STRUCTURE AND DYNAMICS OF INNER COMETARY PLASMAS

The inner cometary coma is a weakly ionized plasma, and its structure and dynamics are governed mainly by ionization due to solar radiation and solar wind electrons and losses due to the radiative processes, re combination, and transport. At comet Halley a narrow ion density deple tion region was observed by spacecraft as well as ground-based instrum ents and has been linked to the dynamics of the plasma and radiation. A model of the cometary plasma consisting of water group ions, bulk el ectrons, and energetic electrons produced mainly by photoionization is presented. The dominant losses in the inner coma are the radiation fr om the excitation of rotational and vibrational levels of water molecu les and the recombination of the plasma. The electron energy losses du e to these processes peak near 4000 K, and at temperatures; higher tha n this value a localized cooling leads to further cooling arising from increased radiation loss and consequently to a thermal instability. T he resulting increase in recombination leads to an ion density depleti on, and the estimates for this depletion at comet Halley agree with th e observations. This instability is sensitive to the plasma conditions and the transport processes, that is, diffusion and thermal conductiv ity. There is no direct measurement of the electron temperature in thi s range, and the electron temperature profile from MHD simulations has been used to develop a model of the inner cometary plasma, which yiel ds the localization of the thermal instability and, hence, the observe d ion density depletion region. The resulting electron temperature pro file is also consistent with that obtained from the temperature depend ence of the electron-ion recombination rate.