While a thunderstorm can put on quite a show for spectators on the ground, the best bits often happen above the clouds. Scientists have now described in detail the most powerful “gigantic jet” of lightning ever observed, which blasted energy equivalent to 60 regular lightning bolts upwards into space.
As the name suggests, gigantic jets are powerful bursts of electrical energy that are thrown from storm clouds – not down to the ground but up to the ionosphere. They belong to a growing group of transient events that occur in the upper atmosphere during storms, and because of that altitude it’s harder to observe them. As such, exactly how and why they occur remains mostly unknown.
Now, scientists have studied one of these gigantic jets in more detail than ever before. The event took place in May 2018 during a storm over Oklahoma, where it was simultaneously captured by several instruments, including the ground-based Lightning Mapping Array, the space-based Geostationary Lightning Mapper (GLM) and Geostationary Operational Environmental Satellite (GOES) network, and a citizen scientist snapping pictures with a low-light camera.
With so many different views of the same event, a team of scientists was able to study the phenomenon in much more detail than ever before. It turns out that this particular gigantic jet was the most powerful one seen so far, throwing about 300 coulombs of electrical charge upwards. For comparison, a normal lightning bolt usually tops out around five coulombs.
Different types of electromagnetic waves were emitted from different structures within the jet. Very high frequency (VHF) radio sources were detected at high altitudes of 22 to 45 km (14 to 28 miles), while optical emissions remained closer to the cloud top from which the jet originated, at altitudes of 15 to 20 km (9.3 to 12.4 miles). This reveals much about the structure of gigantic jets, and lightning in general.
"The VHF and optical signals definitively confirmed what researchers had suspected but not yet proven: that the VHF radio from lightning is emitted by small structures called streamers that are at the very tip of the developing lightning, while the strongest electric current flows significantly behind this tip in an electrically conducting channel called a leader," said Steve Cummer, an author of the study.
The streamers were found to be relatively cool, at around 204 °C (400 °F), while the leaders can reach blistering temperatures of over 4,425 °C (8,000 °F). But there’s still plenty that remains unknown about these gigantic jets, not least of which is why they fire upwards. That said, scientists have their theories.
"For whatever reason, there is usually a suppression of cloud-to-ground discharges," said Levi Boggs, corresponding author of the study. "There is a buildup of negative charge, and then we think that the conditions in the storm top weaken the uppermost charge layer, which is usually positive. In the absence of the lightning discharges we normally see, the gigantic jet may relieve the buildup of excess negative charge in the cloud.”
Further work needs to be done to shed more light on the lightning mystery.
The research was published in the journal Science Advances. The team presents the data in the video below.
Source: Georgia Tech via Phys.org
