Physics
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt.........1v&link_type=abstract
Thesis (PHD). UNIVERSITY OF MARYLAND COLLEGE PARK , Source DAI-B 59/06, p. 2820, Dec 1998, 138 pages.
Physics
Riometry
Scientific paper
This dissertation is a study of disturbances in the polar ionosphere. A relative ionospheric opacity meter (riometer) is a radio frequency instrument that enables the remote sensing of ionospheric disturbances by recording variations in the cosmic radio noise power received at a terrestrial antenna. The Imaging Riometer for Ionospheric Studies (IRIS) produces images of relative ionospheric opacity. In the ionosphere, the attenuation of a radio signal's amplitude is proportional to the electron number density n and the effective collision frequency ν. Therefore, a riometer is sensitive to variations of the product n/nu, but their effects are not separated. The theory of HF radiowave attenuation in a cold magetoplasma and electron continuity yield a pair of uni-directional wave equations that couple the dynamics of cosmic radio noise absorption to the vertical mean value of ν. These equations, and some simplifying assumptions, are the basis of a data analysis that transforms IRIS images into physical quantities related to the absorbing ionospheric electrons: mean velocity, mean effective collision frequency, net production rate and column density. A critical test case and coincident auroral observations support the reliability of the general results of the data analysis. Variations in the mean flow velocity indicate that the ionosphere is not in equilibrium. The mean effective collision frequency shows significant structural variations over 100 km and 1 minute intervals. Column density depletions lead enhancements in a geomagnetic poleward drift, while a net production region moves with the column density enhancement and intensifies as the pole-ward motion ceases. Regions of persistent electron production or loss are found where the collision frequency is relatively low, and specific locations can oscillate between net production and loss with periods of about 1 to 2 minutes. It is found that the spatial structure of a riometer image is chiefly determined by the electron column density, whereas the effective collision frequency affects the time evolution. This dissertation successfully separates the roles that electron column density and mean collision frequency play in radiowave absorption and, thereby, quantitatively extends the physical scope of the IRIS radiowave imager.
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