Modelling the influence of conductivity profiles on red sprite formation and structure Original Research Article

Strong quasi-electrostatic fields, generated in the mesosphere and lower ionosphere after a lightning discharge by a succeeding redistribution of the induced spatial charges, are considered to be responsible for red sprite generation. Factors considered here as important for sprite occurrence, size and shape are the discharge parameters and conductivity profile. Thundercloud charges are assumed to be of hundreds of Coulombs distributed within layers with a horizontal extent of tens of kilometers as typical for big convective multi-cell systems. Cloud-to-ground positive strokes, characterized by the initial charge and the discharge time parameters, are considered. The thermal breakdown mechanism is considered as responsible for sprite onset. The conditions under which sprites initially occur (i.e., when atmospheric parameters are not disturbed) and their spatial and temporal characteristics are studied, depending on the conductivity profiles. A self-consistent analytical modeling is proposed for this purpose. Maxwell’s equations are applied under quasi-electrostatic conditions, when the magnetic field component is neglected and the electric field is assumed to be important. The features of a lightning discharge as well as of the conductivity profiles (including the slight anisotropy in the lower ionosphere) are taken into account in the model. The conductivity profiles are approximated between 0 and 100 km by stepwise profiles, defined as a layered atmosphere with ∼100 layers. Horizontal conductivity variations occurring at sprite heights (50–90 km) due to electron heating and ionization are represented in the model by step functions defined in each layer. Continuity of the Maxwell current density component normal to each boundary is required. The results obtained show that, for sprite occurrence in daytime conditions, a larger lightning charge moment change of the parent discharge is needed, and the sprites are lower than at night. The conditions (concerning the discharge and conductivity parameters) under which the presence of a ‘ledge’ in the conductivity profile in the lower ionosphere can influence the occurrence of a diffuse upper portion of a sprite, are investigated. It is shown that the electric field is intensified near a conductivity profile ‘ledge’ which leads to sprite formation.