The coupled continuity, momentum, and energy equations were solved for ionospheric conditions appropriate for comet Halley at 1 AU. The numerical scheme used is such that any shock transition appears naturally in the solution and no a priori assumptions are necessary. Solutions were obtained for a number of different assumptions concerning electron heating rates, but all showed that the electron temperatures increase rapidly and significantly at a distance from the nucleus where collisional electron‐neutral cooling becomes unimportant. This temperature increase is accompanied by a sharp increase in both the plasma pressure and its associated polarization electric field, causing the supersonic plasma flow to go subsonic. It is not clear at this time whether or not this sonic transition is accompanied by a shock.

The cooling of electrons by vibrational and rotational excitation of water molecules plays an important role in the thermal balance of electrons in cometary ionospheres. The energy loss function for rotational excitation and de-excitation of Hz0 by electron impact is calculated theoretically. The rotational cooling rate is calculated using this loss function for a wide range of electron and neutral temperatures. The vibrational cooling rate is calculated using measured values of electron impact vibrational excitation cross sections. Analytical formulae are provided for some of the cooling rates. The interaction of ions with H,O molecules is also discussed and a formula is suggested for the momentum transfer collision frequency.

The first results of a new time-dependent, axisymmetric dusty gas dynamical model of inner cometary atmospheres are presented. The model solves the coupled, time-dependent continuity, momentum, and energy equations for a gas-dust mixture between the nucleus surface and 100 km using a 40 x 40 axisymmetric grid structure. The timedependent multidimensional partial differential equation system was solved with a new numerical technique employing a second-order accurate Godunov-type scheme with dimensional splitting. It is found that narrow axisymmetric jets generate a subsolar dust spike and a jet cone, where a significant amount of the jet ejecta is accumulated. This subsolar dust spike has not been predicted on earlier calculations. The opening angle of the jet cone depends on the jet strength and it also varies during the time-dependent phase of the jet. For weak jets the steady-state half-opening angle is about 50 °. In the case of the strong jets the jet cone extends to the nightside in good agreement with the Giotto imaging results.

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A numerical solution to the 20‐moment set of transport equations has been found in order to study subauroral ionospheric outflows during periods of enhanced perpendicular ion drifts. The numerical model solves the time‐dependent O+ density, momentum, and both the parallel and perpendicular energy and heat flow equations in the 200–6000 km altitude range. Assuming perpendicular drifts of 3 km/s relative to the neutral atmosphere, we have found that anisotropic heating of O+ (a result of ion‐neutral collisions) leads to a temperature anisotropy, with perpendicular temperatures exceeding 8000 K and parallel temperatures greater than 5000 K (near 200 km altitude). Above approximately 2000 km, transport processes dominate the effects of collisions and wavelike oscillations in O+ velocity, temperature and heat flux were noted.