Plasma waves in the nightside ionosphere of Venus

ince Venus is an unmagnetized planet, the solar wind directly interacts with its atmosphere. An extensive comet-like induced magnetosphere is formed which generally stands off the solar wind well above the planet. A variety of plasma wave emissions are observed in the nightside ionosphere by the Pioneer Venus Orbiter Electric Field Detector with its four narrowband channels. In order to determine the state of the plasma in which those waves occur, we examine first the magnetic and plasma structure of the nightside ionosphere. The magnetic field in the equatorial plane that contains the planet, the solar wind flow and the IMF is well ordered by the upstream IMF as expected for the draping model. The magnetic field is much stronger in the sheath region than in the night ionosphere region directly behind the planet. Near the anti-sunward region the field appears to be more chaotic. In this region there are many density holes inside which the magnetic field is stronger and more radial and plasma...