Modeling of pickup ion distributions in the Halley cometosheath: Empirical limits on rates of ionization, diffusion, loss and creation of fast neutral atoms

Details Modeling of pickup ion distributions in the Halley cometosheath: Empirical limits on rates of ionization, diffusion, loss and creation of fast neutral atoms Modeling of pickup ion distributions in the Halley cometosheath: Empirical limits on rates of ionization, diffusion, loss and creation of fast neutral atoms

Oct 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jgr....9919245h&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. A10, p. 19,245-19,254

Physics

3

Cometary Magnetospheres, Energy Dissipation, Halley'S Comet, Ion Distribution, Ionization, Neutral Atoms, Plasmas (Physics), Diffusion, Giotto Mission, Ions, Velocity Distribution

Scientific paper

The shape of the velocity distribution of water group ions observed by the Giotto ion mass spectrometer on its approach to comet Halley is modeled to derive empirical values for the rates of ionization, energy diffusion, and loss in the midcometosheath. The model includes the effect of rapid pitch angle scattering into a bispherical shell distribution as well as the effect of the magnetization of the plasma on the charge exchange loss rate. It is found that the average rate of ionization of cometary neutrals in this region of the cometosheath appears to be of the order of a factor 3 faster than the `standard' rates approx. 1 x 10-6/s that are generally assumed to model the observations in most regions of the comet environment. For the region of the coma studied in the present work (approx. 1 - 2 x 105 km from the nucleus), the inferred energy diffusion coefficient is D0 approx. equals 0.0002 to 0.0005 sq km/cu s, which is generally lower than values used in other models. The empirically obtained loss rate appears to be about an order of magnitude greater than can be explained by charge exchange with the `standard' cross section of approx. 2 x 10-15sq cm. However such cross sections are not well known and for water group ion/water group neutral interactions, rates as high as 8 x 10-15 sq cm have previously been suggested in the literature. Assuming the entire loss rate is due to charge exchange yields a rate of creation of fast neutral atoms of the order of approx. 10-4/s or higher, depending on the level of velocity diffusion. The fast neutrals may, in turn, be partly responsible for the higher-than-expected ionization rate.

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