The cometary atmosphere and its interaction with the solar wind

Abstract

The present state of both our knowledge and our understanding of cometary atmospheres and their interactions with the solar wind is critically reviewed. Following the introduction (section 1) the spectrophotometry of the cometary atmosphere and the tail is reviewed in section 2. The existing, rather fragmentary, knowledge about atmospheric abundances and the production rates of various molecular species is given in section 3. The deductions that can be made from these values about the chemical composition of the nucleus and their uncertainties are discussed. Existing models of the cometary atmosphere including the ionosphere are critically evaluated in section 4. The continuing dichotomy between models that concentrate on the chemistry while oversimplifying the dynamics and those that concentrate on the dynamics and thermodynamics while minimizing the chemistry is discussed. The need to combine these two approaches in the future is emphasized. The ionization processes in the cometary atmosphere are discussed in section 5. It is emphasized that besides photoionization and charge exchange ionization with the solar wind protons, other (collisional) ionization processes may play a role in the rapid ionization of cometary neutrals in certain situations. The overall nature of the solar wind‐comet interaction is discussed in section 6 with special emphasis on the strong coupling between the solar wind and cometary plasmas, which is responsible for the observed accelerations in the cometary tails. The complex and time‐varying fine structures observed in the plasma tails of comets are discussed next in section 7. The uses of these to infer the strength and morphology of the magnetic field and the associated currents in the plasma tail on one hand, and the varying conditions in the solar wind on the other, are emphasized. The detailed models of the comet‐solar wind interaction, from the earlier fluid dynamic models to the latest MHD models are critically evaluated in section 8, and their limitations are underscored. It is also pointed out that the nature of the solar wind interaction with the cometary atmosphere is highly variable and that in certain circumstances the nature of both the outer and the inner shocks could change. Also, the need for a detailed study of the solar wind interaction with the fledgling cometary atmosphere at larger heliocentric distances is stressed. The review is concluded with section 9, where the emphasis is on the outstanding problems of cometary atmospheres and their interactions with the solar wind.

A magnetohydrodynamical model of the inner coma of comet Halley

“…The inner shock as originally conceived was thought to be located --1000 km inside the diamagnetic cavity boundary surface [Mendis and Houpis, 1982]. The outwardly flowing ionospheric plasma was thought to encounter the shock and then be diverted into the tail.…”

Collisionless coupling in the AMPTE artificial comet

“…The subject is very opportune since it is the controlling factor that determines the momentum coupling process occurring in the interaction of the solar wind with the cometary plasma generated by the ionization of the neutral coma. The overall comet structure, the applicability of MHD or kinetic models, the presence or absence of a cometary shock, and the type of the resulting ionopause [Mendis and Houpis, 1982;Ip and Axford, 1982;Fedder et al, 1986;Sagdeer et al, 1986] depend critically on the wave-particle processes producing the momentum coupling, the thermalization, and the isotropization in the interaction. It is the purpose of the present note to compare the AMPTE in situ observations of the plasma parameters and wave signatures [Gurnett et al, 1985;Haerendel et al, 1986] with the theoretical concepts currently applied to the high Mach number interaction problem.…”

Recent Laboratory Photochemical Studies and Their Relationship to the Photochemical Formation of Cometary Radicals

“…Despite the fact there is general agreement that photochemistry is the most important source of radicals in comets, there have been other models that include ion-molecule reactions (Mendis and Houpis 1982). These models explain some of the ions, but are unlikely to explain most of the neutrals (Mendis and Houpis 1982).…”

“…These models explain some of the ions, but are unlikely to explain most of the neutrals (Mendis and Houpis 1982). One problem all of these models have is accounting for the low-density fluid flow of the gas.…”

“…The chemistry of comets is dominated by photochemistry (Wurm 1943, Mendis andHoupis 1982), so it is natural that a review of laboratory studies as they relate to comets should also be dominated by photochemistry. Most of the cometary photochemistry occurs under conditions that are very different from the typical conditions used for laboratory photochemical studies.…”