| This paper presents a systematic analysis of changes in ionosphere caused by underdense heating using oblique incidence high power radiowave considering the influence of earth's magnetic field. The generalized definition of the plasma field and the differential equation of electron temperature considering the earth magnetic field are presented. The field strength, electron temperature, collision frequency and the heater-induced absorption in the lower ionosphere under different heating modes (X-Mode and O-Mode) in nighttime are investigated on a self-consistent basis. The behavior of sporadic-E layers under the interaction of high power radiowaves is also reviewed and the conditions of possible F-region heating is qualitatively analyzed. Numerical calculations are carried out to investigate how the incident electric field and plasma field change the electron temperature, collision frequency and heater-induced absorption for different heating powers, frequencies and elevation angles. The data and results from a Voice of America (VOA) high power heating experiments are presented. An explanation based on primary heating and secondary heating effects is presented to explain the observed results for both a series of former Soviet Union ionospheric modification experiments and our VOA experiments.; The primary heating effect is produced in the lower ionosphere, resulting in additional absorption and leading to an instantaneous decrease of amplitude of the received signal. The secondary heating effect relates to the perturbation of electron density which is quite different at different altitudes for daytime and nighttime. The analysis of these effects considers both the magnitude of the change in the steady state and the time constant for these changes. |
