Physics
Scientific paper
Mar 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995georl..22..639m&link_type=abstract
Geophysical Research Letters (ISSN 0094-8276), vol. 22, no. 5, p. 639-642
Physics
5
Cometary Atmospheres, Electron Energy, Halley'S Comet, Negative Ions, Plasmas (Physics), Thermal Stability, Energy Dissipation, Excitation, Ion Density (Concentration), Plasma Equilibrium
Scientific paper
The spacecraft and ground based observations of comet Halley inner coma showed a localized ion density depletion region whose origin is not well understood. Although it has been linked to a thermal instability associated with negative ions, the photodetachment lifetime of negative ions (approximately 1 sec) is too short compared to the electron attachment time scale (approximately 100 sec) for this process to have a significant effect. A mechanism for the ion density depletion based on the thermal instability of the cometary plasma due to the excitation of rotational and vibrational levels of water molecules is proposed. The electron energy losses due to these processes peak near 4000 K (0.36 eV) and at temperatures higher than this value a localized cooling leads to further cooling (thermal instability) due to the increased radiation loss. The resulting increase in recombination leads to an ion density depletion and the estimates for this depletion at comet Halley agree with the observations.
Milikh Gennady M.
Sharma Surjalal A.
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