A Study of Hydrogen Diffusion in the Earth's Upper Atmosphere Near the Critical Level

Lew, Stephen K.; Venkateswaran, S.

doi:10.1175/1520-0469(1965)022<0623:asohdi>2.0.co;2

Cited by 16 publications

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“…The reasons for this are not entirely clear, since details of their methods are lacking. The work by Lew and Venkateswaren [1965] suffered from not choosing the lower boundary at a sufficiently low altitude below the critical level. This difficulty was subsequently corrected [Lew, 1967] as discussed later.…”

Section: Departure From Maxwellian At the Exobasementioning

confidence: 99%

Modern exospheric theories and their observational relevance

Fahr¹,

Shizgal²

1983

Reviews of Geophysics

113

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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.

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Section: Departure From Maxwellian At the Exobasementioning

confidence: 99%

Modern exospheric theories and their observational relevance

Fahr¹,

Shizgal²

1983

Reviews of Geophysics

113

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Exospheric models of the topside ionosphere

Lemaire

Schérer

1974

Space Sci Rev

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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.

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О Структуре Неитралянои Верхнеи Атмосферы

Izakov¹

1967

Space Sci Rev

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A Study of Hydrogen Diffusion in the Earth's Upper Atmosphere Near the Critical Level

Cited by 16 publications

References 0 publications

Modern exospheric theories and their observational relevance

Modern exospheric theories and their observational relevance

Exospheric models of the topside ionosphere

О Структуре Неитралянои Верхнеи Атмосферы

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