Augmentation of natural ionospheric plasma turbulence by HF heater waves

HF heating offers a powerful technique for controlled studies of ionospheri c plasma turbulence. Heater waves generate large plasma density depletions and sheet-like ionospheric irregularities in the F region, which can give r ise to spread F backscatter. Electric fields associated with the induced ir regularities can seed plasma instabilities, driven by such environmental ca uses as density gradients and ambient electric fields, to enhance spread F signatures. Significant reductions in the height-integrated radar backscatt er power, measured during the HF heating, indicate the depletion of magneti c flux tubes. Density gradients at the edges of the depletions provide new sources of free energy to augment ionospheric plasma turbulence, enhancing the spread F processes. Furthermore, depleted magnetic flux tubes create an d/or alter ionospheric ducts thus affecting radio wave propagation.