On the efficiency of ionospheric ELF generation

The scaling laws that control the efficiency of converting ground-based HF power to ELF power by using modulation of the polar electrojet current is discussed. The analysis is based on kinetic calculations of the modification of the ionospheric conductivity by HF waves in conjunction with the experimental results reported from the Tromso Max Planck and the Alaska high power auroral simulation (HIPAS) facilities. It is shown that the efficiency can be increased by more than a factor of 10⁴ by (1) using phasing to sweep the antenna beam over an area spanned by a maximum tilt of 35°, on a time scale faster than the cooling rate at the heating altitude (<100 μs) and (2) localization of the heating at the E region (90–100 km) where electron runaway can be induced resulting in substantial modification of the Pederson conductivity.