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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - eLightning (Lightning propagation and high-energy emissions within coupled multi-model simulations)

Teaser

Summary

This project focuses on the physics of lightning. The main feature that makes lightning discharges relevant for our society is their cost on property and human life. But lightning is by itself also a fascinating subject of study with frequent fundamental discoveries being made about its very nature and surrounding phenomena.

One lightning feature that is almost completely mysterious is concerns the propagation of a lightning channel as it progresses from a cloud to the ground or even within the cloud itself. Lightning channels that carry negative charge in their extreme (this includes the majority of cloud-to-ground discharges) advance not continuously but in a series of disconnected steps. A related mystery is that whenever one of these steps takes place the channel emits a bursts of highly energetic radiation in the form of X-rays. A third unsolved problem also involves energetic radiation but in this case hugely intense bursts of radiation that are emitted upwards and routinely detected by orbiting satellites. These are called Terrestrial Gamma-ray Flashes (TGFs).

The ERC-funded eLightning project looks for explanations for all these phenomena by probing the many length and time scales of a lightning discharge within a computer simulation. Due to the large disparity between these scales we use models with different assumptions for each scale and plan couple them in one large, multi-scale model simulation.

Our investigation is also guided by one hypothesis: that the stepped propagation of the lightning channel is explained by how the electrical properties of air change under a strong electrical field. It is possible that the characteristic (non-convex) shape of this dependence leads to a dynamical instability within conducting channels in air. This would explain the emergence of inhomogeneities in electrical discharges. These inhomogeneities do not have a currently accepted explanation but are known to mediate the stepped propagation of lightning and long sparks.

Work performed

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.

One key aspect of our research is the evolution of the electrical properties of air ahead of a propagating lightning channel. The air is converted from being a very poor conductor into a better but still weak conductor by a process that creates thin channels called streamers ahead of the lightning channel. Within these weakly conducting channels the temperature of air raises until it is high enough to turn air into a very good conductor called leader. This process is poorly understood and we have developed new techniques to investigate it. First we have developed and released a computer code called CHEMISE to investigate the chemical properties of a gas such as air. We have coupled this to a model for the electrical properties of thin channels. With this we have determined that water content is an essential feature in the transition of air from poor to good conductor. A second step was to include also the dynamics of the air itself, as it expands when its temperature raises. We have also implemented a model for this and applied it to the study of our hypothesis, concluding that very likely we can explain the initial stages of the transition from streamer to leader.

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 is 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.

Another result 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 better understand the emission of X-rays and the possibly related generation of TGFs, we need models of the interactions between free electrons and air molecules. For this purpose we have collected a set of improved data to incorporate these processes in future models.

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.

Final results

In the second half of the project we expect to reach the main objectives of the grant. Our key results should be theoretical predictions that can be compared with existing or future observations. The expected progress of the project consists of these elements:

1. Further development of models for a system of streamers (corona) and comparison with constrained microscopic models. This will allow us to develop better predictions about the behaviour of a long discharge under laboratory and industrial conditions. We expect it to also help in improving models for the attachment of lightning to standing structures, providing a societal benefit in the form of mitigation of lightning damage.

2. Coupling corona and leader models. This will allow us to model the features of the streamer-to-leader transition and compare with observational constraints. With this we expect to elucidate the reason for stepped leader propagation.

3. Development of hybrid models for streamers and high-energy particles. With this we will predict the intensities and angular distributions of X-ray burst emitted from streamer-collisions. Comparing with the existing observational constraints should clarify whether this is source of energetic radiation in long sparks.

4. Applying our models to the generation mechanisms of TGFs.