Evolution of NWC Transmitter Wave Power Distribution From the Topside Ionosphere Into the Inner Magnetosphere

Ground-based very low frequency transmitters emit signals that primarily propagate within the Earth–ionosphere waveguide, and some of their energy can propagate into the magnetosphere. Ionospheric observations from the DEMETER satellite reveal a distinct concentric rings pattern of the wave power distribution of the North West Cape transmitter on the transverse (longitude–L-shell) plane. Using data from the Van Allen Probes and Arase/energization and Radiation in Geospace satellites, we find that the concentric rings pattern is still visible but becomes less distinct. The pattern shifts outward, and becomes more elliptic as the wave propagates from the southern ionosphere to the Northern Hemisphere. To investigate the cause of this evolution, we performed ray tracing simulations under three conditions: ducted propagation, non-ducted propagation with vertical initial wave normal angles and non-ducted propagation with spread initial wave normal angles. The results show that non-ducted propagation with spread wave normal angles best explains the observed evolution of wave power distribution pattern during the propagation.