Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p51b1194g&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P51B-1194
Other
0343 Planetary Atmospheres (5210, 5405, 5704), 2437 Ionospheric Dynamics, 2499 General Or Miscellaneous, 6295 Venus
Scientific paper
Pioneer Venus Orbiter (PVO) observations have shown that the nightside ionosphere of Venus under solar maximum conditions is maintained by flow from the dayside. However, this is a fragile situation. PVO periapsis passes below 160 km show that far into the nightside, the ionosphere is not typically characterized by smoothly varying densities from the ionopause to the periapsis location. A large number of orbits far into the night traverse abrupt perturbations. The extreme examples singled out in the past were associated with holes, abrupt density depressions with tailward magnetic field enhancements; and with disappearing ionospheres, where the ionospheric densities are severely depleted everywhere. However, there are cases with characteristics intermediate to these extreme states, such as a severe depletion on only part of an orbit. The complex state is seen clearly in a statistical sense by combining all the electron density data from the early phase of PVO when periapsis was below 165 km and binning by altitude and solar zenith angle. The upper density envelope of this distribution provides a measure of the nominal full-up ionosphere with which to reference density depletions. Furthermore, the magnetic field in the depleted regions becomes enhanced and ordered, whereas in the full-up ionosphere regions (above the low altitudes where ion-neutral collisions play a significant role), the field is weak and irregular. In the former case the plasma distribution is dominated by electromagnetic processes that lead to superthermal ions and at times neutral atmosphere perturbations whereas in the latter, low magnitude magnetic field states, the plasma dynamics are driven by plasma pressures. The transition from one state to the other is demonstrated by correlating the plasma density with the magnetic field measured by the magnetometer on PVO. The effects of the electrodynamic forces vs. plasma pressure forces and the transitions from the one ionospheric state to another in the lower nightside ionosphere during the maximum of the solar cycle will be described.
Grebowsky Joseph M.
Hartle Richard E.
Hoegy Walter R.
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