Interplanetary conditions during 3-kHz radio-wave detections in the outer heliosphere

Abstract

A very important observation¹ regarding the plasma physics of the outer heliosphere has been that plasma-wave instruments on both the Voyager 1 and 2 spacecraft beyond ∼ 12 AU detected, during the same interval of time, plasma waves (frequency ∼3 kHz) that could possibly be associated with the turbulence expected at the heliopause. It was suggested that this turbulence had propagated inwards to the spacecraft, which would place the heliosphere boundary at ∼46 AU. To interpret these unusual observations, it is important to understand the characteristics of the interplanetary medium at large heliocentric distances. We draw attention here to the low-energy charged-particle environment in the outer heliosphere during the observations of the unusual plasma-wave signals. These particle data suggest that the outer heliosphere was unusually stable and free of transient shock and particle events for the ∼8 months during the wave observations.