Huddleston, Debbie Ellen; (1990) The interaction between a comet and the solar wind. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Ion 'pickup' is an interesting physical process which occurs when new ions are introduced into a magnetized plasma flow with a motion relative to the bulk plasma. The cometary environment provides an excellent 'laboratory' in which to study such processes. Ions of cometary origin are gradually accomodated into the solar wind flow. The rate of isotropization of the implanted ion velocity distribution in the solar wind frame depends on the level of ambient and self-generated turbulent waves with which the ions interact. The work presented here studies the parallel pickup process to ascertain whether or not pitch-angle diffusion driven by the observed turbulence is fast enough to explain the development of the ion distributions. The theoretical description used is based on a quasilinear approach, and considers the implantation of cometary ions along solar wind flowlines. To make such a study requires some way of extrapolating our measurements on the Giotto trajectory into the upstream region. Models for mass-loading and turbulence are used. A simplified mass-loading model provides the plasma flow parameters which depend upon position relative to the comet, and the cometaxy gas emission characteristics. The turbulent power spectrum is reqmred for the diffusion coefficient of the wave-particle interaction. Theoretical calculations of free energy made available to upstream and downstream propagating Alfvén waves during the pickup process are used to estimate the wave intensity in the region upstream from the spacecraft track. The spectral shape is found to be reasonably constant throughout the region of interest. A kinetic equation describing the source, convection and quasilinear velocity diffusion of the implanted cometary ions is then developed and may be solved numerically using the information outlined above. The present study finds that quasilinear theory gives a level of velocity diffusion which is indeed of the right order.

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