On the size distribution of sunspot groups in the Greenwich sunspot record 1874-1976

We investigate the size distribution of the maximum areas and the instantaneous distribution of areas of sunspot groups using the Greenwich sunspot group record spanning the interval 1874-1976. Both distributions are found to be well described by log-normal functions. Using a simple model we can transform the maximum area distribution into the instantaneous area distribution if the sunspot area decay rates are also distributed log- normally. For single-valued decay rates the resulting snapshot distribution is incompatible with the observations. The current analysis therefore supports the results of Howard (1992, Sol. Phys., 137, 51) and Martinez Pillet et al. ( 1993, A&A, 274, 521). It is not possible to distinguish between a linear and a quadratic decay law, however, with the employed data set.