Differential Rotation Rates of Recurrent Sunspot Groups Lasting Two or Three Passages in the Debrecen Photoheliographic Data Catalogue

We investigate the angular rotation velocities of stable recurrent sunspot groups characterized by a leading unipolar sunspot with an initially well-developed penumbra, similar to the H or J types in the Zürich classification. These structures are tracked for two (class I) or three (class II) solar rotations. The Debrecen Photoheliographic Data sunspot catalogue (1977 – 2017) used in this study provides the daily positions and areas of observable sunspots and sunspot groups with great precision. This allows the calculation of the angular rotation synodic velocities from a least-squares fit to the sunspot positions over a given disk passage. After converting to sidereal coordinates, the velocities were used to obtain the solar rotation parameters via a least-squares fit to the solar differential rotation law. Comparison is made with the solar differential rotation laws obtained in two previous studies considering the same classes of sunspot groups, and over comparable time periods, using the data from the Greenwich Photoelectric Results (GPR) catalogue. We find that, on average, the sunspots exhibit a braking tendency, aligning with previous findings. The common results across the three studies, when examined in the context of simulations for sunspot formation and evolution, suggest a scenario in which recurrent unipolar sunspots are anchored at a shallow subsurface layer. The observed braking effect is attributed to gradual fragmentation, leading to disconnection and a transition to dynamics increasingly influenced by surface flows.