Ionospheric plasma density irregularities measured by stimulated electromagnetic emission

It is well known that ionospheric plasma turbulence can be conveniently generated by controlled injection of powerful high-frequency radio beams from the ground. Irradiation of the ionosphere with such radio waves leads to the formation of plasma density structures, striations, and the generation of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission (SEE). In this paper we present experimental results of the dependence of SEE on decreasing excitation levels of the striations. In the experiments the frequency of the injected radio beam was varied near the fifth harmonic of the local ionospheric electron gyro frequency. We use the SEE measurements to obtain transverse length scales of the striations involved in the generation of the SEE. Our results show that different spectral features of the SEE display different temporal dynamics, suggesting that they are related to striations with different transverse length scales (1 less than or similar to L-perpendicular to less than or similar to 25 m).