An FDTD Investigation of Orthogonality and the Backscattering of HF Waves in the Presence of Ionospheric Irregularities

One of the most successful instruments used to investigate the Earth's ionosphere is coherent scatter radars such as the Super Dual Auroral Radar Network (SuperDARN). Their method of mapping plasma convection at high latitudes is contingent on irregularities that momentarily exist in the ionosphere and are elongated along the direction of the geomagnetic field. Under these conditions, SuperDARN transmits high‐frequency (HF) waves that are then backscattered from these plasma irregularities. In order for maximum backscatter to occur, the wave vector of the incident wave must be orthogonal to the magnetic field lines. This is called the “orthogonality condition.” Over the years, ray tracing results have generated many assumptions, where and how orthogonality occurs. However, since ray tracing only tracks the primary direction of energy flow and typically does not account for diffraction, it provides an incomplete understanding of the interaction of HF waves with small ionospheric irregularities. This work investigates the orthogonality assumptions by modeling at high‐resolution HF electromagnetic wave interactions with small‐scale irregularities in the ionosphere. Specifically, the finite‐difference time‐domain (FDTD) method is employed to determine where orthogonality occurs in the ionosphere and the number of times the condition is satisfied during the simulation. The results provide insights into the orthogonality condition as a function of source frequency and elevation angle for both unperturbed and perturbed ionospheres. From these results, small‐scale irregularities are observed to sometimes contribute significantly to the production of backscatter. A discussion is provided to highlight where three existing orthogonality condition assumptions are incorrect or misleading.