Enhanced ionospheric ELF/VLF generation efficiency by multiple timescale modulated heating

[1] It is shown theoretically that for high power HF ionospheric heaters, such as the current HAARP heater, the HF to VLF conversion can increase significantly by using a two-timescale heating. A long heating pulse, tp 1minute, is followed over the same timescale period bymodulation at the desired ELF/VLF frequency. The long pulse reduces the electron-ion recombination coefficient, resulting in increased ambient electron density and current density. The efficiency increases due to the increase in the current density over the time tp. Besides, the more efficient heating results from sharpening of the ambient density profile at the modified height that reduces low altitude self-absorption. It is shown that following the long preheating pulse by amplitude modulated heating at VLF frequencies can result in efficiency increase of up to 7 dB over the non-preheated case. In addition to the theory the letter describes proof-ofprinciple experiments using the completed HAARP. Citation: Milikh, G. M., and K. Papadopoulos (2007), Enhanced ionospheric ELF/VLF generation efficiency by multiple timescale modulated heating, Geophys. Res. Lett., 34, L20804, doi:10.1029/2007GL031518.