A joint United States - Brazil experiment was conducted near the geomagnetic equator to confirm the theoretically predicted strong dependence of the ionospheric reflection properties on the azimuthal direction of wave propagation. Skywave delays, reflection coefficients, and polarization ellipses were measured at different frequencies, ranges, and azimuths. The results were compared with full-wave computer calculations for several ionospheric models. A series of high resolution waveforms showing the sunrise transition from night- to daytime conditions is included.

Low latitude observations of VLF/LF pulse reflections from the lower ionosphere obtained at nine locations to the east and west of a transmitter in southeastern Brazil are described. The data provide a variety of information on the reflection properties of the ionosphere below about 90 km altitude. Aspects of the data are presented in quasi-dimensional formats useful for identifying ionosphere structure and variability, and detailed analyses of portions of the data provided, which characterize the effective heights of the reflection coefficients of the ionosphere at noon and midnight, over a frequency range from 15 to 65 kHz. Electron density models of the ionosphere, derived from VLF/LF reflection data are also discussed. Keywords: VLF propagation; LF Propagation; Ionosphere reflectivity d region.

In the years since the pioneering efforts of Sir Edward Appleton, M. A. F. Barnett, G. Breit, and M. A. Thve, many radio techniques have been employed to investigate the terrestrial ionosphere. The purposes of this book are to exam ine the basic physical interaction process of radio waves with the ionosphere, scrutinize each of the radio techniques currently in use, and describe the elements of each technique, as well as assess their capabilities and limitations. I have included some of the history of each technique, since we often tend to forget the efforts of the "pioneers". The interaction of radio waves with the terrestrial ionosphere has been described in considerable detail in several "classic" treatments, e.g., Ratcliffe (1959), Al'pert (1963), Budden (1961) and Davies (1965), Rishbeth and e.g., Flock (1979), Davies Garriott (1969), and in other more recent books, (1990), Hargreaves (1979), and Budden (1985). A few of the radio techniques have been described by Hargreaves (1979) and a book by Giraud and Petit (1978) has also included discussion of several of the techniques. The "WITS" handbook No. 2 (1989) also contains description of several radio techniques.