Ground‐based transmitter signals observed from space: Ducted or nonducted?

The principal loss mechanism for electrons from the inner radiation belt (1.2 < L < 2.0) and slot region (2.0 < L < 3.0) is atmospheric precipitation driven by several processes, including coulomb collisions, plasmaspheric hiss, lightning‐generated whistlers, and manmade transmissions. Several studies have shown that ducted and nonducted VLF waves can precipitate radiation belt energetic electrons into the upper atmosphere. Here we investigate the propagation of VLF communication transmitter signals using plasma wave instruments on board the CRRES and DEMETER satellites in order to determine if nonducted transmitter signals are significant in radiation belt loss processes. We investigate the regions where strong transmitter signals are observed in the ionosphere directly above the transmitter, in the magnetosphere near where the signals cross the geomagnetic equator, and in the ionospheric region geomagnetically conjugate to the transmitter. For very low L ‐shell transmitters ( L < 1.5) there is evidence that a significant proportion of the wave energy propagating into the plasmasphere is nonducted. However, at higher L ‐shells the waves become highly ducted in the plasmasphere. Strong evidence for this comes from the lack of significant wave power propagating above the electron half gyrofrequency limit for interhemispherically ducted waves. We conclude that manmade transmissions in the frequency range (18–25 kHz) will be restricted to driving electron precipitation primarily from the inner radiation belt ( L = 1.3–2.5). This will come about through a combination of propagation types, partly through nonducted wave propagation at very low L ‐shells ( L = 1.3–1.5), but predominantly through ducted wave propagation at higher L ‐shells ( L = 1.5–2.5), ultimately limited by the electron half‐gyrofrequency limit for ducted waves.