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ACE News #113 - Jan 18, 2008
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Corotating Interaction Regions (CIRs) arise when fast solar wind streams overtake slower solar wind in the inner heliosphere (<few AU). The interaction between the two streams leads to enhanced levels of turbulence and the eventual formation of shocks which propagate away from the stream interface in both outward (forward) and inward (reverse) directions. Energetic particles are often observed from CIRs, which occur throughout the solar activity cycle but are most prominent during the declining phase. We used ULEIS and SIS to survey 41 CIRs during 1997-2007 obtaining spectra up to a few MeV/nucleon in all cases, and up to >20 MeV/nucleon in the most intense events.
Below ~1 MeV/nucleon the spectra are power laws in kinetic energy/nucleon with average spectral index 2.51 ± 0.10, rolling over above ~1 MeV/nucleon to power law spectra with average index 4.47 ± 0.17 (see left figure). 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. Except for an overabundance of 4He and Ne, the abundances are quite close to the abundance 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 (see right figure). Taken together with previous studies showing the presence of pick-up He+ in CIRs, the observational data provides evidence that CIR energetic particles are accelerated out of a suprathermal ion pool that includes heated solar wind ions, remnant suprathermals from impulsive solar energetic particle events, and pick-up ions.
This item was contributed by G. M. Mason, R. A. Leske, M. I. Desai, C. M. S. Cohen, J. R. Dwyer, J. E. Mazur, R. A. Mewaldt, R. E. Gold, and S. M. Krimigis. Address questions and comments to
Last modified 18 Jan 2008.