The rate of areal decrease of long-lived sunspots

The analysis of 30 long-lived ``naked'' sunspots, observed in the years 1969 -1976 (Tab. 1), and of 30 sunspots of the Zurich H-type, observed during the same period (Tab. 2), yielded the following results: 1. Long-lived ``naked'' sunspots occur in all size categories ranging from small (U = 20 millionths of the visible solar hemisphere) to large sunspots (U = 110 millionths of the visible solar hemisphere). The Zurich H-type sunspots were of medium size (shown in Figs 4-6). Both groups of sunspots represent the final stage of the evolution of an active region. These sunspots are unipolar and have the same magnetic polarity as their neighbourhood. 2. The rate of decrease of the area of umbras of long-lived ``naked'' sunspots per day can be expressed by Eq (1), for the Zurich H-type sunspots by Eq (3). Linear relations of the decrease of areas of whole sunspots W were determined for both groups of sunspots and are expressed by Eqs (2) and (4). They indicate that the larger the initial area U$<sub>o</sub>$ of the sunspot, the larger the absolute value of the daily areal decrease of the sunspot. 3. The average daily relative rate of decay of the ``naked'' sunspots amounts to two hundredths of their initial area (Fig. 3). The average daily relative rate of areal decrease of the Zurich H-type sunspots is equal to two tenths of the area of the initial umbra (Fig. 6). The rate of decay of old unipolar sunspots, assuming they have the same area, depends on another parameters which can so far only be evaluated qualitatively. This parameter is the intensity of the sunspot's interaction with its ambient medium. The interaction mechanism is still not clear, but the ``naked'' sunspots form a group with an extremely low interaction intensity (Fig. 7).