LEAST SQUARES REDUCTION OF BOTTOMSIDE IONOGRAMS.
PENNSYLVANIA STATE UNIV UNIVERSITY PARK IONOSPHERE RESEARCH LAB
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Techniques are presented by which the distribution of electrons in the bottomside ionosphere can be determined using bottomside ionograms. These methods are based upon a least squares solution of the virtual height equations. Using an iterative minimization scheme, the electron density profile can be approximated by a given functional form. For this type of analysis ordinary andor extraordinary mode data are used. Similarly, if the retardation due to the lower portion of a layer is known, the same minimization scheme can be used to approximate the nose of a layer by a parabola using virtual heights sampled near the peak plasma frequency. For the situation in which the virtual heights are not observed below a certain plasma frequency, the group refractive index is approximated by a Chebyshev polynomial expansion in the unseen region. Such an approximation simplifies the virtual height equation and enables one to invert the equation numerically. In this manner accurate true height profiles can be obtained without assuming a given functional form for the electron density distribution in the cutoff region. When a valley is present between two layers of the ionosphere, the ionogram may be reduced by treating it as a low frequency cutoff problem where the critical frequency of the lowest maximum of electron density is considered to be the cutoff frequency. Author