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Published May 10, 2008 | Published
Journal Article Open

Abundances and energy spectra of corotating interaction region heavy ions observed during solar cycle 23


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.

Additional Information

© 2008 The American Astronomical Society. Received 2007 November 5; accepted 2008 January 17. We thank the many individuals at the University of Maryland and the Johns Hopkins Applied Physics Laboratory who constructed the ULEIS instrument. We thank the ACE SWEPAM and MAG teams for solar wind speed and magnetic field data used here, as well as their list of transients and disturbances maintained on the ACE Science Center Web site. J. Giacalone kindly provided the numerical fit data used in Figure 12. Sunspot numbers were obtained from the NOAA Web site at the National Geophysical Sciences Data Center, ftp://ftp.ngdc.noaa.gov/STP/ SOLAR_DATA/SUNSPOT_NUMBERS. This work was supported in part by NASA under grant NNG04GJ51G at the Johns Hopkins University Applied Physics Laboratory.

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