Fine structure of Langmuir waves produced by a solar electron event

Observations of highly structured bursts of Langmuir waves produced by energetic electrons ejected from a solar flare using wideband plasma wave measurements on the Galileo spacecraft are reported. Attention is given to the solar flare that occurred on December 10, 1990, while the spacecraft was at a radial distance of 0.98 AU from the sun. This flare emitted a stream of energetic electrons and an associated type III radio event, both of which were detected by Galileo. A large number of intense Langmuir wave bursts were detected near the local electron plasma frequency, which was about 25 kHz. The bursts, which lasted about 1.5 hr, coincided with the arrival of the solar electrons. The bursts are highly structured and consist mainly of isolated wave packets with durations as short as 1 ms and beat-type waveforms with beat frequencies ranging from 200 to 500 Hz. The highly structured envelopes of these waves are strongly suggestive of nonlinear parametric decay processes such as those predicted by various theories dealing with the saturation of beam-driven electrostatic instabilities.