Towards VLF radio tomography

Summary form only given. The electromagnetic environment of the Earth was recently explored with networks of radio receivers which are distributed over spatial scales from ~1×1-10×10 km² (Mezentsev and Fullekrug, JGR, 2013, Fullekrug et al., SGeo, 2013). These networks were operated in an interferometric configuration to measure the ground and sky waves of 100 kHz radio waves from radio transmitters and lightning discharges. The key parameter for the interferometric signal processing is the frequency dependent slowness of the radio waves within the Earth´s atmosphere with respect to the speed of light in vacuum. Here we extend this interferometric signal processing into the Very-Low Frequency (VLF) range to actually measure the slowness of radio waves from submarine communication transmitters operating around ~20 kHz. The measured values are in good agreement with previously reported results. However, a more detailed analysis shows that the remaining residuals have a significant information content which enables an application of VLF radio tomography. Possible interpretations of the observed distortions of the radio wavefront will be discussed in light of the compelling measurements.