Solar energetic particle flux enhancement as an indicator of halo coronal mass ejection geoeffectiveness

Abstract

Nearly all strong magnetic storms are generated by halo coronal mass ejections (CMEs), but most frontside halo CMEs are not followed by strong storms. Hence additional information is required to discriminate highly geoeffective CMEs from those less geoeffective. There is a tendency for the strongest magnetic storms to be generated by the fastest CMEs, and in the absence of detailed information on the internal structure of an observed CME, the speed is often used to evaluate the risk that a strong storm will follow. We here report on the alternative use of solar energetic particle (SEP) flux as an indicator of the geoeffectiveness of halo CMEs, addressing the question of whether SEP flux is better in this respect than CME speed. On the basis of a list of 137 frontside halo CMEs, we have investigated the relations between CME speed, SEP flux, and strong magnetic storms. We find that enhancements of the ≥10 MeV SEP flux close to CME onset can be used to indicate whether a full halo CME will be followed by Dst below −100 nT within 18 to 72 hours (our definition of a CME–strong magnetic storm association). Ranking the CMEs by speed and by SEP flux enhancement shows that the latter indicator results in a better discrimination between highly geoeffective CMEs and those less geoeffective. The results suggest that SEP flux enhancements may provide a more efficient discrimination than CME speed for any choice of discriminating thresholds, resulting in lower rates of both false alarms and missed predictions.