title: IMP-8 observations of the spectra, composition, and variability of solar heavy ions at high energies relevant to manned space missions.
abstract: In more than 25 years of almost continuous observations, the University of Chicago's Cosmic Ray Telescope (CRT) on IMP-8 has amassed a unique database on high-energy solar heavy ions of potential relevance to manned spaceflight. In the very largest particle events, IMP-8/CRT has even observed solar Fe ions above the Galactic cosmic ray background up to approximately 800 MeV/nucleon, an energy sufficiently high to penetrate nearly 25 g/cm2 of shielding. IMP-8/CRT observations show that high-energy heavy-ion spectra are often surprisingly hard power laws, without the exponential roll-offs suggested by stochastic acceleration fits to lower energy measurements alone. Also, in many solar particle events the Fe/O ratio grows with increasing energy, contrary to the notion that ions with higher mass-to-charge ratios should be less abundant at higher energies. Previous studies of radiation hazards for manned spaceflight have often assumed heavy-ion composition and steeply-falling energy spectra inconsistent with these observations. Conclusions based on such studies should therefore be re-assessed. The significant event-to-event variability observed in the high-energy solar heavy ions also has important implications for strategies in building probabilistic models of solar particle radiation hazards.
authors.affiliation: E.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375, USA. tylka@crs2.nrl.navy.mil
authors.forename: A J
authors.initials: AJ
authors.lastname: Tylka
authors.forename: W F
authors.initials: WF
authors.lastname: Dietrich
mesh_terms: D001256:Astronomy / Q000295:instrumentation; D003359:Cosmic Radiation; D020450:Heavy Ions; D006801:Humans; D007501:Iron; D008962:Models, Theoretical; D010100:Oxygen; D011834:Radiation Monitoring / Q000295:instrumentation*; D011835:Radiation Protection; D019445:Solar Activity; D013026:Space Flight / Q000295:instrumentation*; D018531:Spacecraft / Q000295:instrumentation
chemical_list: D007501:Iron; D010100:Oxygen