Theory for 2–3 kHz radiation from the outer heliosphere

Abstract

High‐intensity radio emission events at 2–3 kHz were observed by the Voyager spacecraft during 1983–1984 and 1992–1993. Such events are thought to occur when shock waves associated with global merged interaction regions (GMIRs) enter a region of the outer heliosheath where the electron speed distribution is primed with a superthermal tail, generated by lower hybrid drive. Previously, this priming mechanism was combined with a theory for type II solar radio bursts to predict the flux of radio emission in the outer heliosphere. Here this theory is extended in a number of ways. First, theoretical arguments regarding the availability of Langmuir and ion sound waves are used to determine whether emission occurs via three‐wave processes or processes involving wave scattering off thermal ions (STI). New expressions for conversion efficiencies into radio emission associated with STI are then implemented where appropriate. Next, the dependence of the predicted fluxes on plasma and shock parameters are determined. Lastly, dynamic spectra are calculated for the radio emission generated by shocks traveling from the inner solar wind to beyond the heliopause and into the very local interstellar medium (VLISM). It is found that the predicted fluxes of fundamental radiation are comparable with those observed for plausible shock and plasma parameters. The theory can also predict radio‐quiet GMIRs to be smaller and slower and to propagate through heliosheath regions with weaker superthermal tails. The calculated dynamic spectra have predicted fluxes below the Voyager detection thresholds in the solar wind, inner heliosheath, and VLISM. However, the predicted fluxes and frequency‐time behavior are very similar to the 2 kHz component observed by the Voyager spacecraft when the GMIR is in the primed region.