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A comprehensive analysis of the ionization rates of air by RF fields is presented. The analysis relies on a time-dependent code which treats the electron energization with a Fokker-Planck type model and the inelastic energy losses with a multiple time scale technique. Derivation of ionization rates for parameters of interest To D region ionospheric by ground-based RF transmitters with frequency much higher than the electron neutral collision frequency is emphasized. The study provides a physical understanding of the ionization proces and its associated efficiency by combining the computational results with analytic theory. It is shown that for values of quiver energies inline image « I, where I is the ionization potential, the electron production time corresponds to the electron energization time from energies below 2 eV to 20–25 eV. The analytic expressions derived are consistent with the computational results over 6 orders of magnitude in ionization rates and over 2 orders of magnitude in values of

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Power threshold definitions are clarified, and the pitfalls of using fluid descriptions or effective electric field notions are discussed. The paper concludes with an assessment the power requirements for ionization at 70-km ionospheric altitude with RF in the 100–900 MHz frequency range.