Criterion of solar energetic particle alerts based on soft X-ray flare data

Solar energetic particle (SEP) alert is one of the most important topic in space weather forecasts because SEP can cause serious radiation hazard to humans and equipment in space. We discuss our statistical investigations into the occurrence of proton events for SEP alerts in space weather forecasts. We analyzed soft X-ray flux and proton intensity data obtained with the GOES satellite in the 23rd solar cycle. We found that the soft X-ray fluence of almost all flares related to proton events exceeded a threshold value, which was about 20 ergs cm^-2. This means that there is a threshold in flare duration in regard to peak X-ray flux and this is about 30 minutes for an M1.0 flare and/or 3 minutes for an X1.0 flare. These results will provide some of the most important criteria for SEP alerts in the future. As a practical application of these findings, we discuss an operational scheme for SEP alerts.