Damir Hržina

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.

An analysis of the solar differential rotation (DR) during solar cycle No. 24 (SC24) (2009–2019), based on the Kanzelhöhe Observatory for Solar and Environmental Research (KSO) data set, is presented. The white-light images were processed and positions of sunspot groups were extracted using the morphological image processing technique. The sample was constrained to ±58° in central meridian distance (CMD). Two methods were applied to derive the sidereal angular rotation rate (ω) and, in turn, the solar rotation parameters A and B: (a) calculating synodic rotation velocities from daily CMD differences and elapsed time (daily shift method); (b) applying a robust linear least-squares fit to the time series CMD(t) for each sunspot group. To assess the relationship between rotation parameters and solar activity, we analyzed the yearly mean total sunspot number from the Sunspot Index and Long-term Solar Observations (SILSO). This study marks the first complete analysis of SC24 using the KSO sunspot groups’ data. Our goal is to extend the previous analysis of DR from the KSO data to the present, especially because the Solar Optical Observing Network/United States Air Force/National Oceanic and Atmospheric Administration data set (SOON/USAF/NOAA) and Debrecen Photoheliographic Data (DPD) catalogues do not provide data after 2018.