A PREDICTION OF THE PEAK SUNSPOT NUMBER FOR SOLAR CYCLE 23 KHALED H. BOUNAR 1 , EDWARD W. CLIVER 2 and VALENT ´ IN BORIAKOFF 2 1 Radex, Inc., Bedford, MA 01730, U.S.A. 2 Air Force Research Laboratory, Hanscom AFB, MA 01731–3010, U.S.A. (Received 9 April 1997; accepted 21 May 1997) Abstract. We use a precursor technique based on the geomagnetic index during the decline (last 30%) of solar cycle 22 to predict a peak sunspot number of 158 ( 18) for cycle 23, under the assumption that solar minimum occurred in May 1996. This method appears to be as reliable as those that require a year of data surrounding the geomagnetic minimum, which typically follows the smoothed sunspot minimum by about six months. 1. Introduction An accurate prediction of the peak amplitude of solar cycle 23 is useful input for planners of upcoming low-Earth orbit space missions. The consensus view of the space physics community is that the maximum of cycle 23 will be relatively high, comparable to the maxima of cycles 21 (13-month smoothed sunspot number SSN 164 5) and 22 (158.5). A panel convened by the NOAA Space Environ- ment Center and the NASA Office of Space Science (Joselyn et al., 1997) surveyed the results of 28 predictions made by six separate techniques and found reasonable agreement for a peak SSN of 160 ( 30). A dissenting view has been offered by Schatten and Sofia (1996) (cf., Schatten, Myers, and Sofia, 1996) who call for a relatively weak cycle (by recent standards) with a peak SSN of 130 ( 30). The Schatten and Sofia prediction is based on the precursor technique pioneered by Ohl (1966) (see Ohl and Ohl, 1979, and references therein) and Brown and Williams (1969). It is generally accepted that the precursor method is the most reliable way to predict the peak SSN (Kunches, 1993; Thompson, 1993). Under this approach, solar and/or geomagnetic indices on the decline or near the minimum of a solar cycle are used to predict the peak SSN of the following solar cycle. Schatten et al. (1978) suggested a theoretical basis for precursor techniques in terms of the solar dynamo through which poloidal solar magnetic fields on the decay of one cycle are transformed via differential rotation into toroidal (sunspot) fields during the subsequent maximum. The poloidal fields are either measured by ground-based magnetographs or inferred from other solar or geomagnetic observations (see Lay- den et al., 1991, for a discussion of input parameters). Schatten and Sofia (1996) based their estimate on Stanford measurements of the solar polar magnetic field, but geomagnetic indices are more commonly used (e.g., Kane, 1989; Wilson, 1990), Also National Research Council, Washington DC, 20418, U.S.A. Solar Physics 176: 211–216, 1997. c 1997 Kluwer Academic Publishers. Printed in Belgium.