A graphical processing unit (GPU)-accelerated finitedifference time-domain (FDTD) scheme for the simulation of radio-frequency (RF) wave propagation in a dynamic, magnetized plasma is presented. This work builds on well-established FDTD techniques with the inclusion of new time advancement equations for the plasma fluid density and temperature. The resulting FDTD formulation is suitable for the simulation of the timedependent behavior of an ionospheric plasma due to interaction with an RF wave and the excitation of plasma waves and instabilities. The stability criteria and the dependence of accuracy on the choice of simulation parameters are analyzed and found to depend on the choice of simulation grid parameters. It is demonstrated that accelerating the FDTD code using GPU technology yields significantly higher performance, with a dual-GPU implementation achieving a rate of node update almost two orders of magnitude faster than a serial implementation. Optimization techniques such as memory coalescence are demonstrated to have a significant effect on code performance. The results of numerical tests performed to validate the FDTD scheme are presented, with a good agreement achieved when the simulation results are compared to both the predictions of plasma theory and to the results of the Tech-X VORPAL 4.2.2 software that was used as a benchmark. Index Terms-Electromagneticpropagation, finite-difference time-domain (FDTD) methods, graphical processing unit (GPU) computing, ionosphere, magnetized plasma. Patrick D. Cannon received the M.S. degree in physics from the University of Oxford, St. Catherine's College, Oxford, U.K., in 2010. He is currently pursuing the Ph.D. degree in physics at Lancaster University, Lancaster, U.K.His research interests include development of a high-performance FDTD code to investigate the nonlinear plasma processes that may occur when a radiofrequency electromagnetic wave interacts with the ionosphere.Farideh Honary received the B.. Her research interests include investigating the complex behavior of energetic particles during magnetic storms and substorms with ground-and space-based instruments, utilizing the ionosphere as a natural plasma laboratory to study a variety of important nonlinear plasma processes associated with the interaction of high-power HF waves and the ionospheric plasma. This is done by employing the EISCAT high-power HF transmitter, and investigation of lunar dust charging and dynamics.

Recent ionospheric modification experiments performed at Tromsø, Norway, have indicated that X‐mode pump wave is capable of stimulating high‐frequency enhanced plasma lines, which manifests the excitation of parametric instability. This paper investigates theoretically how the observation can be explained by the excitation of parametric instability driven by X‐mode pump wave. The threshold of the parametric instability has been calculated for several recent experimental observations at Tromsø, illustrating that our derived equations for the excitation of parametric instability for X‐mode heating can explain the experimental observations. According to our theoretical calculation, a minimum fraction of pump wave electric field needs to be directed along the geomagnetic field direction in order for the parametric instability threshold to be met. A full‐wave finite difference time domain simulation has been performed to demonstrate that a small parallel component of pump wave electric field can be achieved during X‐mode heating in the presence of inhomogeneous plasma.

Experiments in the illumination of the F region of the ionosphere via radio frequency waves polarized in the ordinary mode (O‐mode) have revealed that the magnitude of artificial heating‐induced effects depends strongly on the inclination angle of the pump beam, with a greater modification to the plasma observed when the heating beam is directed close to or along the magnetic zenith direction. Numerical simulations performed using a recently developed finite‐difference time‐domain (FDTD) code are used to investigate the contribution of the O‐mode to Z‐mode conversion process to this effect. The aspect angle dependence and angular size of the radio window for which conversion of an O‐mode pump wave to the Z‐mode occurs is simulated for a variety of plasma density profiles including 2‐D linear gradients representative of large‐scale plasma depletions, density‐depleted plasma ducts, and periodic field‐aligned irregularities. The angular shape of the conversion window is found to be strongly influenced by the background plasma profile. If the Z‐mode wave is reflected, it can propagate back toward the O‐mode reflection region leading to resonant enhancement of the electric field in this region. Simulation results presented in this paper demonstrate that this process can make a significant contribution to the magnitude of electron density depletion and temperature enhancement around the resonance height and contributes to a strong dependence of the magnitude of plasma perturbation with the direction of the pump wave.