Estimates of HZE particle contributions to SPE radiation exposures on interplanetary missions.
Estimates of radiation doses resulting from possible HZE (high energy heavy ion) components of solar particle events (SPEs) are presented for crews of manned interplanetary missions. The calculations assume a model spectrum obtained by folding measured solar flare HZE particle abundances with the measured energy spectra of SPE alpha particles. These hypothetical spectra are then transported through aluminum spacecraft shielding. The results, presented as estimates of absorbed dose and dose equivalent, indicate that HZE components by themselves are not a major concern for crew protection but should be included in any overall risk assessment. The predictions are found to be sensitive to the assumed spectral hardness parameters.
Composition of Solar Energetic Particles
The present measurements of elemental abundances in 15 large solar energetic particle events confirm the existence of two major effects: systematic differences between solar energetic particle abundances and photospheric abundances that are approximately correlated with first ionization potential (implying that the solar corona is the likely source population for these particles), and an enhancement of heavy ion abundances relative to a baseline of solar energetic particle abundances whose magnitude increases with rising atomic number. These data suggest the possibility that the degree of heavy ion enhancement has some correlation with spectral slope, and confirm that the aforementioned effects are not due to time or energy variations within individual events. The maximum column density above the solar acceleration region is less than 0.1 g/sq cm, on the basis of these data.
Abundance of solar cosmic ray alpha particles
The abundance ratio of the fluxes of low-energy (1–10 Mev/nucleon) solar alpha particles to solar protons varies from flare to flare and with time within a single flare event. The fluxes of solar protons and alphas from seven of the large-flare events during the maximum of solar cycle 20 have been integrated over the duration of the events. The event-averaged 4He/¹H ratio is found to vary as 0.026 (Mev/nucleon)−0.2 in the range 1–10 Mev/nucleon. For particles compared as to equal energy per charge (a representation that may have less distortion from propagation effects) the ratio 4He/¹H ∼ 0.05 (Mev/Z)−0.2. In this representation, for 1 Mev/Z particles, the solar cosmic ray 4He/¹H ratio is compatible with (1) the average solar wind helium abundance and (2) a solar helium abundance determined from spectroscopic studies of prominences and the chromosphere but is less than (3) helium abundances predicted from some current models of the solar interior.