Plasma waves and electromagnetic radiation at Venus and Mars

Plasma waves play a significant role in the interaction of the solar wind with the ionospheres of the unmagnetized planets, Venus and Mars. Upstream of the planet plasma oscillations are generated by electrons energized and reflected at the bow shock. The spatial extent of the plasma oscillations is indicative of the amount of energy gained at the shock. Comparison with the Earth's bow shock suggests that the electron energization is controlled by shock curvature. At the dayside ionopause of the unmagnetized planets low frequency waves are observed. These waves were originally identified as whistler-mode waves at Venus, but were subsequently identified as lower hybrid or ion acoustic waves. The waves appear to have an important role in mediating coupling between the solar wind and planetary ionosphere, in terms of either energy or momentum transfer. Plasma waves are also observed in the nightside ionosphere of the planets, and whistler-mode waves have been observed at low altitudes at Venus. These waves have been identified with planetary lightning, although recent observations by the Cassini spacecraft during its Venus flybys suggest that lightning at Venus must have different characteristics than terrestrial lightning. The waves are also subject to collisional Joule dissipation, and considerations of the predicted heating may help constrain the possible source mechanisms for the waves. Given the rich plasma wave environment at the unmagnetized planets, and the importance of plasma waves in the dynamics of the plasma, future aeronomy missions should include some form of wave instrumentation. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.