Outer heliospheric radio emissions: 2. Foreshock source models

Abstract

Low‐frequency radio emissions in the range 2–3 kHz have been observed by the Voyager spacecraft during the intervals 1983–1987 and 1989 to the present while at heliocentric distances greater than 11 AU. New analyses of the wave data are presented, and the characteristics of the radiation are reviewed and discussed. Two classes of events are distinguished: transient events with varying starting frequencies that drift upward in frequency and a relatively continuous component that remains near 2‐kHz. Evidence for multiple transient sources and for extension of the 2‐kHz component above the 2.4‐kHz interference signal is presented. The transient emissions are interpreted in terms of radiation generated at multiples of ƒp when solar wind density enhancements enter one or more regions of a foreshock sunward of the inner heliospheric shock. Theoretically, and based on extrapolated Langmuir levels in planetary foreshocks, the foreshock should contain sufficient levels of Langmuir waves to produce the radio emissions in path lengths small compared with 1 AU and the estimated radial size of the foreshock. Enhancements of the solar wind density by factors greater than 4 are required for the radiation to be generated at the observed frequencies and to propagate into the inner heliosphere, as observed, from a source outside about 50 AU. Solar wind density enhancements by factors of 4–10 are observed. The radiation can therefore be generated in sources beyond the present locations of the Voyager spacecraft. Propagation effects, the number of radiation sources, and the time variability, frequency drift, and varying starting frequencies of the transient events are discussed in terms of foreshock sources. The available data appear to be qualitatively and semiquantitatively consistent with this foreshock theory for transient events. A similar foreshock theory involving transient density enhancements can explain the levels, characteristic frequency, and inward propagation of the 2‐kHz component in terms of a source outside 50 AU. However, the theory encounters significant difficulties in explaining the relatively constant frequency and presence of the radiation, as well as its different properties relative to the transient emissions. Remedies for these problems are briefly explored. However, the 2‐kHz component quite likely has a different generation mechanism and/or a different source region than the transient emissions.