Small-Scale Plasma-Density Depletions in Arecibo High-Frequency Modification Experiments

Observations of Thomson scattering at uhf from Langmuir waves by a scheme involving the linear frequency modulation (chirping) of a uhf transmitter and the demodulation (dechirping) of the received signals show that a high-power hf wave used for ionospheric modification creates small-scale plasma depletions instantly. For a plasma frequency of 5.1 MHz, plasma frequency gradient of about 50 kHz/km, and power density input of 8×${10}^{-5\mathrm{}}$ W/${\mathrm{m}}^2$, the depletion ranged from 3% to 55%. This provides direct evidence that the hf-induced modifications involve Langmuir waves trapped in density cavities.