Physics – Geophysics
Scientific paper
Dec 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995ucla.rept.....s&link_type=abstract
Technical Report, California Univ. Los Angeles, CA United States Inst. of Geophysics and Planetary Physics.
Physics
Geophysics
Pioneer Venus Spacecraft, Electric Fields, Plasma Waves, Whistlers, Venus Atmosphere, Planetary Ionospheres, Lightning, Space Plasmas, Bow Waves, Shock Waves, Ionopause
Scientific paper
The Pioneer Venus Orbiter Electric Field Detector (OEFD) measures plasma wave electric fields at four frequencies 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz. Although limited in frequency resolution, this allows us to study various plasma wave phenomena, including whistler-mode waves (approximately 100 Hz), ion acoustic waves (approximately 730 Hz and 5.4 kHz), and electron plasma oscillations (approximately 30 kHz). The Pioneer Venus OEFD data are often cited as evidence for lightning on Venus. This topic has been a fruitful research topic throughout the lifetime of the grant. The evidence for lightning is now very strong. It is now clear that many of the electric field bursts detected by the Pioneer Venus Orbiter are consistent with radiation propagating through the ionosphere from below. The only likely atmospheric source is planetary lightning. However, any electromagnetic waves generated by lightning will tend to be absorbed by the ionosphere. Only a small fraction of the wave energy could escape to be detected by a spacecraft such as Pioneer Venus. In terms of ionospheric and plasma wave phenomena, a clear demonstration that the waves can escape will probably clinch the debate. We have begun an analysis of the transmission characteristics of the ionosphere, but a more thorough understanding requires further support. One of the major successes of the Pioneer Venus OEFD was the generation of plasma wave maps of the region upstream of the Venus bow shock, known as the foreshock. The Pioneer Venus OEFD data provided an extensive set of data, not available at any other planet. For example, we found that waves generated by electrons streaming away form the shock only extend a finite distance upstream. Presumably the free energy supplied by the back-streaming electrons is removed by the waves. However, comparison with terrestrial observations show that the waves at the earth extend much further (in absolute scale), and it appears the amount of free energy available is closely related to the shock size. The last topic addressed concerns the plasma waves observed above the dayside ionopause of Venus. Although originally cited as a source of energy for the topside ionosphere, much of our more recent work shows that the waves are probably more involved in the coupling of planetary ions to the solar wind. As such, the waves may be part of the process whereby planetary ions are 'picked-up' by the solar wind, and hence lost from the near-Venus environment. This is one mechanism for scavenging the atmosphere, since the planetary ions ultimately come from ionized planetary neutrals within the atmosphere. In addition, similar wave modes appear to occur above the ionopause of Mars. Thus the Pioneer Venus OEFD data provide an important clue for understanding solar-wind ionosphere coupling at the unmagnetized planets.
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