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 HeFe for 41 corotating interaction regions (CIRs) during 1998-2007. Below $1 MeV nucleon À1 the spectra are power laws in kinetic energy nucleon À1 with an average spectral index of 2:51 AE 0:10, rolling over above $1 MeV nucleon À1 to power-law spectra with an average index of 4:47 AE 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 À1 for 4 He, C, N, Ne, Mg, Si, S, Ca, and Fe are 273 AE 72, 0:760 AE 0:023, 0:143 AE 0:005, 0:206 AE 0:009, 0:148 AE 0:006, 0:095 AE 0:005, 0:028 AE 0:002, 0:007 AE 0:001, and 0:088 AE 0:007, respectively. Except for an overabundance of 4 He and Ne, the abundances are quite close to that of the fast solar wind. We have found 3 He/ 4 He 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.

STEREO Observations of Energetic Ions in Corotating Interaction Regions During the May 2007 Solar Events

“…The energetic ions from CIRs are also known to have very characteristic element abundances that distinguish them from other heliospheric energetic particle populations. The average heavy-ion composition of CIR elemental abundances is very close to the average fast solar wind composition (e.g., Mason et al, 2008a). It is known that shock acceleration models require energies well above the solar wind speed.…”

“…However, more recent observations (Sanderson et al, 1995;Torsti, Anttila, and Sahla, 1999;Malandraki et al, 2007Malandraki et al, , 2008, and statistical (Desai et al, 1998;Mason et al, 2008a) studies of corotating energetic particle phenomena suggest that sub-MeV particles from SEP events may provide a seed population for CIR acceleration. Such a sub-MeV particle population appears to be an attractive source since it can supply ions with energies above the threshold energy which is required for the ion acceleration by the CIR shocks.…”

STEREO and ACE Observations of CIR Particles

“…1 shows, the CIR proton spectra are quite steep at energies of several MeV, with spectral indices ranging from ~-3 to as soft as ~-6. A similar range of spectral indices at a comparable energy (0.91-3.6 MeV/nucleon) was found for He spectra in the recent CIR heavy ion survey of Mason et al [4,5]. Note that the two May 2007 SEP events had considerably harder spectra, especially compared with the CIRs immediately preceeding and following them.…”

“…2. Although heavier ion abundance ratios have been found to be nearly independent of energy over this same energy range [4,5], all 6 of these events show a significant energy dependence in the He/H ratio.…”

Evolution of suprathermal seed particle and solar energetic particle abundances

“…2), and our strict quiet-time criteria may eliminate gradual SEP contributions. Mason et al [33] . Also indicated are average abundance ratios for coronal [32], slow solar wind [6], gradual SEP [9], and CIR material [33,9].…”