T. W. Chevalier

Terminal Impedance and Antenna Current Distribution of a VLF Electric Dipole in the Inner Magnetosphere

The current distribution and input impedance of an electric dipole antenna operating in a cold magnetoplasma at very low frequency (VLF) is determined through numerical simulation. A full wave solution of Maxwells equations using a finite-difference frequency-domain (FDFD) method is implemented to simulate electromagnetic wave propagation in this highly anisotropic medium. The classical perfectly matched-layer (PML) boundary condition is found to exhibit instabilities in the form of nonphysical wave amplification in this environment. To circumvent these difficulties, a PML is developed that is tailored to the cold plasma environment at VLF frequencies. It is shown that the current distribution for antennas with length <100 m is approximately triangular for magnetospheric conditions found at L = 2 and L = 3 in the geomagnetic equatorial plane. Calculated variations of input impedance as a function of drive frequency are presented for two case studies and compared with predictions of existing analytical work.

A PML utilizing k-vector information as applied to the whistler mode in a magnetized plasma

The perfectly matched layer (PML) in its most current form has shortcomings in certain cases involving propagation in anisotropic media, such as the whistler mode in a magnetized plasma, for which the vector component normal to the PML of the group velocity vector and the k-vector are anti-parallel with each other. We present a new type of PML that utilizes information on the k-vector direction by applying relevant spatial derivatives to the PML update equations. Derived expressions for the numerical reflection coefficient are used to quantify the performance of the PML for any incident angle. Numerical reflection calculations as well as time domain calculations are performed for whistler mode propagation in a magnetized plasma. We demonstrate the stability of the new PML and find that for the reflection coefficient Gamma, values of up to -40 dB can be realized