Computer Science
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........41d&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, SAN DIEGO, 1993.Source: Dissertation Abstracts International, Volume: 54-11, Section:
Computer Science
2
Scientific paper
The nature of the plasma flow in the cometary environment is examined in two different contexts. In the first study, the Newtonian thin layer approximation is used to describe the structure of the shock layer between the ionopause and the inner shock by assuming that an 'inner' shock exists to break the supersonic outflow of cometary ions and divert them into the cometary tail. We use a three-dimensional axisymmetric photochemical flow model to calculate analytically the size and shape of this layer, as well as the lateral flow profiles within this layer. While sources of ions in this layer are the flow across the inner shock and photoionization of neutrals within it, the sinks are the flow into the flanks and dissociative recombination; the latter being the dominant one. The layer is found to be thin with a calculated thickness of ~ 100 km for the expected value of the dissociative recombination coefficient. This value is consistent with the observations. In the second study, a quasi one-dimensional MHD, single fluid flow model is used to examine the flow in the region between the cometopause and the ionopause. In this model, we used only a single neutral species (the dominant H_2O molecule) and a single ionic species (the dominant ion in the cometosheath, H _2O^+). The effects of massloading which result from the assimilation of the photoionized neutrals, dissociative recombination of the electrons with the ions, magnetic field line curvature, ion-neutral drag and flow divergence on the submagnetosonic flow in this region are discussed. In particular, the inclusion of the flow divergence, which is achieved by allowing the central flow tube area to increase towards the comet, is necessary to bring the cometosheath flow to stagnation at the sub-solar point of the ionopause. The cometosheath flow and field profiles are determined for two assumed values of the electron-ion dissociative-recombination rate. The model calculations performed using the higher recombination rate are shown to be in better agreement with the Giotto data at comet Halley. This second study is extended to a multispecies one, in the final study, by including the detailed chemistry of an H_2O-dominated atmosphere. The following neutral species: H_2O, OH, O, H and ions: H_2O^+, H_3O^+, OH^+ , O^+, and H^+ are considered in this model. The effects of charge exchange and the relevant ion-molecule reactions are included. The results of this model are compared with observations. The calculated and observed magnetic field and flow profiles are qualitatively similar. The quantitative discrepancies are consistent with the fact that the encounter geometry is not along the Sun-comet axis (where our calculations apply) but at a large angle to it.
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