A vertical-plasma-slab model for determining the lower limit to plasma density in sprite columns from VLF scatter measurements

VLF to MF backscatter from “red sprites” is investigated by the use of a one-dimensional model, in which the sprite is considered as an infinite vertical slab of plasma. The reflection (backscatter) and transmission (forward-scatter) coefficients are found for a wide range of slab parameters. For a slab in which such parameters vary with altitude, the approximation is made that the scatter coefficients vary with altitude accordingly. A conductivity of 10^-4 S/m is found sufficient to explain the observed magnitudes of backscatter (large angles up to 180°), provided this or higher conductivity prevails over a substantial part of the cloud-ionosphere altitude range. The effect of the geomagnetic field on the conductivity at altitudes up to 80 km, and wave frequencies up to 10 MHz, is found to be of little consequence