Gradual SEP events:
- electrons up to tens of MeV, ions at GeV
- once per month near solar maximum
- dominated by protons (small e/p fluence ratios)
- variable composition and charge states
- extend over > 100° in solar longitude
- generally associated with large solar flares & fast CMEs
- believed to be due to acceleration by CME-driven shocks and not by flares.
Impulsive SEP events:
- Low energy electrons: 1-100 keV
- Ions: 0.01-1 MeV/nucleon
- >104 events per year near solar maximum
- large e/p ratios
- enhanced abundances of 3He and of heavy ions
- high charge states
- enhanced alpha/proton ratio
- extend over 30° in solar longitude
- associated with type III bursts
- many clearly come from flares/microflares, but often not even a microflare or soft X-ray burst is observed.
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.