FINE-STRUCTURE OF LANGMUIR-WAVES OBSERVED UPSTREAM OF THE BOW SHOCK AT VENUS

Highly structured Langmuir waves, also known as electron plasma oscill ations, have been observed in the foreshock of Venus using the plasma wave experiment on the Galileo spacecraft during the gravity assist fl yby on February 10, 1990. The Galileo wideband sampling system provide s digital electric field waveform measurements at sampling rates up to 201,600 samples per second, much higher than any previous instrument of this type. The main Langmuir wave emission band occurs near the loc al electron plasma frequency, which was approximately 43 kHz. The Lang muir waves are observed to shift above and below the plasma frequency, sometimes by as much as 20 kHz. The shifts in frequency are closely c orrelated with the downstream distance from the tangent field line, im plying that the shifts are controlled by the electron beam velocity. C onsiderable fine structure is also evident, with timescales as short a s 0.15 ms, corresponding to spatial scales of a few tens of Debye leng ths. The frequency spectrum often consists of beat-type waveforms, wit h beat frequencies ranging from 0.2 to 7 kHz, and in a few cases, isol ated wave packets. The peak electric field strengths are approximately 1 mV/m. These field strengths are too small for strongly nonlinear pr ocesses to be important. The beat-type waveforms are suggestive of a p arametric decay process.