Fractionation of solar energetic particles and solar wind according to first ionization potential

Abstract

Although it is well known that solar energetic particles are depleted in elements with first ionization potential (FIP) greater than ∼10 eV, it is less well known that the degree of FIP fractionation varies from event to event. Similar fractionation patterns and variations are observed in the solar wind, suggesting that these variations may have a common origin. We review evidence for the FIP-related fractionation of SEPs and compare it with FIP fractionation effects in the solar wind at 1 AU. On the basis of several significant differences between the solar wind and SEP compositions, we suggest that most solar energetic particles are not simply an accelerated sample of the average solar wind as observed at 1 AU; rather, solar particles and fast and slow solar wind appear to be distinct samples of coronal material with distinctly different FIP-fractionation patterns.