Studies of the ionospheric E-F layer transition.
thesisposted on 19.11.2015, 09:19 by John William Sydney. Hart
The region of the ionosphere situated above the E layer peak and below the F layer is difficult to investigate using conventional ground based techniques, consequently our knowledge of this region is limited. An attempt has been made in this study to construct representative electron density distributions from a consideration of the fine structure observed on ionospheric sounding records and of the variation of the absorption and virtual height of a fixed frequency during the growth of the ionosphere below 200 km following the sunrise. Special reference was made to the results obtained for frequencies reflected near the E layer peak and the validity of this analysis in this region of high deviative absorption and group retardation is discussed. Attention is drawn to the large variation in published values of the collisional frequency above 120 km and an attempt has been made to construct a suitable height variation of this parameter for use in the calculation of absorption. Routine ionospheric soundings were made hourly over an extended period and augmented with continuous soundings on a number of selected days between February and June 1969. Also on 42 days during this period, measurements of absorption and virtual height were carried out on a fixed frequency for several hours following the sunrise. The variations of absorption with time and virtual height with time and frequency were computed from the electron density profiles obtained from 3 independent sources (a) Theoretical post sunrise profiles constructed by Bourne, Setty and Smith. (b) Monotonic profiles computed from Leicester Ionograms (c) Profiles observed by a Thomson Scatter technique at R.R.E. Malvern. The results of these calculations are compared directly with the experimental observations and computations of the variation of absorption with frequency are also discussed. From a synoptic study of the cusp features observed on ionospheric soundings it is found that complex cusp structure around f E is a regular phenomenon at Leicester occurring throughout the day but showing some decrease in occurrence frequency towards noon. The development of this structure can be represented by an expression of the form fcusp = A cosn x where different cusp features take differing values of the constant A. The comparisons made in this study indicate that the polynomial reduction technique produces monotonic electron density profiles which satisfactorily represent conditions when only the virtual height is considered but are not capable of producing calculated absorption values to the same degree of accuracy. The virtual height and absorption values computed from the Thomson Scatter profiles are in good agreement with experimental observations of these parameters at Leicester. These profiles are therefore considered to accurately represent the structure of the electron density distribution. The existence of the small undulations, as evident in these profiles, is shown to account for the multi-cusp phenomenon and various observations concerning the variability and extent of such structures are described. In addition, these comparisons have indicated some inadequacy in the growth theory employed in the derivation of the Bourne, Setty and Smith profiles. As a result of comparing the variation with frequency of both the virtual height and absorption it has been shown experimentally and supported theoretically that indicating that absorption especially in the deviative region may be a more sensitive parameter to profile fine structure. It is anticipated therefore that sweep frequency absorption sounding may indicate more clearly short term perturbations in the electron density profile. (Abstract shortened by UMI.).