Long-term variations in the growth and decay rates of sunspot groups

Abstract

Using the combined Greenwich (1874 – 1976) and Solar Optical Observatories Network (1977 – 2009) data on sunspot groups, we study the long-term variations in the mean daily rates of growth and decay of sunspot groups. We find that the minimum and the maximum values of the annually averaged daily mean growth rates are ¼52% day−1 and ¼183% day−1, respectively, whereas the corresponding values of the annually averaged daily mean decay rates are ¼21% day−1 and ¼44% day−1, respectively. The average value (over the period 1874 – 2009) of the growth rate is about 70% more than that of the decay rate. The growth and the decay rates vary by about 35% and 13%, respectively, on a 60-year time-scale. From the beginning of Cycle 23 the growth rate is substantially decreased and near the end (2007 – 2008) the growth rate is lowest in the past about 100 years. In the extended part of the declining phase of this cycle, the decay rate steeply increased and it is largest in the beginning of the current Cycle 24. These unusual properties of the growth and the decay rates during Cycle 23 may be related to cause of the very long declining phase of this cycle with the unusually deep and prolonged current minimum. A ¼ 11- year periodicity in the growth and the decay rates is found to be highly latitude and time dependent and seems to exist mainly in the 0± − 10± latitude interval of the southern hemisphere. The strength of the known approximate 33 – 44 year modulation in the solar activity seems to be related to the north-south asymmetry in the growth rate. Increasing and decreasing trends in the growth and the decay rates indicate that the next 2 – 3 solar cycles will be relatively weak.