No abstract

The IMPACT investigation for the STEREO Mission includes a complement of Solar Energetic Particle instruments on each of the two STEREO spacecraft. Of these instruments, the High Energy Telescopes (HETs) provide the highest energy measurements. This paper describes the HETs in detail, including the scientific objectives, the sensors, the overall mechanical and electrical design, and the on-board software. The HETs are designed to measure the abundances and energy spectra of electrons, protons, He, and heavier nuclei up to Fe in interplanetary space. For protons and He that stop in the HET, the kinetic energy range corresponds to ∼13 to 40 MeV/n. Protons that do not stop in the telescope (referred to as penetrating protons) are measured up to ∼100 MeV/n, as are penetrating He. For stopping He, the individual isotopes 3 He and 4 He can be distinguished. Stopping electrons are measured in the energy range ∼0.7-6 MeV.

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No abstract

We report on the abundances of energetic particles from impulsive solar flares, including those from a survey of 228 3 He-rich events, with 3 He/ 4 He > 0.1, observed by the ISEE 3 spacecraft from 1978 August through 1991 April. The rate of occurrence of these events corresponds to ~1000 events yr -1 on the solar disk at solar maximum. Thus the resonant plasma processes that enhance 3 He and heavy elements are a common occurrence in impulsive solar flares. To supply the observed fluence of 3 He in large events, the acceleration must be highly efficient and the source region must be relatively deep in the atmosphere at a density of more than 10 10 atoms cm" 3 . 3 He/ 4 He may decrease in very large impulsive events because of depletion of 3 He in the source region.The event-to-event variations in 3 He/ 4 He, H/ 4 He, e/p, and Fe/C are uncorrected in our event sample. Abundances of the elements show a pattern in which, relative to coronal composition, 4 He, C, N, and O have normal abundance ratios, while Ne, Mg, and Si are enhanced by a factor ~2.5 and Fe by a factor ~7 . This pattern suggests that elements are accelerated from a region of the corona with an electron temperature of ~3 -5 MK, where elements in the first group are fully ionized (Q/A = 0.5), those in the second group have two orbital electrons (Q/A ~ 0.43), and Fe has Q/A ~ 0.28. Ions with the same gyrofrequency absorb waves of that frequency and are similarly accelerated and enhanced. Further stripping may occur after acceleration as the ions begin to interact with the streaming electrons that generated the plasma waves.