A New Look at Neon-C and SEP-Neon

Studies of the isotopic composition of neon in lunar soils, meteorites, and interplanetary dust particles have revealed several distinct components. In addition to implanted solar wind, which has a ^(20)Ne/^(22)Ne-abundance ratio of 13.7, there is an additional component with ^(20)Ne/^(22)Ne≈11.2, originally attributed to higher-energy solar energetic particles. Using data from the Advanced Composition Explorer, we have measured the fluence of solar wind, suprathermal particles, solar energetic particles and cosmic rays from ~0.3 keV/nucleon to ~300 MeV/nucleon over an extended time period. We use these measured spectra to simulate the present-day depth distribution of Ne isotopes implanted in the lunar soil. We find that the suprathermal tail of the solar wind, extending from a few keV/nucleon to several MeV/nucleon with a power law spectrum, can produce ^(20)Ne/^(22)Ne abundance ratios in the lunar soil that are similar to the measured composition, although there remain significant questions about the extent to which the present-day intensity of suprathermal ions is sufficient to explain the lunar observations

A Direct Measurement of the Geomagnetic Cutoff for Cosmic Rays at Space Station Latitudes

We report new measurements of the vertical geomagnetic cutoff for cosmic rays with rigidities from ~500 to 1700 MV, made using data from the MAST instrument on SAMPEX. A total of ~10,000 nuclei were used to measure the latitude cutoff in nineteen separate rigidity intervals. These results show that cosmic rays and solar particles can penetrate several degrees lower in latitude than would be estimated from commonly used relations for the geomagnetic cutoff, which has implications for the radiation exposure expected on the Space Station. An excellent fit to our measured cutoffs is given by the relation Rc = 15.062cos4 (Λ) - 0.363 GV, where Rc is the geomagnetic cutoff in rigidity, and λ is the invariant latitude. We suggest that this relation is useful over invariant latitudes from Λ = 0° to 64°, corresponding to rigidity cutoffs from ~0.2 to 15 GV

The Phosphorus, Sulfur, Argon, and Calcium Isotopic Composition of the Galactic Cosmic Ray Source

Galactic cosmic ray (GCR) measurements of the phosphorus, sulfur, argon, and calcium isotopes made by the Cosmic Ray Isotope Spectrometer aboard the Advanced Composition Explorer are reported over the energy range from ~100 to ~400 MeV nucleon^(–1). The propagation of cosmic rays through the Galaxy and heliosphere is modeled with constraints imposed by measurements. Isotopic source abundance ratios ^(31)P/^(32)S, ^(34)S/^(32)S, ^(38)Ar/^(36)Ar, and ^(44)Ca/^(40)Ca are deduced. The derived ^(31)P/^(32)S ratio is 2.34 ± 0.34 times larger than the solar system value, lending further credence to the suggestion that refractory elements are enriched in the GCRs due to the sputtering of ions off grains in the cores of superbubbles. By determining the GCR source abundances of argon (a noble gas) and calcium (a refractory), it is determined that material in grains is accelerated to GCR energies a factor of 6.4 ± 0.3 more efficiently than gas-phase material in this charge range. With this information, the dust fraction of phosphorus and sulfur in the interstellar material that is mixed with stellar ejecta to form the GCR seed material is found to be consistent with astronomical observations

Primary and secondary contributions to arriving abundances of cosmic-ray nuclides

The arriving abundances of a variety of cosmic-ray nuclides consist of comparable amounts of primary material produced by stellar nucleosynthesis and secondary matter resulting from fragmentation of heavier nuclei by collisions during interstellar propagation. In order to utilize such species in studies of cosmic-ray source composition it is necessary to determine the secondary fraction present in the arriving material and to assess the uncertainty in this determination. We have extracted the primary and secondary contributions to the arriving abundances for isotopes of elements between B and Ni by using 1) measurements of cosmic-ray elemental and isotopic composition obtained from the Cosmic Ray Isotope Spectrometer (CRIS) instrument on the Advanced Composition Explorer (ACE) spacecraft, 2) a data base of measured and calculated fragmentation cross sections, and 3) a leaky box model of interstellar propagation. We present derived decompositions and discuss their implications for studies of the composition of cosmic-ray source material. This work was supported by NASA at Caltech (under grant NAG5-12929), JPL, Washington University, and GSFC

Heavy Ion and Electron Release Times in Solar Particle Events

Using data from the SIS and EPAM instruments on ACE we have measured the onset times of 6 - 88 MeV/nuc ions and 38 - 315 keV electrons in 11 solar energetic particle (SEP) events from 1997 through 2002. We find that heavy ions are generally released later than electrons, by as much as ∼50 minutes. There is an apparent correlation between the release times (and the inferred release distances) and the 3He/4He ratio

The P, S, Ar, and Ca isotopic composition of the galactic cosmic ray source

Galactic cosmic ray (OCR) measurements of the phosphorus, sulfur, argon, and calcium isotopes made by the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA's Advanced Composition Explorer are reported aver 1he energy range from ~100 to ~400 MeV/nucleon. The propagation of cosmic rays through the Galaxy and heliosphere is modeled to determine isotopic source abundance ratios ^(31)P/^(32)S, ^(34)S/^(32)S, ^(38)Ar/^(36)Ar, and ^(36)Ar/^(40)Ca. By deriving the OCR source abundance of argon (a noble gas) and calcium (a refractory), it is determined that material in grains is accelerated to OCR energies a factor of ~6.4 more efficiently than gas-phase material in this charge range. With this information the interstellar dust function of phosphorus and sulfur at the cosmic ray source is shown to be consistent with astronomical measurements of hot galactic environments