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Issue A&A Volume 370, Number 2, May I 2001 Page(s) L31 - L34 Section Letters DOI 10.1051/0004-6361:20010319

A&A 370, L31-L34 (2001)
DOI: 10.1051/0004-6361:20010319

Was one sunspot cycle lost in late XVIII century?

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 30 January 2001 / Accepted 1 March 2001)

Abstract
We suggest that one solar cycle was lost in the beginning of the Dalton minimum because of sparse and partly unreliable sunspot observations. So far this cycle was combined with the preceding activity to form the exceptionally long solar cycle #4 in 1784-1799, leading to an irregular phase evolution of sunspot activity (known as a phase catastrophe) and other problems. We reanalyze the available group sunspot numbers and suggest that solar cycle #4 was in fact a superposition of two cycles: a normal cycle in 1784-1793 ending at the start of the Dalton minimum and a new weak cycle in 1793-1800 which was the first full cycle within the Dalton minimum. Including the new cycle resolves the problems mentioned above and leads to a consistent view of sunspot activity around the Dalton minimum. Moreover, it will restore the Gnevyshev-Ohl rule of cycle pairing throughout the 400-year interval of sunspot observations.

Key words: Sun: activity -- Sun: sunspots -- Sun: Dalton minimum -- Sun: solar cycle

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

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