Ionospheric D Region Remote Sensing Using ELF Sferic Group Velocity

Abstract

The lowest region of the ionosphere, the D region, plays an important role in magnetosphere-ionosphere coupling but is challenging to directly observe. The group velocity of the extremely low frequency (ELF; 3–300 Hz) portion of lightning induced electromagnetic radiation can be used to diagnose the D region electron density profile. Day-night conditions can be assessed using ELF receivers and lightning detection networks. Analytical formulations and the Long Wave Propagation Capability software package show that ELF group velocity has particular sensitivity to the sharpness of the exponential electron density profile. Applying the technique to sudden ionospheric disturbances shows that the group velocity increases in response to incidence of solar X-ray flux. A small number of ELF receivers can provide a large-scale diagnostic of the D region. ELF remote sensing using lightning is complementary to very low frequency remote sensing and can be used to assess the Earth-ionosphere propagation channel for very low frequency transmitters.