The latitude and radial dependence of shock acceleration in the heliosphere

Abstract

Voyagers 1 and 2 have observed interplanetary shock acceleration of ≲2 MeV ions at radial distances of >25 AU and heliographic latitudes of up to ∼30°. The previously reported negative latitude gradient in the intensities of accelerated ions at these energies persists over the entire range of latitudes and radial distances measured. The energy spectra of the shock acceleration events at Voyagers 1 and 2 are similar. A dramatic decrease in the flux of ≲ 2 MeV ions in the outer heliosphere was observed by both Voyager spacecraft in 1985. The decrease was observed at ∼30° as well as in the solar equatorial plane. At the time of the decrease, Voyager 1 was at ∼22 and ∼6 AU further from the sun than Voyager 2. However, the decrease began within an ∼12 day interval at both spacecraft. Comparisons with data from the IMP 8 spacecraft near Earth show that the decrease propagated outward from the Sun at ∼570 km/s. The timing of the large decrease between the two Voyager spacecraft implies a latitude gradient in the velocity of interplanetary shocks of ∼4.2 km/s per degree of latitude above the solar equator.