Ray tracing of whistler-mode chorus elements: implications for generation mechanisms of rising and falling tone emissions

Abstract

Using a well-established magnetospheric very-low-frequency (VLF) ray tracing method, in this work we trace the propagation of individual rising- and falling-frequency elements of VLF chorus from their generation point in the equatorial region of the magnetosphere through to at least one reflection at the lower-hybrid resonance point. Unlike recent work by Bortnik and co-workers, whose emphasis was on demonstrating that magnetospheric hiss has its origins in chorus, we here track the motion in the equatorial plane of the whole chorus element, paying particular regard to movement across field lines, rotation, and compression or expansion of the wave pulse. With a generation point for rising chorus at the equator, it was found the element wave pulse remained largely field aligned in the generation region. However, for a falling tone generation point at 4000 km upstream from the equator, by the time the pulse crosses the equator the wavefield had substantial obliquity, displacement, and compression, which has substantial implications for the theory of falling chorus generation.