To address the core questions in our project we have implemented new numerical models and techniques and used them to obtain a preliminary confirmation of our leading hypothesis. We did this by developing chemical, fluid and electrodynamical models of streamer channels. We concluded that the dynamical instability of ionized channels in air is responsible for sharp inhomogeneities present in those channels and that it is likely that it is the first step towards the channel heating and thus explains the stepped propagation of lightning channels.
We have also worked toward understanding large, non-thermal discharges in thunderclouds. Our team has been at the forefront of the one of the latest discoveries in atmospheric electricity: establishing the presence of large streamer coronas in active thunderclouds that can be observed simultaneously in the optical and the radio regions of the electromagnetc spectrum. We also showed that some of the features of these emissions can be explained by assuming the inception of secondary coronas after a primary one has probagated a large distance
Motivated in part by these discoveries, we developed models for light-scattering within thunderclouds. In order to investigate isolated streamer coronas we must understand how light-scattering affects optical observations from space. We developed a numerical code and an analytical framework for this purpose. A methodological outcome of this line of research was the derivation of a procedure to estimate the altitude of a intra-cloud lightning flashes. This method has been employed by our group in work lead by other groups.
The many thin streamers around a leader form a complex system that we do not understand properly and that has thus fas resisted modelling efforts. A large part of our work has been dedicated to develop such kind of models based on the microscopical physics of streamers. One side of this work consist in developing improved streamer models. Another side deals with the development of coarse-grained descriptions that we can the compare, under appropriately constrained conditions, to the microscopic models. We have also developed simplified one-dimensional models for electrical discharges around electrodes or leaders.
Another result from our project concerns the emission of X-rays from a spark or a lightning discharge. Here we have developed a model to derive the properties of radio-frequency emissions associated with streamer collisions, which have been proposed as a source of these X-rays. We propose that by comparing the features that we predict with actual observations we will be able to confirm or discard these collision as a source of X-rays.
To investigate the process of electron acceleration that ultimately produces these X-ray emissions, we have published an improved description of the cross sections relevant for thermal electron runaway. Besides, we developed a particle-management model for numerical simulations that is capable of resolving the energies the involved particles (in our case, electrons) even when some of these energies differ by factors above ten orders of magnitude. This allows us to predict quantitatively the production of relativistic-runaway particles even when it is extremely unlikely but due to subsequent exponential growth may have measurable consequences.
Finally, as a group specializing in lightning physics, we have also tackled related problems such as the propagation of electromagnetic radiation from a lightning stroke and its use in lightning location systems, the features of lightning-induced discharges in the upper atmosphere the possible existence of lightning in other planets.
