EVOLUTION OF SUBKILOMETER SCALE IONOSPHERIC IRREGULARITIES GENERATED BY HIGH-POWER HF WAVES

Electron density irregularities in the F region excited by the Europea n incoherent scatter (EISCAT) high-power high-frequency (HF) facility in Ramfjordmoen, Norway, have been studied in detail by observing scin tillations of 250-MHz satellite signals traversing the HF beam. Spaced antenna measurements were performed to determine the temporal structu re of scintillation and the drift of the irregularities. Studies of sc intillation spectra indicate that even though the onset occurs nearly simultaneously for all irregularity scale sizes, the electron density irregularities with wavelengths in the range of 50-15 m attain their s aturation amplitudes within tens of seconds, whereas longer scale-size irregularities with wavelengths exceeding 100 m saturate in the times cale of minutes. The threshold power density requirement for the gener ation of irregularities of different scales has also been studied by t he variation of the heater power. It is found that for large-scale (>1 00 m) irregularities the threshold power density varies inversely as t he irregularity wavelength with a power law index of 4 as predicted by the theories of self-focusing instability.