Electron density irregularities in the F region excited by the European incoherent scatter (EISCAT) high-power high-frequency (HF) facility in Ramfjordmoen, Norway, have been studied in detail by observing scintillations of 250-MHz satellite signals traversing the HF beam. Spaced antenna measurements were performed to determine the temporal structure of scintillation and the drift of the irregularities. Studies of scintillation 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 saturation amplitudes within tens of seconds, whereas longer scale-size irregularities with wavelengths exceeding 100 m saturate in the timescale of minutes. The threshold power density requirement for the generation of irregularities of different scales has also been studied by the variation of the heater power. It is found that for large-scale (>100 m) irregularities the threshold power density varies inversely as the irregularity wavelength with a power law index of 4 as predicted by the theories of self-focusing instability.¿ 1997 American Geophysical Union