High-Frequency wave propagation in the ionospheric density irregularities*

In this presentation, we investigate the effect of small-scale ionospheric density irregularities, including equatorial plasma bubbles (EPBs), on the propagation of high-frequency (HF, 3–30 MHz) radio waves. HF radio waves play a crucial role in various practical applications. They are commonly used for short-wave international broadcasting and communications by aircraft, military operations, amateur radio operators, and emergency communications. Ground stations can communicate via HF radio waves reflected and refracted in the ionosphere. Therefore, radio communications are sensitive to ionospheric density irregularities like EPBs. By adopting an advanced finite element method (FEM) code, Petra-M, we examine HF wave propagations in the ionospheric density irregularity structure. In particular, we adopt a high-resolution electron density instability simulation result in the Petra-M code and examine the wave properties inside the equatorial plasma bubble. We found that small-scale plasma density irregularities significantly alter the direction of the wave in the ionosphere. More specifically, waves can penetrate the plasma bubble structure and reach higher altitudes. Additionally, the waves are trapped in the bubble, and mode conversion (MC) near the electron plasma frequency occurs near the density gradient. We predict that these phenomena - wave trapping and mode conversion - can lead to a reduction in radio power in addition to the ionospheric collisional absorption. We will also discuss an application of the numerical investigation of HF wave propagation, such as amateur communications and the High-frequency Active Auroral Research Program (HAARP).