Anomalous cosmic ray composition from ACE

Abstract

Abstract. During solar quiet periods, the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE) measures the composition and energy spectra of anomalous cosmic rays (ACRs) with energies > 8 MeV/nucleon in interplanetary space at 1 AU. In particular, the spectra of individual isotopes for the ACR elements N, 0, and Ne are studied with SIS. Intensity enhancements are found in low energy 18 O and 22 Ne, with relative abundances of 18 O/ 16 O ~ 0.002 and 22 Ne/ 20 Ne ~ 0.1. The neon abundance ratio appears more similar to that found in the solar wind than in meteorites and is far below that determined for the galactic cosmic ray (GCR) source, indicating that GCRs contain material from sources other than just the local interstellar medium.

Isotopic abundances in the solar corona as inferred from ACE measurements of solar energetic particles

“…The isotopic composition is highly variable, but using the abundance correlations between different species we empirically correct for the variation and obtain preliminary coronal isotopic abundances from SEPs. Observations in a subset of the events over a more limited energy interval are also reported in [15,16].…”

“…Another complication not addressed in equation (1) is the fact that in gradual SEP events the abundances of elements with low first ionization potential (FIP), such as Fe, are generally enhanced over those with high FIP, such as O, by an amount which also varies from event to event [24,25]. In spite of the above considerations, a reasonable correlation has been shown between isotopic abundances and the Fe/O ratio [12,13,15], but uncertainties in the value of Q(Fe) at SIS energies make it difficult to directly compare the correlation with predictions. 26 Mg/ 24 Mg (right) isotopic ratios versus the Na/Mg elemental abundance ratio in each of the SEP events of Figure 1.…”

STEREO and ACE Observations of CIR Particles

“…As expected, each particle increase is associated with a high-speed stream. However, the converse is not true; often during this period a high-speed stream generates no detectable particle increase above a few MeV [7]. Sometimes a second, smaller particle increase is seen when the solar wind velocity is declining (e.g., panels 1-3 and 5); these generally have a harder spectrum ( Fig.…”

“…4) and the LET Ahead and Behind proton time profiles were nearly identical, but differences in both the timing and intensity are becoming greater as the spacecraft move farther apart. Closer examination [7] shows that CIR onsets early in the mission happened first at Ahead, sometimes by nearly 2 hours before appearing at Behind; since mid-2007 Behind is always first, sometimes by ~3 days. To first order this is as expected.…”

“…H and He spectra in 6 CIR events when ACE and STEREO were still close together in early 2007, covering energies from ~0.1 to >10 MeV/nucleon, are presented and discussed in [7]; the energy dependence of the He/H ratio in these events is shown here in Fig. 2.…”