Evidence for Langmuir Oscillations and a Low Density Cavity in the Venus Magnetotail

We report the discovery of Langmuir oscillations in a very low plasma density region in the Venus magnetotail. These waves are observed more often at 30 kHz, but also at 5.4 kHz indicating densities as low as 0.3/cu cm in the central tail lobe. The Langmuir probe on board the Pioneer Venus Orbiter cannot resolve such a low plasma density. We use the magnetic field strength and the assumption of total pressure balance to infer the electron temperature as a test of the Langmuir wave interpretation. By investigating the spatial distribution of this wave activity we find that the plasma cavity is ordered in a coordinate system defined by the interplanetary magnetic field and is found at either side of the central tail current sheet.