Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p21b0367p&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P21B-0367
Physics
6025 Interactions With Solar Wind Plasma And Fields, 6028 Ionospheres: Structure And Dynamics, 6050 Plasma And Mhd Instabilities, 6295 Venus
Scientific paper
Electron densities in the Venus nightside upper ionosphere that were derived from measurements made with the Orbiting Electron Temperature Probe (OETP) of the Pioneer Venus Orbiter (PVO) have been examined in connection with effects associated with the plasma channels that have been assumed extend downstream from the magnetic polar regions [Perez-de-Tejada, J. Gephys. Res. 106, 211, 2001]. The ionospheric density profiles obtained near the midnight plane have a signature that is different from what it is seen throughout most of the nightside hemisphere. The data in various orbits show that in the vicinity of the midnight plane there is a near constant density plateau that extends across most of the upper ionosphere from the nightside ionopause crossing (up to 2000 km) to very low (200 km) altitudes located near periapsis in the PVO trajectory. The density plateau is limited by a low altitude sharp rise with a slope that is larger than the altitude variation in the density profiles seen away from the midnight plane. It is suggested that the peculiar shape of the electron density profiles near the midnight plane is produced by the displacement of the upper ionosphere adjacent to the plasma channels that is eroded by the solar wind; and that the sharp lower edge of the density plateau results from crossing near the bottom of the plasma channels. It has also been suggested that the plasma channels provide a useful account of the ionospheric holes observed in the OETP data as regions where the ionospheric plasma density in the nightside hemisphere falls to very low values. A set of these latter features was selected to show that when they occur near the midnight plane the magnetic field measured in the solar wind is mostly oriented near the ecliptic plane. Different from this orientation it is found that in the electron density profiles with a near constant density plateau the magnetic field in the solar wind has a significant latitudinal angle with respect to the ecliptic plane. It is argued that the observation of density profiles with ionospheric holes near the midnight plane or with a density plateau in that region results from the different latitudinal orientation of the magnetic field in the solar wind. That orientation defines the position of the magnetic polar regions in the Venus ionosphere and hence that of the plasma channels that extend behind them.
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