The lost sunspot cycle: Reanalysis of sunspot statistics

We have recently suggested (Usoskin et al., 2000) that one low sunspot cycle was possibly lost in 1790s, and argued (Usoskin et al., 2002) that the existence of such a cycle does not contradict with available solar proxies, like auroral observations and cosmogenic isotopes. However, some arguments based on a statistical analysis of sunspot activity have been presented against the lost cycle (Krivova et al., 2002). Since the consequences of a new cycle are significant for solar cycle studies, it is important to try to estimate the probability of such a cycle to exist. Here we present the results of a rigorous statistical analysis of all available sunspot observations around the suggested additional cycle minimum in 1792-1793. We show that the level of sunspot activity in 1792-1793 is statistically similar to that in the minimum phase, but significantly different from that in the mid-declining or maximum phases. Using the estimated uncertainties we also calculate new, weighted annual values of group sunspot numbers in 1790-1796 which show a clear minimum in 1792-1793 and a maximum in 1794-1795, supporting the idea of an additional weak cycle in 1790's.