Large-scale ionospheric modifications by high-power radio waves: Theory and observation.

摘要

This thesis presents an investigation by analysis, computer modelling, and experimental observation of large scale (10's of kilometers) temperature and density modifications produced in the F-region of the ionosphere by a powerful HF wave (frequency 3-8 MHZ, total radiated power.1-1 MW) launched by a ground-based antenna. Comparison of nighttime results from three experimental campaigns at Arecibo covering a wide range (summer, winter and transition from solar minimum to solar maximum) of ionospheric conditions with a two dimension transport code demonstrate that nonlinear refraction of the HF beam is the major factor in production of the observed large perturbations ({dollar}delta{dollar}T{dollar}sb{lcub}rm e{rcub}{dollar}/T{dollar}sb{lcub}rm eo{rcub}sim{dollar} 3, {dollar}delta{dollar}n{dollar}sb{lcub}rm e{rcub}{dollar}/n{dollar}sb{lcub}rm eo{rcub}sim{dollar} 25%). Experimental results show the formation of a universal steady state consisting of a narrow (15-20 km) hot flux tube with a resulting density depression. This state is reproducible and its generation can exhibit large preconditioning effects. Both the transport code and experiments show that time evolution proceeds from a broad (60 km) symmetric heating pattern to a narrow spike (15 km) of intense heating at higher altitudes. Transport code results reveal that the observed time evolution is due to the reorientation of the reflection surface parallel to the geomagnetic field lines.; Additional modelling results demonstrate the saturation by density dependent recombination of ionospheric density cavities at low levels ({dollar}<{dollar}7%) in a daytime polar environment by comparisons between the transport code and a steady-state model previously proposed by Vas'kov and Gurevich.