Abundances and Energy Spectra of Corotating Interaction Region Heavy Ions Observed during Solar Cycle 23

Abstract

Using instruments on the ACE spacecraft, we surveyed the heavy-ion spectra and composition over the range He-Fe for 41 corotating interaction regions (CIRs) during 1998-2007. Below ∼1 MeV nucleon-1 the spectra are power laws in kinetic energy nucleon-1 with an average spectral index of 2.51±0.10, rolling over above ∼1 MeV nucleon-1 to power-law spectra with an average index of 4.47±0.17. The spectral shapes for different species are similar, leading to relative abundances that are constant over our energy range, even though the intensities cover up to 8 orders of magnitude. Relative to oxygen, the measured abundances at 385 keV nucleon-1 for 4He, C, N, Ne, Mg, Si, S, Ca, and Fe are 273±72, 0.760±0.023, 0.143±0.005, 0.206±0.009, 0.148±0.006, 0.095±0.005, 0.028±0.002, 0.007± 0.001, and 0.088±0.007, respectively. Except for an overabundance of 4He and Ne, the abundances are quite close to that of the fast solar wind. We have found 3He/4He ratios to be enhanced over solar wind values in ∼40% of the CIRs. The Fe/O ratio in individual CIRs is observed to vary over a factor of ∼10 and is strongly correlated with the solar wind Fe/O ratio measured 2-4 days preceding each CIR. Taken together with previous studies showing the presence of pickup He+ in CIRs, the observational data provide evidence that CIR energetic particles are accelerated out of a suprathermal ion pool that includes heated solar wind ions, pickup ions, and remnant suprathermals from impulsive solar energetic particle events. © 2008. The American Astronomical Society. All rights reserved.