The dependence of solar energetic particle fluences on suprathermal seed-particle densities

Measurements during solar cycle 23 showed that most solar energetic particles (SEPs) are accelerated from a seed-population of suprathermal ions (e.g., >10 keV/nuc) rather than from the bulk solar wind. In this case the SEP fluence should depend on the pre-existing density of suprathermal ions. Lacking near-Sun measurements of suprathermal ion densities we have used ACE/ULEIS daily-average densities of suprathermal Fe at 1 AU during 1998-2005 as a proxy. We find that the maximum Fe daily-average SEP fluences measured by ACE/SIS are apparently limited by the pre-existing suprathermal number density. Similarly, large fluences of Fe in solar energetic particle events only occurred when there was a pre-existing high density of suprathermal Fe. We conclude that in situ suprathermal ion data can play a key role in estimating the probability of large SEP events, or in forecasting all-clear periods.