High-power high-frequency transmitters near Arecibo were used to generate artificial ionospheric irregularities in the F region. Radio star scintillation observations at 430 MHz were performed at Arecibo Ionospheric Observatory with the 305-m antenna, and radar backscatter measurements at 50 MHz were simultaneously made from Guadeloupe Island to probe the subkilometer and 3-m irregularities in the heated volume. Scintillation studies indicate a low-frequency modulation of the faster intensity fluctuation structure. By the use of plasma drift data this low-frequency modulation of the faster intensity fluctuation structure. By the use of plasma drift data this low-frequency temporal structure translates to spatial dimensions of 1--2 km. The frequency of the modulation envelope is found to be controlled by the heater power and is related to the dominant irregularity wavelength generated by the self-focusing instability. Scintillation spectra imply a steep power law index of ~5 in the scale length range of about 300 m to 150 m and a shallow index of ~2 at less than 150 m. The steep power law index may arise from a aperture averaging effect of the large 305-m antenna at Arecibo. Simultaneous measurements of 430-MHz scintillations and 50-MHz radar backscatter from field-aligned striations were performed to show that subkilometer irregularities can be generated by both O and X mode heating whereas the 3-m irregularities are excited only by the O mode heating, as is predicted by the theories of self-focusing and parametric instability. The width of the 50-MHz echo Doppler spectra is observed to be very narrow, only ~2-3 Hz, and independent of the background plasma drift, implying that the frequency bandwidth of the scattered signal is probably controlled by the instability process.