A prediction of geomagnetic activity for solar cycle 23

Using a database of 13 solar cycles of geomagnetic aa data, we obtained cor relations between cycle averages of geomagnetic activity (and sunspot numbe r) and the numbers of days with disturbance levels above certain aa thresho lds. We then used a precursor-type relation to predict an average aa index of 23.1 nT for cycle 23 and inserted this average aa value into the above c orrelations to forecast the integral size distribution of geomagnetic activ ity for the new cycle. The predicted size distribution is similar to that o bserved for cycles 21 and 22 but most closely resembles that of solar cycle 18 (1944-1954), which was slightly smaller than cycles 21 and 22. Our pred iction agrees reasonably well with the "climatology-based" forecast made by the intergovernmental panel tasked to predict geomagnetic activity far the coming solar cycle and is significantly different from their "precursor-ba sed" prediction.