The energy dependence of relative ion abundances during quiet-time periods in the 23rd solar cycle

Abstract

The origin of 0.03-1 MeV/nucleon ions is investigated on the basis of ULEIS (ACE) data during quiet time periods in 1998-2011. The energy spectra of 3He, 4He, O, and Fe ions and the energy dependence of their relative abundances are obtained. The comparison of Fe/O abundance ratios during quiet time periods with the average relative ion abundances in various phenomena of solar activity shows that the ion fluxes tend to split into 3 groups. Over the entire solar cycle excluding minimum the Fe/O values corresponded to those observed in impulsive solar energetic particle (SEP) events (group I) and in the solar corona (group II). Near the activity minimum, however, the Fe/O values were usually near their solar wind values (group III). The energy spectra of suprathermal ions and the energy dependence of ion abundance ratios in different groups are also different depending on the ion first ionization potential. These results suggest that various acceleration mechanisms are at work in the different groups. The nearly constant values Fe/O 1 and 3He/4He 0.065 up to 0.8 MeV/nucleon obtained for ion group I possibly result from ion acceleration in small impulsive SEP events rich in Fe and 3He. This ratio Fe/O is about 15 times higher than the photospheric value and corresponds to the Fe abundance in the upper corona in old active regions. We suggest that ions in these coronal regions represent the population accelerated in impulsive SEP events.