FINE-STRUCTURE OF LANGMUIR-WAVES PRODUCED BY A SOLAR ELECTRON EVENT

Highly structured bursts of Langmuir waves produced by energetic electrons ejected from a solar flare have been observed using wideband plasma wave measurements on the Galileo spacecraft. The wideband sampling system on Galileo provides digital electric field waveforms at sampling rates up to 201,600 samples s-1, much higher than any previous instrument of this type. The solar flare of interest 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. Starting about 1 hour after the onset of the flare, a large number of intense Langmuir wave bursts were detected near the local electron plasma frequency, which was about 25 kHz. The Langmuir wave bursts. which lasted about 1.5 hours, 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 peak electric field strengths are about 1.7 mV m-1. 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. However, the intensities are too low for strong turbulence effects to be important.