THE USE OF INTERPLANETARY SCINTILLATION MAPS IN THE PREDICTION OF GEOMAGNETIC-ACTIVITY

Interplanetary scintillation (IFS), the twinkling of small angular dia meter radio sources, arises from the interaction of the signal with sm all-scale plasma irregularities in the solar wind. The technique may b e used to sense remotely the near-Earth heliosphere and has potential for tracking large-scale interplanetary disturbances from close to the Sun to the Earth. Such observations might be useful within routine-ge omagnetic forecasts, and we use data from the Mullard Radio Astronomy Observatory to test this suggestion. A forecast was based on the visua l evaluation of each daily map. If an IFS event was observed then we p roposed that any associated geomagnetic activity would occur either on that day, or during the following two days. We consider the success o f these forecasts in predicting days when either an SSC/SI or an Ap va lue exceeding 30 were recorded. The identification of IFS events is ne cessarily subjective and so two observers compiled independent events lists; and the results were compared. Approximately half of the IFS ev ents in each list were followed by a geomagnetic signature but compari son of the two lists showed that different days were being chosen. We also found that the forecasts had very high false alarm rates. Since I FS is sensitive to a volume we did not expect all events to be associa ted with a geomagnetic signature. However, the technique failed to for ecast a large proportion of geomagnetic events and the association bet ween IFS events and geomagnetic activity is not much better than would be expected by chance. Comparing the IFS forecasts with forecasts of Ap released by the Space Environment Services Center (SESC) we found t hat SESC correctly predicted a similar proportion of days when Ap grea ter than or equal to 30, but that the performance was significantly be tter than would be expected by chance, and had a much lower false alar m rate. We conclude that these IFS data cannot be used alone to produc e reliable geomagnetic activity forecasts.