A&A 396, 235-242 (2002)
DOI: 10.1051/0004-6361:20021340

Was one sunspot cycle in the 18th century really lost?

N. A. Krivova^{1, 2}, S. K. Solanki¹ and J. Beer<sup>3

1</sup>  Max-Planck-Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany
    e-mail: natalie@linmpi.mpg.de; solanki@linmpi.mpg.de
²  On leave from Astronomical Institute, St. Petersburg University, 198504 St. Petersburg, Russia
³  Swiss Federal Institute of Environmental Science and Technology (EAWAG), 8600 Duebendorf, Switzerland
    e-mail: beer@eawag.ch

(Received 17 July 2002 / Accepted 12 September 2002)

Abstract
The unusually long 4th solar cycle has recently been proposed by Usoskin et al. (2001) to be composed of two cycles. They argue that a weak and short cycle might have been lost in sparse sunspot data at the end of the 18th century. Here we check this hypothesis in different ways. First, we consider the sunspot number record in greater detail and compare in a statistical sense the sunspot observations of the period in question with those at other times. In a statistical sense the sunspot numbers recorded at the time of the proposed new cycle minimum are extremely untypical for other minima in the solar cycle record, but quite usual for the declining phase of the solar cycle. We also analyse other available proxies of solar activity, such as variations of the cosmogenic nuclides ¹⁰Beand ¹⁴Cas well as auroral activity. These historical records are sufficiently long and provide an independent testimony of the cyclic behaviour of solar activity at the end of the 18th century. We found no evidence for a lost cycle in any of these data sets. Finally, we compare the proposed new cycle with the other cycles in the sunspot record. This reveals that the proposed "missing" cycle has very unusual properties, much more so than the original, standard cycle 4. Taken together, the evidence from these various tests strongly suggests that no cycle was missed and that the official sunspot cycle numbering and parameters are correct.

Key words: Sun: activity  -- Sun: magnetic fields -- sunspots

Offprint request: N. A. Krivova, natalie@linmpi.mpg.de

© ESO 2002

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