Coordinated study of subkilometer and 3‐m irregularities in the F region generated by high‐power HF heating at Arecibo

Abstract

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 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 an 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.