Nonthermal Messages from Thermal Planetary Atmospheric Systems

Abstract

Hans J. Fahr, Michael Heyl

Light atmospheric gas constituents tend to evaporate from the planetary gravitational fields. The point is that not only the uppermost atmospheric layer contributes to this gas escape, but the lower layers contribute their share as well and give the outcoming particle flow a nonthermal, non-Maxwellian character. In this article we do study the outflow of hydrogen atoms from a planetary oxygen atmosphere assumed to be one-dimensionally stratified by the action of the planet‘s gravitational field. This outflow is modified by local elastic collisions of upwards flying H-atoms with the heavy major atmospheric background constituent, as in case of the terrestrial atmosphere, the monoatomic oxygen atoms. This shock-modulation of the upwards particle flow produces nonthermal kinetic features of the particle distribution which we want to decribe. Since angle-integrated elastic collision cross sections are velocity-dependent, falling off with the velocity v like (1/v), the occuring collision-modulation of the H-atom flow does change the kinetic velocity profile of the escaping H-atoms. Deeply down in the lower atmophere the local H-atoms, like the O-atoms as well, are in thermodynamical equilibrium characterized by Maxwell Boltzmann distributions with a common temperature TH = TO. Nevertheless, at the upper exobase border of the atmosphere the resulting H-atom escape flow is shown to be a nonMaxwellian, non-equilibrium flow with non-thermal escape-relevant properties. We describe this collisional modification of the H-escape flow and can quantify the upcome of kinetic non-equilibrium features like power laws in the H-distribution function. Thus, as we demonstrate in this article, this collisional modulation effect via velocity-dependent collision cross-sections acts as a typical process to convert equilibrium distributions into non-equilibrium distribution functions. On the basis of this new kinetic theoretical approach we then calculate the effective escape flux of H- atoms to open space, demonstrate its nonthermal character, and quantify its difference with respect to the classical Jeans escape value. We find that with the classical Jeans formula one slightly overestimates the actual escape flux by an amount that varies between 30 - 80 percent, depending on the temperature of the lower atmosphere of the planet which varies with the solar 10.7-cm- radioflux.

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