Satellite particle exospheres of planets: Application to Earth

Richter, Enrico; Fahr, H. J.; Nass, H. U.

doi:10.1016/0032-0633(79)90136-3

Cited by 13 publications

(14 citation statements)

References 16 publications

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“…At larger geocentric distances of r ≥ 4 R E , hydrogen atoms of the so-called "elliptic" type may be the dominant contributors to the exospheric H-density (Fahr and Shizgal, 1983;Richter et al, 1979). Elliptic-or satellite-type particles are injected into their orbits above the exobase and can orbit Earth several times without undergoing collisions.…”

Section: Dawn/dusk Asymmetrymentioning

confidence: 99%

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

2013

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The Lyman-α Detectors (LAD) on board the two TWINS 1/2-satellites allow for the simultaneous stereo imaging of the resonant emission glow of the H-geocorona from very different orbital positions. Terrestrial exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation. During the past solar minimum, relevant solar parameters that influence these emissions were quite stable. Here, we use simultaneous LAD1/2-observations from TWINS1 and TWINS2 between June 2008 and June 2010 to study seasonal variations in the H-geocorona. Data are combined to produce two datasets containing (summer) solstice and (combined spring and fall) equinox emissions. In the range from 3 to 10 Earth radii (R_E), a three-dimensional (3-D) mathematical model is used that allows for density asymmetries in longitude and latitude. At lower geocentric distances (< 3 R_E), a best fitting r-dependent (Chamberlain, 1963)-like model is adapted to enable extrapolation of our information to lower heights. We find that dawn and dusk H-geocoronal densities differ by up to a factor of 1.3 with higher densities on the dawn side. Also, noon densities are greater by up to a factor of 2 compared to the dawn and dusk densities. The density profiles are aligned well with the Earth–Sun line and there are clear density depletions over both poles that show additional seasonal effects. These solstice and equinox empirical fits can be used to determine H-geocoronal densities for any day of the year for solar minimum conditions

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Section: Dawn/dusk Asymmetrymentioning

confidence: 99%

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

2013

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“…a component of H-atoms orbiting the earth in long living Keplerian trajectories with perigees above the exobase (see Chamberlain, 1963). Such satellite-type H-atoms are likely to dominate in the uppermost regions of the geocorona (see Richter et al, 1979;Fahr and Shizgal, 1983), and with the densities derived from TWINS-LAD data one should feel encouraged to investigate mechanisms to populate these Hatom satellite orbits. The simple r-dependent H-density model, as we have shown, could furthermore not explain the observed scatter in the data of the LAD-1/2 Ly-α-intensity for lines of sight with identical LOS impact altitudes but different viewing directions.…”

Section: Conclusion and Comparison With Other Planetary Exospheresmentioning

confidence: 99%

3-D-geocoronal hydrogen density derived from TWINS Ly-α-data

et al. 2010

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Abstract. Based on Ly-α-line-of-sight measurements taken with two Ly-α detectors onboard of the satellite TWINS1 (Two Wide-angle Imaging Neutral-atom Spectrometers) density profiles of the exospheric, neutral geocoronal hydrogen were derived for the time period between summer solstice and fall equinox 2008. With the help of specifically developed inversion programs from Ly-α line of sight intensities the three-dimensional density structure of the geocoronal hydrogen at geocentric distances r > 3 R E could be derived for the period mentioned characterized by very low solar 10.7 cm radiofluxes of ≈65-70 [10 −22 W m −2 Hz −1 ]. The time-variable, solar "line-centered"-Ly-α-flux was extracted on the basis of daily (terrestrial) NGDC 10.7 cm radioflux data using the models from Barth et al. (1990) and VidalMadjar (1975).The results for the geocoronal H-densities are compared here both with theoretical calculations based on a MonteCarlo model by Hodges (1994) and with density profiles obtained with the Geocoronal Imager (GEO) by Østgaard and Mende (2003). In our results we find a remarkably more pronounced day-/night-side asymmetry which clearly hints to the existence of a hydrogen geotail (i.e. a tail structure with comparatively higher hydrogen densities on the night side of the earth for geocenctric distances >4 R E ), and a only weakly pronounced polar depletion. These unexpected features we try to explain by new models in the near future. The derived 3-D-H-density structures are able to explain the line-of-sight (LOS) dependent Ly-α intensity variations for all LOS seen up to now with TWINS-LAD. The presented results are valid for the region with geocentric distances 3 R E < r < 7 R E and are based on the reasonable assumption of an optically thin H-exosphere with respect to resonant Ly-α-scattering allowing the use of single scattering calculations.

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“…The details of the calculation of the rates of the reactions listed in Table 4 have been given elsewhere [Richter et al, 1979;Richter, 1980]. Here we give a brief summary of their Figures 20 and 21 we give, as an example, the height dependence of the production rates and destruction frequencies of the processes listed in Table 4.…”

Section: Hs + O-->h + O Hs + He-->h + He H• + O + --> H + + O H• + H+mentioning

confidence: 99%

Modern exospheric theories and their observational relevance

Fahr¹,

Shizgal²

1983

Reviews of Geophysics

114

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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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Satellite particle exospheres of planets: Application to Earth

Cited by 13 publications

References 16 publications

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

3-D-geocoronal hydrogen density derived from TWINS Ly-α-data

Modern exospheric theories and their observational relevance

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