Modern exospheric theories and their observational relevance

Abstract

A review and critique of present‐day kinetic theory models of planetary exospheres is presented. Models of ionized exospheres, specifically the solar and the terrestrial polar wind, are also discussed. The objective of the paper is to point out the need for a rigorous kinetic theory treatment of the atmosphere in the altitude region between the thermosphere and the exosphere. This is the region where the atmosphere undergoes a transition from a collision‐dominated to a collisionless situation. The various aspects of the exospheric problem are introduced and developed around this main theme. The exospheric problem and its considerable overlap with several related problems in physics and chemistry are noted. The calculation of the velocity distribution function of exospheric constituents with the spherically symmetric collisionless model is then presented. Various modifications of this standard model with regard to planetary rotation, nonuniform exobasic density and temperature distributions, and time‐dependent effects are subsequently discussed and compared. The extent to which collisionless models suffice to explain observational data of planetary exospheres is assessed. This assessment is given in terms of a comparison of density profiles or reduction factors determined with hydrogen and/or helium cells. A comparison of experimental measurements of the diurnal variation of terrestrial atomic hydrogen densities and theoretical models to explain the asymmetry is presented. The collisionless approach is criticized on theoretical grounds for the neglect of a consideration of the transition region and the resulting artificial boundary between the thermosphere and the exosphere. The aim of collisional models with regard to escape‐induced non‐Maxwellian effects, modifications of the temperature structure, nonthermal escape processes, and satellite particle populations from Monte Carlo simulations and/or kinetic theory models based on a Boltzmann equation is discussed, and the various results are compared. The various attempts to introduce collisional concepts are shown to be incomplete, each considering the problem with different objectives. Throughout, the importance of a more rigorous collisional treatment is stressed, in particular with respect to the description of nonthermal escape processes and satellite particle populations which cannot be considered in a collisionless context. The effects of charge exchange reactions, photoionization, and solar radiation pressure on satellite particles are examined. With regard to nonthermal escape processes, the absence of a detailed kinetic theory of product velocity distributions based on a collisional theory is also noted.

Effects of M dwarf magnetic fields on potentially habitable planets

“…The Earth magnetosphere is located at r M 10−15 R ⊕ (Bagenal 1992), where planetary thermal pressure can safely be neglected. The Earth also does not have a significant atmospheric escape ( a few kg/s, Fahr & Szizgal 1983). Mass loss from planets can be significant, however, especially for close-in gas giant planets orbiting solar-type stars (or hotter), as a consequence of high stellar irradiation (Lecavelier des Etangs 2007;Murray-Clay et al 2009;Ehrenreich & Désert 2011;Owen & Jackson 2012).…”

The response of the H geocorona between 3 and 8 <i>R</i><sub>e</sub> to geomagnetic disturbances studied using TWINS stereo Lyman-<i>α</i> data

“…In the past, observations have been carried out with high-altitude rockets or satellites (e.g., Kupperian et al, 1959;Johnson, 1961;Rairden et al, 1986;Østgaard et al, 2003) and even by Apollo 16 from the Moon (Carruthers et al, 1976). Numerous theoretical studies on this subject have been published (e.g., Chamberlain, 1963; J. H. Zoennchen et al: The response of the H geocorona between 3 and 8 R e to geomagnetic disturbances Thomas and Bohlin, 1972;Fahr and Shizgal, 1983;Bishop, 1991;Hodges Jr., 1994).…”

Modeling of Venus, Mars, and Titan

“…Once this equation is solved, the velocity distribution is integrated in the region of velocity space consistent with trajectories coming from the exobase in a gravitational field. The velocity space can be divided into different regions, which represent different classes of trajectories (Fahr and Shizgal 1983). Figure 8 displays a cut of the velocity space.…”

“…8 Repartition of the different classes of particle in the velocity space adapted from Fahr and Shizgal (1983). The reduced velocity w is defined as the ratio of the velocity and the thermal velocity.…”