Free access article

A&A 403, 743-748 (2003)
DOI: 10.1051/0004-6361:20030398

The lost sunspot cycle: Reanalysis of sunspot statistics

I. G. Usoskin¹, K. Mursula² and G. A. Kovaltsov<sup>3

1</sup>  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

(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.

Key words: Sun: activity -- Sun: sunspots -- Sun: solar-terrestrial relations

Offprint request: I. Usoskin, ilya.usoskin@oulu.fi

© ESO 2003

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