Numerical investigation on the advance of leader channels in lightning and long sparks

Resum

This thesis aims to study the stepped propagation of lightning channels with a prominently numerical approach. We have built a self-consistent state-ofthe- art 2D cylindrically symmetric model that accounts for charge transport, electrostatic interactions, gas heating, and expansion. In our efforts for improving our model, we have implemented a numerical method to solve the Poisson equation that allows us to reduce the size of the computational domain, speeding up our simulations. This method is also valid to optimize the calculation of the photoionization term in streamer discharge codes. We have used our model to investigate the emergence of space stems. These are luminous spots that appear ahead of an advancing leader mediating the leader’s stepped propagation. We show that space stems start as regions of locally depleted conductivity that form in the streamers of the corona around the leader. An attachment instability enhances the electric field leading to strongly inhomogeneous, bright, and locally warmer regions ahead of the leader that explain the existing observations. Space stems are known to readily launch counter-propagating streamers. These are believed to heat the space stem close to leader temperatures. Similarly, high-altitude electric discharges such as sprites develop a non-thermal version of a space stem known as glow. As space stems, glows also shoot counterpropagating streamers. We have studied the mechanism underlying the onset of these counter-propagating streamers with the AFIVO 3D streamer model. Our results show that an attachment instability leads to a charge accumulation at the boundaries of the glow, which enables it to shoot these counterpropagating streamers. This explains the characteristic shape of carrot sprites. Finally, we have studied the effect of a forced electric current through a spacestem- like structure. The source of this electric current could be the counterpropagating streamers mentioned above. In the same way, we have studied the influence of water in the development of a space stem under such conditions. Our results show that water enhances the initially small plasma inhomogeneities. This explains some of the features observed in the leader stepping and highlights the relevance of water chemistry models to address leader propagation.