2006A&A...460..875P

It has recently been claimed that analysis of Greenwich sunspot data over 120 years reveals that sunspot activity clusters around two longitudes separated by 180° (``active longitudes'') with clearly defined differential rotation during activity cycles. In previous work we demonstrated that such effects can be observed in synthetic data without such features, as an artefact of the method of analysis. In the present work we extend this critical examination of methodology to the actual Greenwich sunspot data and also consider newly proposed methods of analysis claiming to confirm the original identification of active longitudes. We performed fits of different kinematic frames onto the actual sunspot data. Firstly, a cell-counting statistic was used to analyse a comoving system of frames and show that such frames extract useful information from the data. Secondly, to check the claim of century-scale persistent active longitudes in a contramoving frame system, we made a comprehensive exploration of parameter space following the original methodology as closely as possible. Our analysis revealed that values obtained for the parameters of differential rotation are not stable across different methods of analysis proposed to track persistent active longitudes. Also, despite a very thorough search in parameter space, we were unable to reproduce results claiming to reveal the century-persistent active longitudes. Previous parameter space exploration has been restricted to frames whose latitudinal profile is opposite to solar surface differential rotation. Relaxing this restriction we found that the highest values of nonaxisymmetry occur for frames comoving with the solar surface flow. Further analysis indicates that even these solutions are the result of purely statistical fluctuations. We can therefore say that strong and well substantiated evidence for an essential and century-scale persistent nonaxisymmetry in the sunspot distribution does not exist.