Speaker
Description
Most predictions of space climate, that is, the long-term behavior of the solar-terrestrial environment, have focused on forecasting the 11-year sunspot cycle. Geomagnetic activity, on the other hand, has mainly been predicted in shorter, space weather timescales of up to days to weeks. Using a 180-year composite aa index, we aim to predict here the temporal behavior of geomagnetic activity over the last 16 solar cycles. By identifying activity peaks in the ascending and declining phases of the cycle and an activity minimum between the two, we represent each aa cycle with two triangular peaks. The large-scale features of the aa cycle depicted by the model are related to changes in the occurrence of coronal mass ejections and high-speed solar wind streams which drive geomagnetic activity. Using past aa and sunspot observations, as well as a recent sunspot cycle prediction model, we find interesting relationships for the predictability of the aa peak amplitudes and timings, which suggest intrinsic differences between even and odd cycles and give strong support to the so-called Gnevyshev-Ohl rule ordering of cycles to even-odd cycle pairs. Finally, we attempt to hindcast each past aa cycle, including the ongoing cycle 25, while also estimating the prediction uncertainty using a leave-one-out cross-validation methodology. Prediction of a new cycle is made at the time of aa minimum at the start of the cycle, which occurs typically a few months after the sunspot minimum.