Abstract
Abstract We analyze the temporal distribution of sunspot groups for even and odd cycles in the range SC12-SC24. It seems that cycle 24 is a characteristic even cycle, although with low amplitude. The number of large sunspot groups for cycle 24 is relatively smaller than for the average of both even and odd cycles SC12-SC23, and there is a deep decline of the large groups in the middle of the cycle. Temporal evolution of the sunspot groups of the even cycles is non-synchronous such that the northern hemisphere distribution of groups maximizes earlier that the southern hemisphere groups. This leads to a double-peak structure for the average even cycle. On the other hand, the distributions of the sunspot groups of odd cycles maximize simultaneously. We show that this double-peak structure intensifies the Gnevyshev gap (GG) for the even cycles, but is not its primary cause. On the contrary, we show that the GG exists for even and odd cycles, and separately on both hemispheres. We resample all cycles to have equal number of 3945 days and study the difference in the evolution of average total group area and average group area of the even and odd cycles separately. The analysis shows that there is a decline in both total area and average area in the even cycles 1445 days (about four years) after the beginning of the cycle, which is at least 99 % significant for both total and average area. The odd cycles do not have such a clear decline.
Highlights
Most of the studies of solar cycle 24 have been, far, predictions of the size and timing of the cycle
We show the distributions of sunspot groups of both hemispheres for all groups located at absolute latitude greater and smaller than 15 degrees for M set and Leussu et al [2017] (L set) of odd cycles SC12-SC23 in Figs. 6a and 6b, respectively
It seems that cycle 24 is a characteristic even cycle, with low amplitude and longer than average decline between the two maxima
Summary
Most of the studies of solar cycle 24 have been, far, predictions of the size and timing of the cycle. Choudhuri et al [2007] modelled the last few solar cycles by “feeding” observational data of the Sun’s polar magnetic field into their solar dynamo model Their results fitted to the observed sunspot numbers of cycles 21–23 reasonably well and predicted that cycle 24 will be about 35% weaker than cycle 23, which was extremely near the actual maximum. Richardson and Cane [2012a] and Richardson [2013] have studied the drop of geomagnetic disturbance during the maximum of the solar cycle They found, especially, a tendency for the Kp-index storm rate to fall during fourth year of the cycle. They associated several occasions of decline in geomagnetic activity aa-index around solar maximum to the Gnevyshev gap during solar cycles 20 - 24 [Richardson and Cane, 2012b].
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