Abstract
The interaction of very low frequency transmitter signals with radiation belt electrons depends ultimately on their wave normal angles. In the equatorial interaction region, these can be either low (ducted propagation) or comparatively large (nonducted propagation).
Experimentally distinguishing the two modes is complicated, as multicomponent spacecraft data typically do not extend to high enough frequencies with a sufficient frequency resolution. One exception that we exploit are 11.9 kHz signals from Alpha transmitters detectable by the Van Allen Probes spacecraft.
We use multicomponent burst mode measurements to distinguish between the ducted and nonducted modes of propagation and to evaluate their relative importance. While the ducted waves are detected less often, they tend to have larger Poynting fluxes.
The total power propagating in the two modes is thus comparable. Magnetic local time and in-situ density fluctuations are main parameters controlling the relative fraction of ducted waves.