Precursor analysis and prediction of large‐amplitude relativistic electron fluxes

Abstract

An analysis of large‐amplitude relativistic electron fluxes, Je, is given using models that predict their appearance based on solar wind precursors. It has been noted that relativistic electron flux bursts in the magnetosphere are associated with high‐speed solar wind streams. This observation is used to motivate several models that quantify the association in terms of 2 × 2 contingency tables. For each model, the minimal cost structure for which the model would be useful is computed as a function of large threshold values of Je. The first model is based on the observation that a threshold crossing in the daily averaged solar wind velocity, VSW, tends to precede large relativistic electron fluxes. The optimal ratio of correct to false alarms forecasts found using this algorithm is 18:4 for a threshold corresponding to amplitudes of Je at L = 4.4 above Jc = 103 particles/str·cm2·s (corresponding to 110 total events). The second model allows for jumps in the solar wind to be an event indicator and yields slight improvements in the forecast ratio for larger values of Jc. The dependence of the optimal forecast ratio on L shell is also considered. It is shown that there are L values for which a threshold crossing of the daily average of VSW from below to above 600 km/sec is a sufficient condition for the appearance of large‐amplitude relativistic electron fluxes on one of the following three days. It is also shown that the condition of a threshold crossing of VSW above 600 km/s is not a necessary condition, because ≃80% of events were not preceded by this condition.