Influence of Photoionization on the Morphology, Electric Field, and Stability of Sprite Streamers under Mesospheric Conditions

Sprites are transient luminous events produced in the mesosphere by quasi-electrostatic fields generated above intense thunderstorms. In this work, sprite streamer discharges are simulated using the Afivo-Streamer framework under mesospheric conditions, employing a fluid plasma model based on the drift–diffusion approximation and the Local Field Approximation (LFA). Photoionization is modeled through the three-term Helmholtz approximation of the Zhelezniak formulation, and its influence is evaluated by comparing simulations performed with and without the photoionization source term under identical physical and numerical conditions. The results show that photoionization promotes longer and more uniform streamer channels, reduces peak electric fields, and suppresses fine branching by pre-ionizing the medium ahead of the discharge front. In contrast, simulations without photoionization develop fragmented structures, multiple sub-heads, and stronger local electric-field enhancements associated with Laplacian instability. These findings confirm the stabilizing role of nonlocal ionization mechanisms in sprite streamer dynamics and highlight the importance of accurately modeling photoionization in numerical studies of transient luminous events.