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.