Observations of nuclei heavier than iron in the primary cosmic radiation

Charge and energy spectra of primary cosmic rays made with large area Cerenkov scintillation counter on baloon flights - heavier than iron nucle

The Ionic Charge State Composition at High Energies in Large Solar Energetic Particle Events in Solar Cycle 23

The ionic charge states of solar energetic particles (SEPs) depend upon the temperature of the source material and on the environment encountered during acceleration and transport during which electron stripping may occur. Measurements of SEP charge states at relatively high energies (≳15 MeV/nucleon) are possible with the Mass Spectrometer Telescope (MAST) on the Solar, Anomalous, and Magnetospheric Particle Explorer satellite by using the Earth's magnetic field as a particle rigidity filter. Using MAST data, we have determined ionic charge states of Fe and other elements in several of the largest SEP events of solar cycle 23. The charge states appear to be correlated with elemental abundances, with high charge states (~20 for Fe) for all elements in large Fe-rich events. We review the geomagnetic filter technique and summarize the results from MAST to date, with particular emphasis on new measurements in the very large 14 July 2000 SEP event. We compare the charge states determined by MAST with other measurements and with those expected from equilibrium calculations

Observations of geomagnetic cutoff variations during solar energetic particle events and implications for the radiation environment at the Space Station

Data from the polar-orbiting Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite have been used to measure the location of the geomagnetic cutoff for low-energy protons and alpha particles during several large solar energetic particle events from mid-1992 to late 1998. When fluxes are sufficiently high, the cutoff latitude can be measured up to four times per orbit, allowing the variability of the cutoff to be observed on relatively short timescales. We find significant changes in the cutoff location, often by more than 5° in less than 1 day, and these changes are well correlated with geomagnetic activity as measured by either Dst or Kp. Spacecraft in intermediate-inclination orbits such as the International Space Station (ISS) graze the geomagnetic polar cap at certain longitudes each day. Calculations show that a 5° suppression in the average geomagnetic cutoff increases by more than a factor of 2.5 the time that the ISS spends in the polar cap exposed to energetic particles. Since the Station is only vulnerable at certain longitudes, however, real-time monitoring of the cutoff location from a polar-orbiting spacecraft could be used to provide advance notice of the polar cap location and conditions, sometimes hours before the Space Station itself reaches high magnetic latitudes

Geomagnetic Cutoff Variations During Solar Energetic Particle Events - Implications for the Space Station

Measurements of the time-variability of the location of the geomagnetic cutoff during the large solar energetic particle events of October and November 1992 using count rate data from the polar orbiting SAMPEX satellite are reviewed. Significant changes in the cutoff location of up to ~ 5° in less than one day are observed, even during periods with only moderate geomagnetic disturbances. We discuss the implications of such variations for the radiation hazard at the International Space Station, and we note that real-time monitoring of the cutoff location might be u~ed to provide a warning of the increased radiation levels, sometimes hours before the Space Station itself reaches high magnetic latitudes

Solar energetic particle characteristics and their dependence on longitude in solar cycle 24

In previous solar cycles, most studies examining the longitude dependence of solar energetic particle (SEP) event characteristics (such as composition and spectral hardness) have involved statistical analysis of single-point measurements. With the significant separation between the two STEREO and near-Earth spacecraft during solar cycle 24, these SEP characteristics can be examined simultaneously from multiple vantage points. Using SEP measurements from sensors on STEREO and ACE, we have examined the longitude dependence of the Fe/O abundance ratio at 10 MeV/nuc and the oxygen spectral index for energies above 10 MeV/nuc. Longitudinal patterns were sought that support or refute the scenarios put forth by Tylka et al. and Cane et al. to explain the Fe-enriched large SEP events of cycle 23. Unfortunately few Fe-enriched events have occurred in cycle 24 and their longitudinal behavior is not entirely consistent with either of the proposed scenarios

Role of Flares and Shocks in Determining Solar Energetic Particle Abundances

We examine solar energetic particle (SEP) event-averaged abundances of Fe relative to O and intensity versus time profiles at energies above 25 MeV/nucleon using the SIS instrument on ACE. These data are compared with solar wind conditions during each event and with estimates of the strength of the associated shock based on average travel times to 1 AU. We find that the majority of events with an Fe to O abundance ratio greater than two times the average 5–12 MeV/nuc value for large SEP events (0.134) occur in the western hemisphere. Furthermore, in most of these Fe-rich events the profiles peak within 12 hours of the associated flare, suggesting that some of the observed interplanetary particles are accelerated in these flares. The vast majority of events with Fe/O below 0.134 are influenced by interplanetary shock acceleration. We suggest that variations in elemental composition in SEP events mainly arise from the combination of flare particles and shock acceleration of these particles and/or the ambient medium

Heavy-ion Fractionation in the Impulsive Solar Energetic Particle Event of 2002 August 20: Elements, Isotopes, and Inferred Charge States

Measurements of heavy-ion elemental and isotopic composition in the energy range ~12-60 MeV nucleon^(–1) are reported from the Advanced Composition Explorer/Solar Isotope Spectrometer (ACE/SIS) instrument for the solar energetic particle (SEP) event of 2002 August 20. We investigate fractionation in this particularly intense impulsive event by examining the enhancements of elemental and isotopic abundance ratios relative to corresponding values in the solar wind. The elemental enhancement pattern is similar to those in other impulsive events detected by ACE/SIS and in compilations of average impulsive-event composition. For individual elements, the abundance of a heavy isotope (mass M_2) is enhanced relative to that of a lighter isotope (M_1) by a factor ~(M_(1)/M_2)^α with α ≃ 15. Previous studies have reported elemental abundance enhancements organized as a power law in Q/M, the ratio of estimated ionic charge to mass in the material being fractionated. We consider the possibility that a fractionation law of this form could be responsible for the isotopic fractionation as a power law in the mass ratio and then explore the implications it would have for the ionic charge states in the source material. Assuming that carbon is fully stripped (Q_C = 6), we infer mean values of the ionic charge during the fractionation process, Q_Z , for a variety of elements with atomic numbers 7 ≤ Z ≤ 28. We find that Q_(Fe) ≃ 21-22, comparable to the highest observed values that have been reported at lower energies in impulsive SEP events from direct measurements near 1 AU. The inferred charge states as a function of Z are characterized by several step increases in the number of attached electrons, Z – Q_Z . We discuss how this step structure, together with the known masses of the elements, might account for a variety of features in the observed pattern of elemental abundance enhancements. We also briefly consider alternative fractionation laws and the relationship between the charge states we infer in the source material and those derived from in situ observations

Elemental and isotopic fractionation in 3He-rich solar energetic particle events

Using data from the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE) mission, heavy ion composition measurements have been made in 26^3He-rich solar energetic particle (SEP) events that occurred between 1998 and 2004. Relative abundances of 13 elements from C through Ni have been investigated, as have the isotopic compositions of the elements Ne and Mg. We find a general tendency for the abundances to follow trends similar to those found in gradual SEP events, in which fractionation can be represented in the form of a power-law in Q/M. However several deviations from this pattern are noted that may provide useful diagnostics of the acceleration process occurring in solar flares