Physics
Scientific paper
Jun 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........20n&link_type=abstract
Thesis (PhD). BOSTON UNIVERSITY, Source DAI-B 58/12, p. 6628, Jun 1998, 162 pages.
Physics
Scientific paper
The thermal and dynamical properties of hydrogen and helium in the Earth's atmosphere above 300 km are investigated using optical, infrared and radar frequencies. Hydrogen and Helium are the dominant neutral and ionized species above about 600 km. A better understanding of their spatial and temporal distributions and velocity are important to spacecraft activities at these altitudes. The physics of the escape of light gases from terrestrial planetary atmospheres is central to the atmospheric evolution of terrestrial planets. Observational and analysis techniques are established that permit the direct measurement of the total escape flux. The discrepancy between the limiting flux, which is the rate at which a chemical species can be transported from the lower atmosphere to a point where it can escape the Earth's gravitational potential, and the Jeans or thermal evaporative escape flux of telluric hydrogen has been known since observations and theoretical calculations of the early 1970's. Results using 1986, 1991 and 1993 Balmer Alpha spectral line (hereafter Hα) data from Arecibo indicate a local escape flux similar to the theoretical limiting flux and about 1.5 times greater than the thermal escape flux. Inbound energetic populations in the vertical Hα profiles have also been detected. A correlation between inbound proton flux and hydrogen escape is shown, illustrating the importance of non-thermal plasma interactions as a source of Hydrogen escape flux. The helium production rate and its Jeans escape flux differ by two orders of magnitude. Observations of metastable helium in this dissertation provide a measurement of exobase temperature to help untangle the helium escape mystery. These infrared measurements represent a new approach to the problem using ground-based remote sensing. Temperatures derived from these observations increase with increasing shadow height and are higher then expected of a neutral species. Using recent ionized helium (He+) measurements from Arecibo, our results show that He+ recombination results in a much greater contribution to the thermospheric metastable helium population then previously thought. Metastables produced through recombination are thermalized to the ion population and not the neutral population, creating a metastable helium temperature enhancement in agreement with observation.
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