On the Relation between Moving Magnetic Features and the Decay Rates of Sunspots

Cornelis Zwaan
1974 Symposium - International astronomical union  
Bumba (1963) inferred from plots of areas of sunspot groups against time that stable spots may show a phase of slowest decay which is independent of the spot's area. From the Greenwich Photoheliographic Results we analysed the areas of individual spots. Indeed there is a clear lower limit in the decay rates close to Bumba's value dA/dt≃ −4.0 × 10−6visible hemisphere/dayHowever, only a small fraction of regular spots and an even smaller fraction of the irregular spots pass through the phase of slowest decay.
doi:10.1017/s0074180900069114 fatcat:jyoa6otaijbpvan2ezaqyinncq