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Shock Acceleration Within 1 to 36 Au During a Five Year Interval Centered on the 1986 Solar Minimum

05 January 1990

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We have used intensities of low energy ions from the LECP instrument on the interplanetary probes Voyagers 1 and 2 (E >= 30 keV) and on the earth-orbiting satellite IMP-8 (E >= 300 keV) to examine the variation of shock-accelerated ions during the 5 year interval centered on the solar minimum of September 1986. In early 1985 there was a 50-fold decrease in the flux of ~ 1 MeV ions, observed first at 1 AU, and then ~ 50 days later at Voyager 2 (at 16 AU). This delay corresponds to a reasonable outward convection speed ~ 550 km/s. The flux decrease was a major depletion in the background level of low energy ions within the heliosphere, and was evidently caused by a marked reduction in the frequency of interplanetary shocks, in particular, recurrent corotating shocks. There was a resurgence of shock acceleration activity during the periods late 1985 to early 1986 and late 1987 to early 1988, but it was not until at least May 1988 that the background flux of ~ 1 MeV ions recovered to its pre-1985 level at 1 AU.