The production and modification of ionospheric irregularities by powerful HF radio transmissions

Several types of active experiments were performed with the high power HF heater near Arecibo, Puerto Rico. The temporal behavior of suprathermal electron fluxes was studied by observing the 430 MHz radar backscatter from ionospheric Langmuir waves that were Cerenkov emitted by the suprathermal electrons during pulse operation of the HF heater. Coded radar pulse techniques were used to study the height dependence of radar backscatter from Langmuir turbulence for both CW and pulsed HF heating. Observations with the 430 MHz radar of the power backscattered from HF induced Langmuir turbulence with a temporal resolution of 1 ms sometimes showed almost periodic short bursts of power at time intervals of 20 to 50 ms. The observational limitations on the role of strong Langmuir caviton turbulence were pointed out and the important role of field aligned depletions of the plasma density were stressed in accounting for the observed height dependence of the 430 MHz radar backscatter and for nature of the observed 46.8 MHz radar backscatter.