Simulation study of the interaction between large‐amplitude HF radio waves and the ionosphere

The time evolution of a large‐amplitude electromagnetic (EM) wave injected vertically into the overhead ionosphere is studied numerically. The EM wave has a carrier frequency of 5 MHz and is modulated as a Gaussian pulse with a width of approximately 0.1 milliseconds and a vacuum amplitude of 1.5 V/m at 50 km. This is a fair representation of a modulated radio wave transmitted from a typical high‐power HF broadcast station on the ground. The pulse is propagated through the neutral atmosphere to the critical points of the ionosphere, where the L‐O and R‐X modes are reflected, and back to the neutral atmosphere. We observe mode conversion of the L‐O mode to electrostatic waves, as well as harmonic generation at the turning points of both the R‐X and L‐O modes, where their amplitudes rise to several times the original ones. The study has relevance for ionospheric interaction experiments in combination with ground‐based and satellite or rocket observations.