The multiscale nature of spark precursors and high altitude lightning

The formation of ionized channels in electric breakdown is a generic dynamic process that occurs in many natural and technological phenomena. Well known examples are the formation of spark channels through the streamer process and the formation of much larger lightning channels through the somewhat complexer leader process. Other very actively studied phenomena are transient luminous events above thunderclouds (so-called "high altitude lighting'') that were documented for the first time only 15 years ago; recent detailed observations of spatial and temporal structures of sprite discharges in high layers of the atmosphere lead to the conclusion that sprites are possibly just up-scaled versions of streamers. Streamers are also used in many technical processes in so-called corona reactors, where the non-equilibrium plasma at the tip of the channel is used for very efficient gas processing and other applications.

As is clear from the different application fields, the investigation of such transient discharges is spread over many disciplines (applied physics, plasma physics, electro-engineering, geophysics) and their theoretical understanding requires plasma physical modeling, nonlinear dynamics and pattern formation, large computations and model reductions. Quantitative understanding has long been hindered by the rapidity of the process that made good experimental characterization of the transient states difficult to impossible, and by the multiscale nonlinear dynamical structure even of a single channel that poses a major challenge to analysis and computations. All fields and methods have made considerable progress in recent years. Nonlinear analysis and large computations become ripe to tackle this multiscale problem. On the experimental side, new plasma diagnostics with nanosecond temporal resolution is becoming available that allows a proper view on the dynamics, and also the knowledge on lightning related discharges below and above thunderclouds is growing through time resolved and telescopic observations, measurements of electromagnetic radiation etc.

The aim of the workshop is to bring experts from relevant disciplines together, to represent and share the newly gained knowledge and methods of the respective disciplines, and to stimulate cross-disciplinary research. In particular, we invite geophysicists working on sprite discharges above thunderclouds and on ordinary lightning, experimental physicists and electro-engineers studying streamer and corona discharges, and theoreticians from pattern formation and nonlinear dynamics, low temperature plasma physics, non-equilibrium statistical physics and scientific computing.

Key issues:

The new access to the short-time dynamics and microscopy of this multiscale process is an incentive to reconsider previous phenomenological concepts on the large scale properties of such discharge channels and to work a multiscale ladder of quantitative predictions upwards from micro- to meso- and macroscopic scales. This goal requires a strong interaction of modeling, nonlinear analysis and pattern formation, large-scale simulations, well designed experiments, most advanced experimental diagnostics as well as phenomenological knowledge on atmospheric electricity or technical operation conditions of corona reactors.

Central topics of the workshop are

·        Recent experimental access to streamer discharges, characterization for different voltages, pressures, boundary conditions and gasses, relation to leader discharges in lightning.

·        Present observational knowledge on sprite discharges.

·        What can experiments on streamer discharges tell us about sprite discharges in high layers of the atmosphere that are difficult to access? About energy and charge transport, electromagnetic radiation and changes of the chemical composition (ozone, nitrogen oxides)?

·        Exchange on numerical modeling of sprites or streamers, PIC versus PDE codes, how to proceed to larger systems, stronger potentials, multiple branching of channels and large scale structures.

·        Microscopic modeling: Role of additional microscopic mechanisms for channel propagation like runaway electrons etc. in laboratory or atmospheric discharges.

·        Towards macroscopic modeling: properties of ionization fronts, and their dynamics as moving boundaries between ionized and non-ionized regions. Channel branching and large scale structures.

For more information (in Dutch) on this topic go to http://homepages.cwi.nl/~ebert/Zenit4.pdf