Shock‐associated low‐energy ion enhancements observed by Voyagers 1 and 2

Abstract

Observations of shock‐associated ≥30 keV ion enhancements are presented using data from the Low Energy Charged Particle (LECP) experiment on Voyagers 1 and 2, launched on days 248 and 232, 1977, respectively. The observations include examples of energetic storm particle (ESP) events associated with flare‐produced shocks and examples of corotating particle events (CPE) associated with forward and reverse shocks that bound corotating interaction regions in the outer heliosphere. The first well‐defined CPE are not seen at Voyagers 1 and 2 until late in 1978 when each spacecraft was at a heliocentric radial distance of ∼4 AU. Thus far, seven CPE have been identified in the Voyager 1 LECP data from launch through day 170, 1979. Most of these CPE show features similar to many of those observed at Pioneers 10 and 11, e.g., they have recurrent double‐peaked intensity enhancements showing little or no velocity dispersion at peak intensities, time durations of several days, soft (γ ≥4 for protons ∼1 MeV) energy spectra extending up to ∼5 MeV/nucleon and p/α ratios that are lowest at the reverse peaks. The new LECP measurements also show for the first time that the CPE ion spectra extend, with no sign of a low‐energy turnover, to energies as low as 30 keV/ion. If the CPE are produced by shock acceleration of an ambient particle source, the fact that the observed CPE exhibit well‐formed high intensity peaks at 30 keV/ion means that the pre‐acceleration energy of the source particles was <30 keV/ion. In several of the shock events, the lowest energy ion enhancements are confined mainly downstream of the CIR shocks, with the magnitude and duration of the upstream enhancements increasing with increasing ion energy. The similarities in the low energy ion morphologies observed near the shock during an ESP event and a CPE suggest that the same shock acceleration mechanism and propagation processes were operative in each case. These observations are consistent with the dynamical and kinematical effects expected when low energy ions are accelerated at quasi‐perpendicular shocks.