36th DPS Meeting, 8-12 November 2004
Session 33 Comets: Comae
Poster II, Thursday, November 11, 2004, 4:15-7:00pm, Exhibition Hall 1A

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[33.19] Radial Distribution and Chemistry of Ion Masses <=40 amu in Comet Halley's Ionosphere

S. A. Haider (PRL), A. Bhardwaj (NASA MSFC)

We have developed a model to study the chemistry of 46 ions corresponding to masses <=40 amu in the inner coma of comet 1P/Halley. The production rates, loss rates and ion mass densities are calculated using the Analytical Yield Spectrum approach and solving coupled continuity equation controlled by steady state photochemical equilibrium condition. The ionization sources in the model are solar EUV photon, photoelectron, and auroral electron of solar wind origin. The chemical model couples ions, neutrals, electrons, and photons through over 600 chemical reactions. Of the 46 ions considered in the model the chemistry of 24 important ions are discussed in detail in the paper. At radial distances <1000 km, the electron density is mainly controlled by 6 ions, viz., NH4+, H3O+, CH3OH2+, H3S+, H2CN+, and H2O+. However, at distances >1000 km, the 6 major ions are H3O+, CH3OH2+, H2O+, H3CO+, C2H2+, and NH4+. It is found that at radial distances greater than ~1000 km (+/-500 km) the major chemical processes that govern the production and loss of several of the important ions in the inner coma are different from those that dominate at distances below this value. The calculated ion mass densities are compared with Giotto Ion Mass Spectrometer (IMS) and Neutral Mass Spectrometer (NMS) data at radial distances 1500, 3500, and 6000 km. The nine major peaks at 12, 15, 19, 26, 28, 31, 33, 35, and 39 amu observed in IMS/NMS spectra are reproduced well by model calculations inside the ionopause. We present simple formulae for calculating densities of the 7 major ions in the inner coma, which will be useful in estimating their densities without running the complex chemical models.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.