Lightning: cannonades of electrons

Summer is the season in which lightning occurs most frequently in Catalonia. The most common case is lightning generated by a large cloud. "The lightning is generated by the cloud's electrification," explains Joan Montanyà, director of the UPC Lightning Research Group (Grup de recerca i estudi dels llamps de la UPC).

"Not all clouds can be electrified. In order to do so, they need to display a series of characteristics. In this case, we're dealing with summer clouds with a strong vertical development, clouds that can grow to heights of over 10 or 15 kilometers. Basically, the mechanisms of electrification are triggered by the interaction of the hydrometeors present in the cloud and the amount of water it contains. These mechanisms are not yet fully understood, though there are different theories," indicates Joan Montanyà.

"To ascertain the effects of lightning striking an electrical cable, an airplane or a wind turbine, you must first understand the parameters, in this case electric ones, relative to what the current is like, how it evolves, what electrical charge it has and so on. They can be measured directly, through a tower equipped to measure the current with which the bolt strikes, or indirectly, through electromagnetic fields," explains Montanyà. Recording lightning and its electromagnetic fields is the primary activity of the Lightning Research Group, which uses a high-energy detector to capture the X-rays and gamma rays produced by lightning.

Joan Montanyà, a professor at the UPC College of Industrial Engineering of Terrassa (EUETIT), explains that "the electrification of a cloud causes part of it (the lower portion, up to a height of eight kilometers) to gain a negative charge, and the upper part to have a positive charge. We know that for cloud-to-ground lightning to occur there also needs to be a certain degree of positive charge in the lower reaches of the cloud. Once the cloud is electrified, a preliminary discharge occurs in the form of a movement of electrons (called a streamer), accompanied by a series of ionization and deionization processes, which is precisely one of the factors we are studying. Then a leader appears, that is to say, an electrical arc that moves from the cloud towards the ground," he concludes.

How is lightning produced?

There are three types of lightning: cloud-to-ground, also called descending lightning; ground-to-cloud or ascending lightning; and intra-cloud lightning, which is lightning taking place within a cloud. "Around 80% of lightning occurs within the cloud. Only 20% ever reaches the ground. Intra-cloud lightning is the least understood, due to the obvious difficulty of measuring it," Montanyà points out.

In irregular places such as buildings and electricity or telecommunications towers, leaders or electrical arcs can also appear from different spots. These leaders attempt to 'catch' the leader traveling towards the ground. This is cloud-to-ground lightning. "Each discharge lasts only a few hundred microseconds whereas a single lightning strike can involve a number of discharges," explains the researcher.

Can we predict where lightning will strike? The reply at present is a resounding no. As Montanyà asserts: "You can't predict the exact location from which it will strike. What does exist is a remote sensing system by which to monitor the evolution of lightning strokes during a storm. This technology can be used in outdoor research and activities. If, for instance, a thunder storm is approaching when workers are repairing an electricity tower, they can be informed so they may leave the area."

Catalonia is not an area particularly prone to lightning. "There are regions that receive more or less lightning -- it depends on the climate." Tropical areas get more lightning than areas in higher latitudes. Where lightning will strike, however, cannot be predicted, nor can its energy be harvested. "We're talking about some 20,000 to 30,000 amperes," states Montanyà.

And can this current be captured? "Lightning is a process involving a great deal of energy, but very little electrical energy reaches the ground. Some 20,000 to 30,000 amperes in a few hundred microseconds. The majority of the energy winds up in the temperature of the lightning channel, heating it to some 30,000 ºC and producing thunder. This is where there really is a lot of energy, but from an electrical perspective, we cannot harvest it," Montanyà indicates.

Lightning hunters The UPC Lightning Research Group began working in this field early last decade. They've worked hard and can now be considered leaders in many of the aspects they're researching. The effort has been worthwhile.

This is the first time the University has what could be called 'lightning hunters'. Many of the activities carried out by this research group require the measurement of certain lightning parameters. Lightning, however, does not always occur near the tower equipped with measuring devices on the Niu de l'Àliga Peak in La Cerdanya where the team has their research base, which means they have to go out and look for them. "According to the forecasts provided by the Meteorological Service of Catalonia as well as our own, we travel to prospective areas in a van fully equipped with the necessary devices: antennae, a high-speed video camera, high voltage detectors and other instruments," explains Montanyà. "Over the past year, we have basically performed high-speed video measurements."

Recording lightning, however, is not the only task carried out by this group, comprised by David Romero, Óscar van der Velde, Nicolau Pineda, Glòria Solà, Víctor Marchen and Daniel Aranguren, as well as Joan Montanyà. They also research physics and engineering aspects, the latter focusing on protection issues.

"The issue of protection, such as the protection required by wind turbines, forces us to go back to the source, to physics. The phenomenon must be understood in order to study its possible effects or how to protect a specific device," indicates Montanyà. "We must explore the physics of lightning. This means, for instance, learning about the electrical discharge mechanism or the electrification processes in a storm," he adds.

Another sphere of their research, which was what actually led to the creation of the group, is the one dealing with measuring the electrostatic and electromagnetic fields generated by lightning. They also attempt to find out why storms produce more or less lightning, or if there is a relationship between lightning and tornadoes or hail. By the same token, they research other, more basic aspects, such as modeling the electrical discharge (streamer modeling), which is a relatively unexplored area "in which many processes intervene and where the modeling is, for now, still very basic," explains Montanyà. Moreover, the Group studies so-called "transient luminous events," electrical discharges occurring in the upper atmosphere. Their dimensions are enormous, reaching over 40 km in height. In this regard, the team also studies the high energy aspect, that is, the emission of X-rays and gamma rays by storms and lightning, which they hope to soon measure from space.

Insofar as protection, the group is mainly working on protecting wind turbines, although they also do research into airplane protection and other more traditional spheres, such as protecting power lines. "It is not only direct lightning strokes on power lines that cause overvoltage, but also leaders striking points near the lines," he adds.

The group is making continuous progress. At present, thanks to a project financed by the Spanish Ministry of Science and Innovation relating to the European Space Agency's ASIM mission (Atmosphere-Space Interaction Monitor), they are working on installing a 3D lightning detection system. "We hope to have the system operating this year, as it will be the first one available in Europe and will allow us to record half a million images per second," explains Montanyà. This mission will allow us to ascertain the origins of the gamma ray emissions coming from Earth.

Along these lines, the group is also working on expanding the new LINET lightning detection network, which was developed at the University of Munich in Germany and has now spread throughout Europe. "We are in charge of installing and administering the network on the Iberian Peninsula. At present, there are already a number of companies using the data from this network," asserts the professor.