Evaluation of nighttime exponential ionospheric models using VLF atmospherics

Measurements of very low frequency (VLF) atmospherics were made simultaneously at a station close to the lightning sources (Norman, Oklahoma) and at stations 1787 km (La Posta, California) and 5912 km (Kamuela, Hawaii) from Norman. Timing synchronization was maintained between stations to better than 10 μsec which, coupled with the source bearing from Norman, allowed accurate fixing of source locations. Using the numerical techniques of Pappert et al. for propagation parameter calculations, an appropriate exponential ionospheric conductivity model was chosen which best fit the modal interference structure near 10 kHz as a function of source distance as measured at La Posta. From source current moment spectra measured at Norman, the electric field strengths at La Posta and Kamuela were predicted for a wide frequency band (2 to 30 kHz) using the selected conductivity profile. For β = 0.6 km−1 and h′ = 84 km (in the notation of Wait and Spies) good agreement between the numerical calculations and the average measured field strengths was obtained.