Deterministic Chaos in Ionospheric Plasma Pumped by Radio Waves

A commonly observed spectral feature of electromagnetic emissions stimulated by powerful radio waves in ionospheric plasma is found to have an exponential frequency spectrum. This so called broad upshifted maximum (BUM) is excited for pump frequencies near and slightly above an harmonic of the ionospheric electron gyro frequency. A simplified model of lower hybrid oscillations driven by parametric four‐wave interaction localized in a cylindrical density depletion shows how the plasma drift can exhibit deterministic chaos. The associated exponential spectra are consistent with the experimental results. Plain Language Summary Radio waves transmitted into the ionosphere can excite turbulence in the ionospheric plasma. This plasma is a result of the solar radiation that during daytime partially ionizes the thin gas of the atmosphere, thereby creating a gas of electrically charged particles (electrons and ions). One object of these experiments is to understand how plasma, the first and most common state of visible matter in the universe, responds to flows of energy, particularly the interplay of order and disorder in the turbulence. A commonly observed spectral feature of the electromagnetic emissions from the turbulence that can be stimulated by radio waves in the ionosphere is found to have an exponential frequency spectrum. This so called broad upshifted maximum (BUM) is excited for radio‐wave frequencies near and slightly above an harmonic of the ionospheric electron gyro frequency which is a result of the earth's magnetic field in the ionosphere. A simplified model is presented of four different waves interacting in a filamentary cylindrical plasma density depletion and driven by the transmitted radio wave. It shows how the plasma motion can exhibit deterministic chaos that is consistent with the experimental results. The chaotic motion thus resides inside the ordered plasma density filament. Key Points Electromagnetic emissions stimulated by powerful radio waves in the ionosphere commonly exhibit an exponential frequency spectrum Exponential fluctuation power spectra are a characteristic of deterministic chaos A simplified model of parametric four‐wave interaction in two‐dimensional geometry shows how the chaotic dynamics can occur