Evidence for electrostatic decay in the solar wind at 5.2 AU

The Unified Radio and Plasma wave Experiment (URAP) on the Ulysses spacecraft provides in situ observations of Langmuir waves and ion‐acoustic waves in the solar wind. The observations presented in this paper were obtained at 5.2 AU from the Sun. Low‐frequency (20–200 Hz in the spacecraft frame) electric field signals are observed coincident in time with the most intense Langmuir waves. The low‐frequency wave signals are identified as long‐wavelength ion‐acoustic waves. These observations provide evidence for the decay of Langmuir waves into daughter Langmuir and ion‐acoustic waves (the electrostatic decay process) in the solar wind.