Simulating the possible regions of delayed sprite inception above thunderstorms using piecewise-varying lightning current time dependence

The appearance of transient luminous events (TLEs) in the mesosphere is known to be associated with strong (almost exclusively) positive cloud-to-ground (+CG) strokes with large charge moment change (CMC) values in tropospheric thunderstorms. Nevertheless, despite numerous observational campaigns from ground and space-based platforms, robust theoretical models, and laboratory experiments, there are lingering open questions concerning the exact circumstances for the appearance of sprites, among which is the cause for the observed delay in sprite appearance relative to the onset of the current in the parent stroke. Curiously, seemingly identical +CG discharges with the same CMC that should lead to a mesospheric discharge do not initiate sprites, while sometimes even weaker +CG discharges are able to do so. Previous studies aiming to resolve this issue have investigated different effects, such as mesospheric inhomogeneities, the presence of meteoritic ablation products, discharges in neighboring cloud cells, associative detachment of electrons from atomic oxygen ions, and long continuing currents. Here, we investigate the properties of the parent +CG's continuing current by suggesting piecewise-varying discharge time dependence. We present the results of simulations using a 3D quasi-electrostatic model (Haspel and Yair, 2024) with various patterns of the parent flash discharge current. We show how short, moderate, and long delayed sprites can be incepted due to piecewise-varying discharge current time dependence, and how discharges possessing low iCMC values can still produce electric fields in the mesosphere with magnitudes above the conventional electrical breakdown field. The model is validated by simulating two sprite events observed from the International Space Station during the ILAN-ES campaigns in April 2022 (on AX-1) and February 2024 (on AX-3), showing how a delayed sprite is incepted by a prolonged piecewise pattern of the current in the parent +CG flash.

 

Haspel, C. and Y. Yair (2024), Numerical Simulations of the region of possible sprite inception in the mesosphere above winter thunderstorms under windshear. Ad. Spa. Res.., 74, 11, 5548-4468, doi:10.1016/j.asr.2024.08.050