Using Eccentricity to Locate Ionospheric Exit Points of Magnetospheric Whistler Mode Waves

Whistler mode waves are a type of electromagnetic plasma wave and play a dominant role in the energy dynamics of the near-earth space environment and associated space weather processes. Ground-based observations of these waves are important for numerical model validation, space weather monitoring, and discovery science. Unfortunately, ground-based observations are often challenging to interpret since information about traversal through the ionosphere and distance propagated in the earth-ionosphere waveguide is rarely available. A new approach to identifying the ionospheric exit points of the magnetospheric whistler mode emissions is presented, which takes into account the observed wave polarization. The method relies on the initial circular polarization of the ducted magnetospheric emissions which is shown to convert to linear polarization after propagation in the earth-ionosphere waveguide at a predictable rate. Finite-difference time domain modeling of observed eccentricity provides a metric for determining distance from observer to ionospheric exit point. The method is shown to produce good agreement with observations of magnetospheric chorus emissions, artificially triggered emissions, and lightning-induced whistlers.