Exospheric models of the topside ionosphere

Abstract

The historical evolution of the study of escape of light gases from planetary atmospheres is delineated, and the application of kinetic theory to the ionsphere is discussed. Ionospheric plasma becomes collisionless above the ion-exobase which is located near 1000 km altitude in the trough and polar regions, and which coincides with the plasmapause at lower latitudes. When the boundary conditions at conjugate points of a closed magnetic field line are different, interhemispheric particle fluxes exist from the high temperature point to the low temperature point, and from the point of larger concentrations to the point of smaller concentrations; therefore the charge separation electric field in the exosphere is no longer given by the Pannekoek-Rosseland field. For non-uniform number densities and temperatures at the exobase, the observed r-4 variation of the equatorial density distribution is recovered in the calculated density distributions. Taking account of plasmasheet particle precipitation does not change very much the electric field and ionospheric ion distributions, at least for reasonable densities and temperatures of the plasmasheet electrons and protons. For field aligned current densities along auroral field lines smaller than 10 -5 Am -~, the potential difference between the ion-exobase and plasmasheet is about --3V. In the case of open magnetic field lines the flow speed of hydrogen and helium ions in the exosphere becomes rapidly supersonic as a consequence of the upward directed charge separation electric field, whereas the oxygen ions have a negligible small bulk velocity. Adding a photoelectron etttux decreases the thermal electron escape but does not change significantly the number density distributions.

Ionospheric plasma of comet 67P probed byRosettaat 3 au from the Sun

“…(c) The location of Rosetta with respect to the so-called ion exobase (Lemaire & Scherer 1974) has been assessed. On the one hand, the mean free path of the ions is given by…”

Current–voltage relationship

“…It takes into account the partial current of the escaping H + ions that balances the partial current of the ionospheric electrons in the polar wind models where J J ¼ 0. As a consequence of the additional current carried upward by the ionospheric H + ions, the value of f E Àfs for which the FAC vanishes is smaller (V 0 ¼ 2.7 V) in the model of Lemaire and Scherer (1974) than in that of Knight. From Fig.…”

“…Therefore, ignoring the ionospheric O + , H + and He + ions as well as the plasmasheet proton density in both of these equations does not provide an accurate distribution of the electric potential between the ionosphere and infinity. When realistic concentrations of ions are added to the collisionless electron model of Knight (1973), the exospheric model developed by Lemaire and Scherer (1974) is obtained. The current-voltage relationship obtained in this more comprehensive collisionless-exospheric model of an auroral flux tube is shown in Fig.…”

Titan's ionosphere in the magnetosheath: Cassini RPWS results during the T32 flyby

“…Actually, the altitude of the ion exobase, where the mean free path of the ions λ i is equal to the density scale height H i , is slightly higher than the neutral exobase (Lemaire and Scherer, 1974) and depends on each species. The knowledge of the collision frequencies (previously given) and of the density scale height (calculated from the Langmuir probe data), combined with the ion temperature, leads to a rough estimation of this critical level for the main ion species HCNH + .…”