What is solar cycle minimum?

Establishing the time of minimum between cycles 22 and 23 is complicated because there are two periods of low solar activity during 1996. To resolve this controversy, we studied the time of minimum in terms of the historical basis for defining this fiducial point in the solar cycle using several measures of solar activity, as well as the cycle membership of active regions observed during the minimum phase between these two cycles. Our conclusion is that cycle minimum is not defined solely on the basis of the occurrence of the minimum in the smoothed sunspot number, but rather by several additional parameters, including the monthly (or rotationally) averaged sunspot number, the number of regions, and the number of spotless days. Using these specific measures of solar activity, we recommend that the minimum between cycles 22 and 23 occurred in September 1996 and not in May 1996. (Author)