Solar cycle prediction of geomagnetic activity

The vast majority of solar cycle predictions focus on the 11-year sunspot cycle, while space weather and geomagnetic activity predictions are typically made for short time scales, from hours up to a month. Here, we aim to predict geomagnetic activity in the solar cycle time scale. We use a 180-year composite of the geomagnetic aa index and fit each aa cycle between two successive sunspot minima with a parameterized asymmetric Gaussian curve. We show that this curve can be represented with two free parameters and the model closely depicts the cyclic behavior of aa index in about 5-year timescale. However, it is unable to accurately represent shorter term variability.

We show how the two parameters can be forecasted using past aa values and a recently developed sunspot prediction model. Employing these estimated parameter values, we hindcast each past aa cycle from Solar Cycle 10 onwards and make a prediction for Solar Cycle 25, also estimating the uncertainties. Each cycle prediction is made at the time of minimum aa starting the respective cycle.

For Solar Cycle 25, our prediction gives the (5-year smoothed) aa index maximum of 21+/-3 nT, which is slightly higher than, e.g., in Cycle 24 (18.9). However, our model suggests that the overall aa cycle maximum has already been reached quite early in the cycle in July 2022. This suggests that Solar Cycle 25, similarly to Solar Cycles 11 and 13, will probably not have a strong, long-lasting peak of geomagnetic activity in the late declining phase.