The lost sunspot cycle: Reanalysis of sunspot statistics

¹ Sodankylä Geophysical Observatory (Oulu unit), 90014 University of Oulu, Finland
² Department of Physical Sciences, 90014 University of Oulu, Finland e-mail: kalevi.mursula@oulu.fi
³ Ioffe Physical-Techical Institute, 194021 St. Petersburg, Russia e-mail: gena.kovaltsov@pop.ioffe.rssi.ru


Corresponding author: I. Usoskin, ilya.usoskin@oulu.fi

Received: 28 January 2003
Accepted: 11 March 2003

Abstract

We have recently suggested that one low sunspot cycle was possibly lost in 1790s (Usoskin et al. 2001, A&A, 370, L31). In this paper we present the results of a rigorous statistical analysis of all available sunspot observations around the suggested additional cycle minimum in 1792–1793. First we estimate the uncertainty of a monthly mean sunspot number reconstructed from a single daily observation. Then we compare, using quantitative statistical tests, the average level of sunspot activity in 1792–1793 with the average activity during the minimum, mid-declining and maximum phases of cycles in the well-measured reference period 1850–1996. We show that, contrary to the results by Krivova et al. (2002), the level of sunspot activity in 1792–1793 is statistically similar to that in the minimum phase, and significantly different from that in the mid-declining and maximum phases. Using the estimated uncertainties, we also calculate new, weighted annual values of 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.