It is a very hot day in September. As far as the eye can see, there is hardly any shelter from the harsh sun. Nettles sting my ankles. My supervisor admits that some trimming is needed. I’m in the heart of Drenthe, and this is mainly the domain of insects and birds, and day-trippers cycle there, perhaps in search of the many dolmens in the area. It is also the place where more than two thousand antennas of the LOFAR (Low Frequency Array) radio telescope are located, which are shaped like short, straight flagpoles. There are more antennas packed in black boxes.
It’s hot days like these, and sometimes they end in heavy rain and a few good claps of thunder. This open field seems like the last place you want to be. The former agricultural area near Exlo, now a nature reserve, offers no protection against rain and thunder.
Antennas of the LOFAR radio telescope, which entered use in 2010.
Roel van der Heijden for Nemo Kinslink
However, scientists are still wringing their hands as thunder gathers over Lovar. So does Brian Hare. It uses radio telescope antennas to look for lightning. In recent years, the astronomical observatory has demonstrated its ability to map lightning strikes in great detail. Herr, of the ASTRON Research Institute and the University of Groningen, who co-established the lightning research group in 2017, hopes to discover exactly how thunderstorms arise. After decades of rapid research, this remains unclear.
Miles of structures
The discharge occurs in the form of a lightning bolt inside the cloud or toward the ground.
According to Hare, it’s safe to say that lightning is a mystery. “Every time I see new LOFAR images of a lightning bolt, I discover something I haven’t seen before. We understand relatively little about lightning and don’t know why it behaves the way it does. Using LOFAR, Hare has a tool with which he can investigate how a bolt begins high up in the clouds Then they grow into structures tens of kilometers long.
Of course there are ideas about how thunderstorms form. Electric fields in thunderclouds likely play a role, as do ice crystals and raindrops moving up and down that transfer charge and ensure that there are areas of positive charge (normally high in the cloud) and areas of negative charge (low in the cloud). cloud) created. Eventually, the accumulated voltage becomes so great that a discharge occurs. It is not clear how this begins. The discharge occurs in the form of a lightning bolt inside the cloud or towards the ground. This is what the LOFAR antennas record. They pick up radio waves from the tip of the lightning bolt as it travels through the atmosphere.
Point in the picture
LOFAR consists of several measuring stations. At the heart of the telescope I’m visiting today, they’re spread out over an area of about 1 by 2 kilometers. In the Netherlands there are several stations tens of kilometers away that researchers also use. By looking at the direction and arrival times of thunderstorm signals, they can create a 3D image of the lightning, about one meter away. The process can be followed at lightning speed, down to nanoseconds.
LOFAR records lightning strikes in detail over a distance of about 50 km. The radio telescope has antenna stations all over Europe, but for thunderstorm measurements the researchers only use Dutch antennas, which are mainly located in Drenthe.
Lightning produces a lot of radio emissions. Hare says researchers don’t fully understand the fact that it does and that it is measurable to LOF. “It’s certainly not the stream of electrons that is released by a lightning bolt. This results in longer radio waves that LOFAR does not record. The higher frequencies – between thirty and eighty megahertz – that we measure come from much smaller processes, such as sparks that cause cold air to turn into hot plasma.” And conductive. This happens on top of the lightning and creates a conducting channel. This is a complex and misunderstood process, which makes it difficult to interpret the lightning images captured by LOFAR. In a sense, we see lightning, but we don’t know exactly what we’re looking at.
The fact that little is known about thunderstorms is partly due to their unpredictable nature and speed. Of course, you can find beautiful photos or videos of lightning, but to study it in detail you need special high-speed cameras. “However, those cameras that take a million images per second are still too slow to capture the details of lightning like LOFAR does. “We’re taking a billion images per second and we also see what’s happening in the clouds,” Hare says. “By the way, people have been studying storms.” Thunderstorms to predict the weather for a long time. This is done using a global network of special radio antennas. However, these structures are unable to determine the shape of lightning the way LOFAR does.
a lot of information
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Birds that often perch on some of the countless LOFAR sites around us, due to bird droppings, have no idea they are taking a rest at an advanced astronomical observatory. The antennas collect data day and night about galaxies and black holes billions of light-years away. Researching thunderstorms is a home game by comparison.
When a thunderstorm approaches, Hare and his colleagues send a request to the LOFAR control room for measurements. The telescope goes into a special thunderstorm mode and astronomical observations stop. The latter isn’t so bad according to Hare, since LOFAR isn’t usable during a thunderstorm anyway. Lightning researchers use only 6 of the 96 receivers per antenna station; This is more than enough to get a picture of the storm. Of the 38 Dutch stations, they use approximately 230 antennas. They use this to inspect approximately 100 lightning strikes per year. “Every lightning produces a lot of information; this always keeps us busy for some time.” Herr and his colleagues are also working with meteorologists from the Dutch weather institute KNMI, among others. LOFAR measurements can be used to test existing thunderstorm detection systems. This in turn leads to better weather forecasts.
But ultimately, it’s basic thunder research that matters to researchers. The questionnaire is long. There’s also something strange going on with lightning in Holland; This appears to be “inverted”. “This is certainly what I hear when I show our images to colleagues in the United States, for example, where a lot of cursory research is done,” Hare says. “This reversal probably has something to do with the accumulation of storm clouds. In cold regions, such as in the Netherlands, the positively charged region could be low in the cloud, making it easier for lightning to develop downward from the central negative region. This is our hypothesis anyway.” In addition, we recently introduced the so-called Sparkle Small lightning bolts are found in the cloud that do not grow to become larger. And no one else can see it in as much detail as we can with LOFAR.
According to Hare, it shows that lightning research using LOFAR is producing a lot of new knowledge about thunderstorms. Much of the research is being done in areas with warmer climates, for example Florida, New Mexico, Spain and southern France, which produce a different type of lightning. In this sense, thunderstorms are at least as changeable as the weather.
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