Effects of interplanetary transport on derived energetic particle source strengths

We study the transport of solar energetic particles (SEPs) in the inner heliosphere in order to relate observations made by an observer at 1 AU to the number and total energy content of accelerated particles at the source, assumed to be near the Sun. We use a numerical simulation that integrates the trajectories of a large number of individual particles moving in the interplanetary magnetic field. We model pitch angle scattering and adiabatic cooling of energetic ions with energies from 50 keV nucleon^(−1) to 100 MeV nucleon^(−1). Among other things, we determine the number of times that particles of a given energy cross 1 AU and the average energy loss that they suffer because of adiabatic deceleration in the solar wind. We use a number of different forms of the interplanetary spatial diffusion coefficient and a wide range of scattering mean-free paths and consider a number of different ion species in order to generate a wide range of simulation results that can be applied to individual SEP events. We apply our simulation results to observations made at 1 AU of the 20 February 2002 solar energetic particle event, finding the original energy content of several species. We find that estimates of the source energy based on SEP measurements at 1 AU are relatively insensitive to the mean-free path and scattering scheme if adiabatic cooling and multiple crossings are taken into account

Reduction and analysis of data from experiment CAI on the IMP-8 mission

The Caltech Electron/Isotope Spectrometer (EIS) on the Interplanetary Monitoring Platform 8 (IMP-8) has provided precise measurements of the energy spectra and time variations of low energy electrons (0.16 to 6 MeV), the isotopes of hydrogen and helium (approximately 2 to 40 MeV/nucleon), and the elements from lithium through oxygen (approximately 5 to 50 MeV/nucleon) in energetic particle fluxes of solar, galactic, interplanetary, and magnetospheric origin since 1973. The accomplishments that have resulted from EIS measurements during the period March 24, 1980 to December 31, 1984 are summarized

Solar cycle variations of the anomalous cosmic ray component

The intensity of the anomalous cosmic ray component, consisting of He, N, O, and Ne, has long been known to be especially sensitive to the effects of solar modulation. Following its discovery in 1972, this component dominated the quiet time flux of cosmic ray nuclei below approx. 30 MeV/nucleon during the 1972 to 1978 solar minimum, but then became essentially unobservable at 1 AU with the approach of solar maximum conditions. One recent theoretical model predicts substantial differences in the intensity of the anomalous fluxes from one solar minimum period to the next because of the reversal of the solar magnetic field. Using data from the Caltech experiments on IMP-8 and ICE (ISEE-3), the intensity of anomalous O and He at 1 AU during the years 1972 to 1985 is reported in particular. Whether the anomalous fluxes will return to their 1972-1978 levels, as predicted by spherically symmetric modulation models, or whether they will fail to return to 1 AU, as suggested by modulation models in which gradient and curvature drifts dominate are to be determined. The preliminary analysis of data from 1984 shows that the intensity of 8 to 27 MeV/nucleon O is still more than an order of magnitude below its 1972 to 1978 levels, while the intensity of 25 to 43 MeV/nucleon He is a factor of Approx. 8 below its maximum level in 1977

Isotope Abundances of Solar Coronal Material Derived from Solar Energetic Particle Measurements

Coronal isotopic abundances for the elements He, C, N, 0, Ne, and Mg are derived from previously published measurements of solar energetic particles by first measuring, and then correcting for the charge-to-mass-dependent fractionation due to solar flare acceleration and propagation processes. The resulting coronal composition generally agrees with that of other samples of solar system material, but the previously noted difference between the solar flare and solar wind ^(22)Ne/^(26)Ne ratios remains unresolved

Contribution to Cherenkov Resolution from Knock-on Electrons

Calculations of the mean and standard error of the added Cerenkov component from knock-on electrons for sample counters with refractive indices ranging from n = 1.03 to n = 1.49 are presented. We find that this contribution to the Cerenkov resolution is significant, but not dominant, for typical detector parameters

A Search for ^2H, ^3H, and ^3He in Large Solar Flares

The results of a new study of solar flare H and He isotopes imply that earlier observations have significantly overestimated the abundances of ^2H, ^3H, and ^3He in large solar flares. We find no evidence that solar flare nuclei have suffered any significant amount of fragmentation before escaping from the Sun

Isotope Abundances of Solar Coronal Material Derived from Solar Energetic Particle Measurements

Coronal isotopic abundances for the elements He, C, N, O, Ne, and Mg are derived from previously published measurements of the isotopic composition of solar energetic particles by first measuring, and then correcting for, the charge-to-mass-dependent fractionation due to solar flare acceleration and propagation processes. The resulting coronal composition generally agrees with that of other samples of solar system material, but the previously noted difference between the solar flare and solar wind ^(22)Ne/^(20)Ne ratios remains unresolved

Implications of Time Variations for the Origin of Low Energy Cosmic Ray Nitrogen and Oxygen Nuclei

We report observations of time variations in the quiet time intensity of 5-27 MeV/nuc nitrogen and oxygen and 13-25 MeV/nuc helium nuclei, obtained with the Caltech Electron/Isotope Spectrometers on IMP 7 and 8. We find no significant correlation of the O variations with variations in the low intensity fluxes of 1-2 MeV solar protons. However, we do find the O intensity to be well correlated with the modulation of galactic cosmic rays as measured by neutron monitors. When compared with the neutron monitor, the factor of ~ 3 change in the 0 intensity during 1972 through 1974 is consistent with the factor of ~ 30 increase observed since 1969. These observations imply that the enhanced low energy N and O fluxes are not of solar origin, but originate far enough from the sun that they undergo substantial modulation and have significantly increased access to 1 AU at times of minimum solar modulation. It may be possible to determine the charge state of the He and O by comparing their time variations. The He and O variations are strongly correlated, and both exhibit similar hysteresis effects when compared to the neutron monitor

The isotopic composition of hydrogen and helium in low-energy cosmic rays

The isotopes ^2H and ^3He have been identified in low-energy cosmic rays during solar-quiet periods from 1973 January to 1974 October. These observations, made with the Caltech Electron/Isotope Spectrometer on IMP-7, cover the energy intervals 5-29 MeV per nucleon for ^2H and 7-50 MeV per nucleon for ^3He. We find that the energy spectra of ^1H, ^2H, and ^3He all fall rapidly with decreasing energy, giving ^2H/^1H and ^3He/^1H ratios which are essentially independent of energy as expected from current theories of the solar modulation of galactic cosmic rays. The measured ^4He spectrum, however, is essentially flat below ~40 MeV per nucleon, suggesting that there may be contributions from a local, nonsolar source of ^4He. Comparisons of the ^1H, ^2H, and ^3He observations with calculated spectra at 1 AU imply a mean interstellar path length of 7 ± 2 g cm^(-2). However, present low-energy measurements of Hand He isotopes at 1 AU do not discriminate between possible cosmic-ray source spectra