Geomagnetic activity and solar cycles V.M. Silbergleit a,b, * , P.A. Larocca a a Departamento de Fı ´sica, Facultad de Ingenierı ´a, Universidad de Buenos Aires, Av. Paseo Colo ´ n 850 – Piso 2, 1063-Buenos Aires, Argentina b CONICET of Argentina Received 19 October 2002; received in revised form 20 July 2003; accepted 7 August 2003 Abstract Multiple linear regression method is used to estimate the total number of geomagnetic storms per solar cycle and to predict cycle 23Õs maximum amplitude. A recurrence trend of 7 and 12 cycles are used as lagged values to estimate the amplitude of the current cycle, which resulted to be equal to 137 (±65) smoothed monthly mean sunspots. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Storms; Sunspots; Multiple linear regression method 1. Introduction In the past three decades, a growing group of solar forecasters has tried to predict the amplitude of next solar activity cycle with the precursors technique, pio- neered by the works of Soviet and British Astrophysi- cists in the sixties. As an useful input for space mission planning, the topic is still under development with the introduction of other forecasting methods. For the current cycle numerous studies to predict the maxi- mum solar activity level have been published (e.g. Schatten and Pesnell, 1993; Li, 1997; Bravo and Stew- art, 1997; Bounar et al., 1997; Jain, 1997; Badalayan et al., 2001). To predict solar cycle amplitude, useful for forecast- ing solar-terrestrial relations, many authors considered the ‘‘predictor technique’’. This method is based on correlative studies between selected parameters of phe- nomena observed on the Sun (or originating from the Sun) and geomagnetic disturbance indices, which are indicative of solar phenomena occurrence. Here it is used for the solar and the geomagnetic activity levels as they are represented by the sunspot numbers and the AA * index, covering the period 1868–2002 (solar cycles 11–23). Comparing the years when AA * and Ap values over- lap, it is shown that the relative frequency of storms per year is the same. Another reason to differentiate them is that an index derived from only two observatories clearly shows more extreme values, because single mea- surement places are affected by local magnetic distur- bances induced by the geomagnetic configuration. The aa indices are taken in a 3-h period; when their average exceeds 60 nT a magnetic storm is regarded as in progress. It is considered that a storm ceases when the mean value goes below 60 nT. The maximum value obtained is referred to as AA * . Moreover, we used the pre- dictor technique to re-obtain the number of storms oc- curred during the solar cycles 4–11. Once established, the series can be extrapolated to generate a prediction of future cycles. The ‘‘precursor method’’ has been used among oth- ers, by Thompson (1992). There exist different publica- tions on the forecast of the maximum amplitudes of the solar cycles using the most diverse methods (see Gle- issberg, 1942; Joselyn et al., 1997). 0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.08.084 * Corresponding author. Tel./fax: 54 11 4963 2062. E-mail address: vsilber@fi.uba.ar (V.M. Silbergleit). www.elsevier.com/locate/asr Advances in Space Research 36 (2005) 2384–2387