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Session 59 - HEAD II: Cosmic Particle Accelerators.
Oral session, Tuesday, January 14
Somewhere beyond 70 AU, where the solar wind pressure is balanced by that of the local interstellar medium, is a shock beyond which the solar wind is no longer supersonic. There is now strong evidence that this solar wind termination shock is an efficient particle accelerator, capable of accelerating ions to beyond 1 GeV on a time scale of \sim1 year. The observed ions are called "anomalous cosmic rays" (ACRs) because of their unusual composition, which includes H, He, C, N, O, Ne, and Ar. ACRs are mainly singly-charged, andoriginate from interstellar neutral particles that have been swept into the heliosphere, ionized by solar UV or charge exchange with the solar wind, and then picked up by the solar wind and convected to the termination shock where they are accelerated to energies of 1 to 50 MeV/nuc. Data from a number of spacecraft now verify most of the details of this picture. Ulysses has observed the incoming neutrals and freshly-ionized "pick-up" ions directly. SAMPEX has used the Earth's field as a magnetic spectrometer to show that the highest energy ACRs are multiply-charged, thereby providing important new tests of shock acceleration models. Voyager observations imply that the shock is presently located at about 85 AU, which Voyager-1 will reach within the decade. We review this new evidence and discuss the implications for shock acceleration models, for the nature and composition of the local interstellar medium, and for models of galactic cosmic ray origin and acceleration.
Program listing for Tuesday