VLF scattering from red sprites: Application of numerical modeling

Red sprites were discovered by chance in 1989 when a low-light TV system was pointed above an active thunderstorm. Red sprites are associated with columns of ionization in the Earth-ionosphere waveguide, from above the thunderstorm into the D region of the ionosphere. The ionized columns have been detected through “VLF sprites,” perturbations of the phase and/or amplitude of subionospheric VLF transmissions, which can be used to study the electrical properties of red sprites. There is extensive experimental evidence that VLF sprites may involve wide scattering angles and can produce back scattered radiation. Here we present a numerical and theoretical study of me scattering of subionospheric VLF transmissions caused by the plasma columns associated with red sprites. Comparison of the VLF scattering from sprites is made between a non-Born rigorous model which assumes the sprites are infinite columns of constant conductivity, and a three dimensional Born scattering code. Both formulations show excellent agreement with one another. The formulations predict VLF sprites similar to those experimentally observed for all scattering angles. This shows that the conclusions of previous studies 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 sprite plasma is highly ionized in comparison with the ambient nighttime ionosphere, being nearly 5 orders of magnitude greater than the ambient at some heights.