Gradual SEP events:

Impulsive SEP events:



Expected number of events

(To be verified for current sensitivity of STEP & update the table for current mission phases definitions. Also per orbit.)

Observations from the Wind 3DP instrument over the solar cycle 1995 -2006 show that the occurrence frequency distribution for impulsive electron events observed at ~ 1AU follows a power law with an average exponent of –1.4, with the rate varying with solar activity. In 2004-2005 and 1995-1998, the same near-solar minimum phase as the SolO prime mission, WIND detected 376 events. Taking into account that STEP is about 50 times more sensitive than Wind 3DP in the key ~2-20 keV energy range, and assuming a conservative inverse-square radial dependence of the peak electron flux and an average radial distance of ~0.7 AU for SolO, we expect STEP to detect ~2400 impulsive electron events during the prime mission, with about 50 events during closest approach (<~0.4 AU, about 142 days around solar minimum). We estimate that >~90% of these impulsive electron events will be accompanied by a solar type III radio burst, and STIX will provide X-ray locations for more than half, or ~25 events during closest approach, and ~1000 events over the nominal mission.