Whistler Mode Chorus Propagation in Inhomogeneous Small Scale Density Structures

Whistler mode chorus waves are among the most intense electromagnetic waves in the Earth's magnetosphere and a key driver of radiation belt dynamics. Chorus waves are known to be generated within a few degrees of the geomagnetic equator from a non-linear cyclotron resonance interaction involving phase trapping of counter streaming electrons. Once they are generated chorus waves' propagation characteristics are strongly influenced by the background plasma density.

Field aligned plasma density irregularities can significantly modify whistler mode trajectories and in general focus wave energy along the geomagnetic field line and enforce small wave normal angles. The density structures can also affect the occurrence of chorus waves and their intensity, by lowering the required minimum wave energy threshold for the wave-particle interaction to access the nonlinear regime. In the context of non-diffusive particle acceleration to relativistic energies that requires a minimum level of coherence, density structures can affect the coherence of chorus waves. On the other hand, spatially limited or short ducts near the equator could guide waves at moderately oblique wave normal angles depending on their size. The outer boundary of the plasmapause itself can be a source of these irregularities.

Although classical ray tracing approximations can be applied if the density structure dimensions are large compare to a wavelength, however, to model wave propagation in the smaller scale density irregularities a full wave solver must be employed with no assumptions on the propagation medium. We use a two dimensional finite difference time domain (FDTD) scheme to solve the Maxwell's equation and fluid momentum equation linking through the displacement current. This model is used to quantify the effects of small density irregularities on chorus wave properties, namely the energy trajectory, degree of amplitude focusing, and wave normal angle.