Other
Scientific paper
Sep 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987phdt........65p&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, LOS ANGELES, 1987.Source: Dissertation Abstracts International, Volume: 48-06, Sectio
Other
Scientific paper
This dissertation examines the solar wind interaction with Venus, with emphasis on the role of the interplanetary magnetic field (IMF), using observations from Pioneer Venus Orbiter (PVO). The intrinsic magnetic field of Venus is much smaller than that of Earth. Based on a survey of low altitude PVO magnetometer observations, the planetary magnetic moment is less than 8.4 times 10^{10}Tm^ 3, or roughly 1/100,000 of the terrestrial moment. As a result, the impact of changing IMF orientation on the interaction is very different at Venus than at the magnetized planets. The solar plasma interacts directly with the ionospheric plasma, which generally is able to maintain a shielding current system which constitutes an impenetrable boundary. This "ionopause" has an average subsolar altitude of roughly 300 km, increasing to ~900 km near the terminator, and is consistently higher in the dawn hemisphere than in the dusk hemisphere. This observed asymmetry is shown to result from flow aberration due to planetary motion and from orbital bias. The IMF is compressed and draped around the boundary, reaching a maximum strength just above the ionopause. This field can in certain circumstances enter the ionosphere and be carried downward by ionospheric convection. In the magnetosheath, the region between the ionosphere and the bow shock, the magnetic field has a characteristic draping pattern. Magnetic fluctuations are distributed in a way that suggests a source at the quasi-parallel portion of the shock. The magnetosheath field magnitude is controlled by the clock angle of the IMF, or equivalently by the motional electric field of the solar wind. This control suggests that pickup of newly ionized exospheric particles is occurring preferentially on one side of the planet. A numerical experiment is performed to analyze this hypothesis. Ion pickup events are observed in the magnetosheath in locations consistent with this model. Another effect of the magnetic asymmetry is to depress the ionopause on the side of the planet with enhanced magnetic field. This effect is due to transfer of momentum between the exospheric ions and the magnetosheath plasma.
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