Concepts of medium-range (1–3 days) geomagnetic forecasting

To make deterministic medium-range (1 to 3 days ahead) forecasts would require prediction of the detailed near-Earth solar-wind conditions – including the out-of-ecliptic interplanetary magnetic field component – days ahead from remote observations of the inner heliosphere. This is, however, not feasible and will not be in the foreseeable future. Nevertheless, important steps can be taken towards a more physics based approach to geomagnetic forecasting. In this report from an ongoing study within the ESA Space Weather Applications Pilot Project, we discuss presently used concepts and new ideas for fully automatic provision of geomagnetic activity forecasts hours to days ahead. Observations of eruptive events on the Sun are nowadays routinely used as predictors of strong (Dst < −100 nT) magnetic storms, but these predictions are still hampered by many false alarms. We here suggest to use the flux of >10 MeV solar energetic particles as a discriminator of potentially geoeffective halo coronal mass ejections, substantially reducing the false alarm rate. We also describe how low speed/high speed solar-wind stream interfaces can be automatically detected in currently available solar-wind models, and point out that, in the absence of eruptive events on the Sun, these may be used as predictors of moderate (Dst < −50 nT) magnetic storms, effectively introducing a deterministic element into the mainly probabilistic forecast schemes.