. Their temporal evolution and relationship with solar activity were analyzed using yearly mean Solar Index and Long-term Solar Observations (SILSO) sunspot numbers. Rotation velocity residuals were also analyzed as a function of time and heliographic latitude in search of possible torsional oscillation (TO) signatures. The obtained DR parameters are consistent with previously published results. For SC25, a strong anticorrelation between the yearly fitted parameters A and B was found, in agreement with earlier studies, although its interpretation is affected by the covariance between the two parameters introduced by the fitting procedure. The residual rotation velocities exhibit temporal behaviour similar to that reported in previous sunspot-group studies. Latitude–time residual maps reveal patterns that may represent TO signatures, however the limited temporal coverage of SC25, the characteristics of sunspot group tracers, and the use of interpolated data require a cautious interpretation.