Lightning can shoot upwards from tall towers – and researchers now know that it produces X-rays at the very beginning of its ascent into the clouds.
During a thunderstorm, different parts of a cloud become positively and negatively charged. This separation of charge across the cloud leads to a dramatic electrical discharge that manifests as a column of hot ionised gas and electrons, or plasma, reaching for the ground: a lightning strike.
But when charged-up clouds hover over a tall structure made predominantly from metal or other conductive materials, lighting can also go the other way, emerging from the building’s tip and flashing upwards. at the Swiss Federal Institute of Technology Lausanne and their colleagues wanted to understand how this process begins.
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The researchers studied four upward lightning flashes from the 124-metre-tall Säntis Tower. The building, which is used for telecommunications and as a weather station, sits at the top of Mount Säntis, a peak in the Appenzell Alps in Switzerland that is 2502 metres above sea level. The tower’s interior is metallic and hollow. It contains instruments that measure both electric currents running through the structure and the changes in the magnetic field that accompany them. The researchers used these instruments – as well as electric field and X-ray detectors and a high-speed camera – to study the four upward lightning flashes.
Oregel-Chaumont says past observations indicated that upward flashes may produce X-rays. The new measurements showed that they are produced at the very beginning of the lightning’s journey upwards. This process is discontinuous: the electricity jumps up in twisting, turning steps that build a column of plasma. The new measurements clearly showed that X-rays were produced along with such leaps, they say.
at the University of New Hampshire says the new measurements may improve our understanding of how X-rays arise during a lightning flash. They may also offer insights into a rare but even more dramatic phenomenon where thunderstorms produce gamma ray flashes, which are more energetic than X-rays.
Because the team collected different kinds of data from a variety of instruments, the new observations could be a good test for theories about what electrons and other particles are doing at the moment lightning begins to form, says Oregel-Chaumont. They say one popular idea is that accelerated lightning electrons hit nearby air particles and “brake” by emitting some of their energy as X-rays. The Säntis measurements may help fill in the details of that process.
arXiv
