VLF scattering from red sprites: Application of numerical modeling

Red sprites were discovered by chance in 1989 when a low-fight TV system wa s pointed above an active thunderstorm. Red sprites are associated with col umns of ionization in the Earth-ionosphere waveguide, from above the thunde rstorm into the D region of the ionosphere. The ionized columns have been d etected through "VLF sprites," perturbations of the phase and/or amplitude of subionospheric VLF transmissions, which can be used to study the electri cal properties of red sprites. There is extensive experimental evidence tha t VLF sprites may involve wide scattering angles and can produce back scatt ered radiation. Here we present a numerical and theoretical study of the sc attering of subionospheric VLF transmissions caused by the plasma columns a ssociated with red sprites. Comparison of the VLF scattering from sprites i s made between a non-Born rigorous model which assumes the sprites are infi nite columns of constant conductivity, and a three dimensional Born scatter ing code. Both formulations show excellent agreement with one another. The formulations predict VLF sprites similar to those experimentally observed f or all scattering angles. This shows that the conclusions of previous studi es into VLF sprites making use of the non-Born formulations of Rodger et al . [1997a, b] are valid. The modeling provides strong evidence that red spri te plasma is highly ionized in comparison with the ambient nighttime ionosp here, being nearly 5 orders of magnitude greater than the ambient at some h eights.