Abstract
The vast majority of solar cycle predictions focus on predicting the
11-year sunspot cycle, while space weather and geomagnetic activity
predictions are largely 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 the model
reasonably describes the cyclic behavior of $aa$ using only two free
parameters. We present how these 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 using a leave-one-out
cross-validation methodology. Each cycle prediction is made at the time
of minimum $aa$ starting the respective cycle. For Solar Cycle 25, our
prediction gives the $aa$ index maximum of $21 \pm
3\, \mathrm{nT}$ (at the original $aa$
index level) early in the cycle in July 2022, suggesting that Solar
Cycle 25, similarly to Solar Cycles 11 and 13, will not have a strong,
long-lasting peak of geomagnetic activity in the late declining phase.