Modeling HF Waves Through Plasma Irregularities Using the FDTD Method

Radio scintillation and scattering at high latitudes of the earth can occur when polar cap patches are present. These patches, usually formed during periods of southward interplanetary magnetic field (IMF), enter by the dayside cusp and convect in the antisunward direction having densities 2-10 times the background densities. Irregularities are caused by gradient drift instabilities (GDI) and are observed throughout the patch. While the patch can range from 100 to 1000 km in size, the irregularities can range from meters to 10's of km. Various difficulties may arise in attempting to model scintillation and scattering of electromagnetic waves by these irregularities due to (1) modeling the complicated geometric structures of the irregularities, (2) simulating electromagnetic waves in a magnetized plasma environment, (3) and accounting for computational efficiency and accuracy. The finite difference time domain (FDTD) method is a new approach to simulate propagation of electromagnetic waves throughout the patch. Irregularities and patch profiles were generated based on Defense Meteorological Satellite Program (DMSP) data. The sizes of the irregularities can be defined by a power law function which is used to create a probability density function for irregularities in a given patch size. The patch profiles were added to a FDTD model having a dipole source in the high frequency region. Features such as spread F were captured in the simulation as small sized irregularities scattered the waves. Experimental spread F observations were used to validate the FDTD model. A ray tracing model was compared to the FDTD model with the same irregularity and patch profile to give contrast between the two modeling methods.