Radio Frequency Wave Propagation in the Ionospheric Density Irregularities: Ray tracing and Full-wave Simulations

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We investigate the impact of small-scale ionospheric density irregularities, specifically equatorial plasma bubbles (EPBs), on the propagation of radio frequency waves. High-frequency communications depend on the reflection of signals off the ionosphere, making them sensitive to varying ionospheric conditions, such as density irregularities like EPBs. In this study, we perform ray tracing simulations to consider larger-scale EPBs and utilize full-wave simulations to analyze smaller-scale plasma irregularities. We first employ the PHaRLAP ray tracing code to model individual rays and track their paths through the ionosphere. For this, we adopt a two-dimensional electron density profile derived from SAMI3 simulations. To explore wave propagation, we examine one-hop transmissions across various elevation angles and frequencies. This analysis enables us to assess how irregularities affect wave accessibility and signal delay. Additionally, we evaluate the implications of phase difference statistics on the scintillation index, $\sigma_{\varphi}$. By using PHaRLAP to model individual ray propagations within the ionosphere, we aim to gain a deeper understanding of how EPBs influence high-frequency long-distance communication through empirical and statistical calculations of values such as $\sigma_{\varphi}$. Furthermore, we introduce an advanced full-wave simulation code called Petra-M. This code is based on a scalable MFEM finite element library, and its accuracy has been thoroughly verified through several benchmarking activities. Petra-M has already been widely used in various fusion devices and space plasma studies. By employing Petra-M, we examine radio frequency wave propagation within small density irregularity structures. We utilize high-resolution electron density instability simulation results in the Petra-M code to investigate wave properties within these small structured fluctuations. Our findings indicate that small-scale plasma density irregularities significantly alter the direction of waves in the ionosphere. We also observe that mode conversion can occur from an incoming electromagnetic wave to localized electrostatic waves at the electron plasma frequency near the density gradient. We predict that both large and small plasma irregularities can lead to a reduction in radio power, in addition to the ionospheric collisional absorption.

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