Variability of sprite discharges from ionospheric conductivity perturbations

Abstract We study sprite initiation and propagation by using 3D computer calculations, with a focus on the initiation dynamics in irregular ionospheres. In the computer simulations, the ledge in the plasma density in the ionospheric D-region is stochastically perturbed at spatial scales of 1-5 km. Calculations show that these perturbations become nuclei for sprite initiation. We then show that the steepness of the ledge has a pronounced effect on sprite initiation, as it influences the dielectric screening of the electric field with altitude. We also demonstrate that the ionospheric conductivity can be modified by subcritical lightning flashes that do not produce sprites, but which are intense enough to lower the conductivity via dissociative electron attachment. Our analysis shows that the initiation dynamics are determined by the steepness of the D-region ledge as well as the presence of ionospheric fluctuations.