Energetic protons associated with interplanetary active regions 1–5 AU from the Sun

Abstract

About 100 energetic proton (0.61 ≤ Ep ≤ 3.41 MeV) events have been observed at heliocentric distances between 1 and 5 AU with the University of Iowa instrument on Pioneer 11. Absolute intensities, anisotropies, and crude energy spectra are measured. The particle intensity in the events has no significant dependence on heliocentric distances. Beyond ∼2.4 AU, power spectra of the counting rates show strong peaks at about 26 and also usually 13 days corresponding to corotation of the solar wind structure. The number of events observed per unit time interval is 10 times greater in the close vicinity of interplanetary active regions (IAR) (Smith and Wolfe, 1976) than elsewhere in interplanetary space. The frequency of occurrence of events has a marked tendency to peak within ±5 hours of the time that IAR edges are crossed. Events observed in the vicinity of IAR have, on the average, greater particle intensities, softer energy spectra, and smaller time widths at half maximum intensity than events observed elsewhere. Many of the IAR‐associated events have particle anisotropies corresponding to the net flow of particles toward the sun along the interplanetary magnetic field. The phenomenological evidence strongly favors local acceleration in the interplanetary medium by an as yet unidentified mechanism in magnetohydrpdynamic shocks. The sources of the shock‐accelerated particles are apparently corotating streams of solar emitted protons, most of which have energies Ep < 0.61 MeV.