Harnessing Lightning and Hiding from Sound
Steve Cummer jokingly calls himself something of a Luddite because of his stubborn refusal to give up pencil and paper as his main medium for working through ideas. But in reality, that quirk is hardly enough to justify such a title, particularly when you consider that some of those ideas he fiddles with on paper are being transformed into some of the most technically advanced and futuristic materials ever devised. Oddly enough, Cummer's involvement in materials research and development actually began with lightning research. Cummer, who came to Duke in 1999, began graduate school at Stanford with a general interest in electromagnetic fields and waves. Once there he decided that the most interesting work under that heading was by a team studying lightning using the radio waves they produce. His attraction to both lightning and what he calls the "physicy" side of electrical engineering are two factors that have continued to drive his work ever since. For those who directly study individual bolts, lightning can be a frustratingly fickle target for research. But Cummer's work is greatly simplified by his not needing to get close to any single, unpredictable lightning bolt. He focuses instead on lightning's production of powerful radio waves that travel great distances. Cummer uses the measurements to more directly study lightning-related phenomenon, including a bizarre form of upper atmospheric lightning known as sprites, as well as gamma rays. Once thought to be produced only by far off astronomical processes, more recent research has shown that at least at times, gamma rays are being produced by lightning. He also uses radio measurements of lightning as a tool for understanding some phenomena that really have no direct connection to lightning, such as energetic particle radiation in the upper atmosphere. About half of Cummer's research attention is focused on the study and development of futuristic new materials. This research isn't tied to lightning itself, but rather the way that Cummer and his colleagues study it. The basics of detecting, studying, and simulating radio waves are the same whether they come from lightning or some other source, and they also apply to other forms of electromagnetic radiation such as microwaves and X-rays. So, Cummer's expertise in the field has led to work with fellow Duke professor and colleague David Smith to study and design advanced materials that artificially manipulate electromagnetic radiation. Goals for the work include better understand the basic physics and engineering involved as well as the development of materials for a variety of applications. These include use in antennas and other communications devices and possibly someday even creating cloaking systems that might hide a submarine from sonar view by manipulating the sound waves used for sonar or someday even hide a person from visual view like Harry Potter's famous cloak.
A Stationary Lightning Chase
Most of Cummer's lightning-related research focuses on the fact that lightning is a very strong source of electromagnetic radiation in the form of radio waves. To measure the waves, the Cummer team currently uses sensors that include large coils of hundreds of thousands of wire wrappings that constantly measure the electric and magnetic fields from radio waves produced by lightning as far away as halfway around the planet. When he first arrived at Duke he built all the electronics needed to drive the detectors and wrote his own data acquisition software, but since then has established collaborations with companies that now produce some of these components commercially. The ability to detect and measure lightning from such a great distance stems from some fortuitous qualities of the radio waves produced. The waves radiate in a broad range of frequencies, simplifying detection, and on the global scale lightning is common, meaning plenty of opportunities for detection if you monitor over large distances. The radio waves are also strongly reflected by the upper atmosphere, which, conveniently, helps them travel great distances to detectors. The strength of these radio waves provides information about the electric currents contained in lightning, which can answer a range of questions about the size, speed, and power of a particular lightning event. Just as teams that run seismic sensors can go to their data to answer questions about far away earthquakes or other loud events such as bomb tests, Cummer's team can go to its data to answer questions about events teams at far away locations may have observed. The trick for enabling such collaborative efforts is that the records are time stamped to the level of microseconds based on the precise clocks on GPS satellites. That makes it possible to find a single lightning event even at times when the instrumentation is recording 100 lightning signals per second.
Sprites: Figuring Out What's Up in the Sky
Researchers first discovered the fleeting form of lightning known as sprites occurring above clouds in 1989 and detailed studies began while Cummer was in graduate school at Stanford. Sprites are energetic electrical bursts that are actually fairly common above powerful thunderstorms, but they happen so quickly--typically lasting only 10 to 100 milliseconds--they can barely be seen by the naked eye. Their occurrence between 40 to 90 kilometers above the ground made them even harder to spot, explaining why no one had confirmed their existence until so recently. Pilots had seen them routinely, but tended to shy away from reporting them because when pilots admit to seeing otherwise unexplained strange flashes of light, their fitness for duty may at times be questioned. Cummer began studying sprites soon after their discovery, and his interest in and research on them has continued ever since. Though scientists are interested in sprites' potential effects on atmospheric chemistry, Cummer says much of the motivation is pure science. "There's this giant thing that fills up a 1,000 cubic miles of space- that we didn't even know of, and it's pretty fantastic looking," he says, "so we try to get better looking measurements that improve our understanding of what's actually happening up there." The instruments Cummer uses can capture readings of sprites in the same way as regular lightning because they too produce bursts of radio waves. Perhaps more importantly, he is gathering detailed information about the lightning that actually generates the sprites, which is critical to understanding the phenomenon. Cummer's team gets timing information on sprite events from groups that observe them and is then able to find these same events in their own data and answer such questions as how powerful the lightning and the storm that generated it were. Cummer has also worked with collaborators to make visual observations. With a team in Colorado, working 5,000 feet up at the edge of the Rockies, he and his collaborators used an ultra-high-speed camera in 2005 to capture sprites created by storms at altitudes below their position. With the camera taking images at an astonishing 7,000 frames per second, they gathered some of the most detailed photos of sprites ever captured. This and related work has revealed many of the sprites' intricate details, including how they form. Lightning that produces sprites can be 100 to 1,000 times more powerful than a normal bolt of lightning that travels from a cloud to the ground. Sprites form in a similar fashion to regular lightning when electric fields above clouds build up enough to create a spark, which then shoots along in the direction of the electric field, forming an intense display in the process. Though initial photos looked like blobs, the images captured by Cummer's collaborative group and others has more recently captured reveal a large pattern of light streamers that branch down like tree roots, with each of these lines as narrow as 10 to 20 meters. An entire display is about 50 kilometers wide.
Sprites: Figuring Out What They're Doing to the Sky
Cummer and others have also begun considering the impacts sprites have on the atmosphere. The electrical discharges from sprites are highly energetic process that have the potential for significant chemical effects, possibly even leading to the production of pollutants such as nitrogen dioxide. Such gases could be generated at high sprite altitudes and then travel down to lower altitudes where they are known to play important roles. No one imagines that sprites could be a major driver in climate change, however, given that they are common, it's possible some of their impacts are significant, and they would be a source that has never before been considered. The same types of electric discharge processes that might be of interest in the stratosphere are also used in a huge range of industrial applications, and some researchers in this field have discussed potential uses of Cummer's research with him. "It was actually sort of shocking to learn that people had real reasons for caring about these sorts of things that I thought were pure science," he says with a laugh. The reactions that cause a fluorescent bulb to light, for instance, are similar to those that occur in sprites. Work by Cummer and his colleagues on sprites and related phenomena could offer insights into how to control these types of electrical discharges in the laboratory, which could lead to improvements at the industrial scale, such as improved lifetime for fluorescent bulbs and improved combustion in engines.
Gamma Rays: Solving the Mysterious Source
Scientists have long been interested in the production in space of gamma rays, which are a very high-energy form of nuclear radiation, similar to X-rays, that are sometimes formed by astronomical events such as supernovas. Besides the interesting physics involved, gamma rays are of interest because they can be a threat to astronauts, particularly if longer trips are taken to destinations such as Mars. Like radiation in the radiation belt, gamma rays can also damage spacecraft. In the 1990s, a satellite measuring gamma rays from space recorded some then unexplainable bursts that seemed to be originating from the direction of Earth rather than from space. Though the idea was controversial at first, over time researchers have shown that the mysterious rays were generated by, or at least near, lightning. Cummer uses his equipment to measure the radio emissions from the lightning processes associated with these gamma ray, and he is working on the theoretical side to better understand what these measurements say about the processes that might be responsible for the gamma ray emissions. Creating gamma rays requires strong electric fields that can accelerate electrons enough that they can create more high-energy electrons when they collide with gas molecules in air. These electrons subsequently slow down through friction with the air, and this braking process actually creates the gamma rays. Where in the atmosphere this process takes place was an open question for many years. It was originally thought that the strong high altitude electric fields created by the lightning responsible for sprites might also be responsible for these terrestrial gamma ray flashes. But, using lightning measurements, Cummer was able to show that the lightning associated gamma rays were much too small to create sufficiently strong high altitude fields. This suggested that they were probably produced from strong electric fields at much lower altitudes, probably in the thunderclouds themselves. That the gamma rays are produced at low altitudes means that a tremendous number of them must be created in order for satellites to detect them hundreds of kilometers up. One of the biggest open questions is whether gamma rays are associated with only a small fraction of lightning events, or whether this process might be more universally present in lightning. More measurements are needed to answer this question, and for reasons still not fully understood, one of the places in the world where gamma ray flashes are most often seen by satellites is the Caribbean. Cummer and his colleagues are looking at the possibility of setting up equipment closer to this region in hopes of gathering better observations to illuminate the connection between lightning and gamma rays.
Lightning as a Tool: Probing the Upper Atmosphere
Though some of Cummer's research is focused directly on processes related to lighting itself, another interest is using lighting simply as a tool. Electromagnetic radiation given off by lightning travels through the upper atmosphere in ways that provide a wealth of information about this zone otherwise difficult to study, making these waves a free research tool worth their volts in gold. One research area where lighting can help is, oddly enough, better understanding of Earth's radiation belts above 1,000 kilometers up, where the space shuttle and telecommunications satellites roam, but hundreds of kilometers above where the lightning actually happens. These belts of very energetic electrons and protons are maintained through processes simlar to those that create the aurora at high latitudes. This high altitude radiation environment is highly variable and can significantly damage satellites over time, even leading to their premature failure. So, designers have to shield satellites against this radiation, and doing so optimally requires better understanding of the phenomena involved. Electromagnetic radiation from lightning is strongly reflected by the upper atmosphere between 60 and 120 kilometers above the Earth, which wouldn't seem to be very tightly connected to the 1,000-kilometer and higher radiation belts, except that Earth has natural windows for observing this phenomenon. As it happens, there are places around the planet where Earth's magnetic field is weaker than others. Energetic particles are always raining into Earth's atmosphere to a degree. However, at these weak points, especially high numbers of radiation particles from the radiation belts penetrate into Earth's atmosphere, and down into and past the range of lightning's electromagnetic signals. When there are high concentrations of radiation belt particles entering the atmosphere, there are frequent collisions between them and the low-oxygen air at high altitudes. These collisions ionize the air, altering the lightning electromagnetic signals. These changes are recorded using Cummer's equipment, giving clues to the frequency and energy of radiation belt events. "Monitoring what is coming out of the bottom end is a pretty decent proxy for what is happening inside the belt, " says Cummer, "so it's a very good way of to study the belt that is cheap and versatile and provides information that is fundamentally different from what you can get from a satellite." The places where the magnetic field is weakest and so the radiation effect is most pronounced is in the southern part of Brazil, a spot known as the South Atlantic Anomaly, and Cummer and his team have traveled there with their equipment to gather data. "We're trying to understand whether we can get good continuous measurements of what's happening to the radiation belts by sitting down here in what is effectively the drain in a lot of those particles," says Cummer. Major goals include understanding the mechanics of how these particles are generated, which is still not completely understood, and how radiation levels in the belt vary over time. As understanding increases, such work could inform improved construction techniques for satellites by allowing improved calculation of how much shielding against the radiation is required according to the intended life of a satellite.
Acoustic Cloaking: Hiding in the Sound of Silence
Besides collaborating with Smith on work to develop new materials and to further explore the possibilities of cloaking for electromagnetic radiation such as microwaves, Cummer's group has also branched into a study of the potential to create materials that similarly manipulate sound waves. Soon after Smith's first theoretical paper appeared about the concept of cloaking against microwaves and other electromagnetic waves, a mathematical paper examined the possibility that materials could be designed that would accomplish similar cloaking against sound waves. It concluded it wasn't possible. But using metamaterials to control sound waves was something Cummer had already been pondering for years, and he wasn't willing to give up on the idea. When the paper was published, he says, he wondered if there might still be a way to manipulate sound waves enough to effectively, if imperfectly, accomplish cloaking--even if the theoretical work in the paper was correct. He jokes that such musings may have been due in part to an inability to follow every aspect of the mathematicians complex work on the topic, but his research to date strongly suggests the acoustic cloaking is worth pursuing. In 2007, he and his team published a theory paper that showed there is in fact what Cummer calls a "perfect analogy" between electromagnetic and acoustic waves that would allow sound wave cloaking, at least in two dimensions--for instance with sound waves traveling toward the edge of a long cylinder. More recently, he published another paper showing that three-dimensional sound cloaking should also be an option. "The 3-D case we showed was not a quirky, special case," says Cummer, "It really is doable." Though the doing may take some time yet. The basic idea behind sound cloaking materials would be similar to that used for electromagnetic cloaking. Cummer envisions using a contained liquid embedded with pieces of metal or other material that's much denser than the liquid and that are much smaller than the wavelengths of sound. By precisely controlling the amount of each material in each part of the shell, the sound waves can be smoothly bent around an object in the interior of the shell. "The key will be figuring out the right shapes and right arrangements," says Cummer. But Cummer says such sound-shifting materials are not yet as advanced as those he, Smith, and others are using in their electromagnetic cloaking work. Still, he predicts that within the next couple of years the group should be able to move beyond theory to produce a concrete proof of concept for acoustic cloaking.