An active region quasi-biennial oscillation

A quasi-biennial fluctuation has been detected in the time-series of the sunspot umbra/penumbra (U/P) area ratio on the basis of the Debrecen Photoheliographic Data. The study is based on an intermittent period of nearly eight years; the material comprises more than 18,000 individual sunspots. The present contribution reports preliminary pieces of information about the temporal behaviour of the U/P ratio and period analyses. The physical background of the phenomenon is as yet unclear but it seems to belong to the growing family of mid-term fluctuations.