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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
  • >10events 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. 




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