OBSERVATIONS OF HYSTERESIS IN SOLAR-CYCLE VARIATIONS AMONG M SOLAR-ACTIVITY INDICATORS

We show that smoothed time series of 7 indices of solar activity exhib it significant solar cycle dependent differences in their relative var iations during the past 20 years. In some cases these observed hystere sis patterns start to repeat over more than one solar cycle, giving ev idence that this is a normal feature of solar variability. Among the i ndices we study, we find that the hysteresis effects are approximately simple phase shifts, and we quantify these phase shifts in terms of l ag times behind the leading index, the International Sunspot Number. O ur measured lag times range from less than one month to greater than f our months and can be much larger than lag times estimated from short- term variations of these same activity indices during the emergence an d decay of major active regions. We argue that hysteresis represents a real delay in the onset and decline of solar activity and is an impor tant clue in the search for physical processes responsible for changin g solar emission at various wavelengths.