HR: 13:40h
AN: P52B-01    [PDF]
TI: Evidence of ionospheric Holes in the Venus Nigthtside Ionosphere as Crossings of the Pioneer Venus Orbiter (PVO) Through Plasma Channels Near the Magnetic Polar Regions
AU: * Perez de Tejada, H A
EM: perezdet@igeofcu.unam.mx
AB: Data obtained from the PVO orbiter electron temperature probe (OETP) and magnetic field observations conducted in the Venus nightside ionosphere reveal that the ionospheric holes detected in that region represent crossings of the PVO through plasma channels that extend downstream from the magnetic polar regions. The electron density profile and the magnetic field signature measured across the nightside ionosphere in orbit 85 show that the spacecraft moved through plasma channels (in an ionosheath-plasma channel-ionosphere transit) in which the magnetic field magnitude and its orientation are similar to those encountered within the ionospheric holes. The enhanced (~30 nT) magnetic field intensity measured within the channels, which accounts for that present within the holes, results from the accumulation of the magnetic field that is forced by the solar wind thermal pressure on the polar upper ionosphere in a manner similar to what occurs near the subsolar region where the solar wind kinetic energy density is replaced by the accumulation of magnetic field fluxes around the ionosphere. The large [~10(exp-7) ergs cm-3] kinetic energy density that the solar wind maintained in orbit 85 suggest wide and deep plasma channels extending downstream from the magnetic polar regions of the Venus ionosphere and that resulted from the extensive local erosion produced by the solar wind. The fact that an intense [~10(exp 5) cm-3] peak electron density was measured deep in the umbra by the midnight plane near the trajectory periapsis (~200 km) during that orbit indicates that the nightside ionosphere did not disappear despite the large value of the solar wind dynamic pressure. A geometry of the ionopause traced across the magnetic polar regions is suggested to account for the maintenance of large ionospheric electron densities near the midnight plane and the absence of such densities far away from that plane when there are large solar wind dynamic pressures.
DE: 5405 Atmospheres--composition and chemistry
DE: 5407 Atmospheres--evolution
DE: 5409 Atmospheres--structure and dynamics
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting