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 in finite vertical slab of plasma. The reflection (backscatter) and trans mission (forward-scatter) coefficients are found for a wide range of s lab parameters. For a slab in which such parameters vary with altitude , the approximation is made that the scatter coefficients vary with al titude accordingly. A conductivity of 10(-4) S/m is found sufficient t o explain the observed magnitudes of backscatter (large angles up to 1 80 degrees), provided this or higher conductivity prevails over a subs tantial part of the cloud-ionosphere altitude range. The effect of the geomagnetic field on the conductivity at altitudes up to 80 km, and w ave frequencies up to 10 MHz, is found to be of little consequence.