Every now and then, Earth reminds us it's capable of releasing some furious energy.
Case in point: scientists have just detected a new extreme in hotspots of lightning activity called 'superbolts': intense lightning strikes that shine up to 1,000 times brighter than typical lightning strikes.
The observations come from researchers at the US Los Alamos National Laboratory, who used satellites to measure the extreme lightning events. The results force a rethink on what constitutes a superbolt, and shed new light on how and where superbolts originate.
"We want[ed] to see what the boundaries [of superbolts] really are," atmospheric scientist Michael Peterson told The Washington Post. "It's about how big and how bright they can get."
Superbolts were first detected from satellite data in the 1970s, being described as lighting that outshines average bolts by a factor of 100 or more.
Since then, atmospheric scientists have been debating what really counts as a superbolt, because measurements taken by different instruments can vary.
"When you see a lightning flash from space, it will look a lot dimmer than if you were to see it from ground level because the clouds block some of the light," Peterson said, explaining how satellite measurements can differ from ground-based detectors.
There's also the question of whether superbolts are supercharged by some unique phenomenon, or if they're just bigger, brighter strikes of the usual lightning variety.
"Understanding these extreme events is important because it tells us what lightning is capable of," said Peterson, who has detected some record-breaking lightning strikes in recent years – including one 2018 megaflash (long-duration lightning burst) that stretched some 700 kilometres (440 miles) across the sky and lasted nearly 17 seconds.
In a new study, Peterson and his colleague Erin Lay analysed data collected by NASA's Geostationary Lightning Mapper, a detector strapped to weather satellites and sent into orbit to record flashes of lightning, day and night, over the Americas and the adjacent oceans every two milliseconds.
Unlike ground-based monitoring systems, which detect radio waves, the GLM measures the total brightness (optical energy) of lightning bolts within clouds, between clouds, plus lightning that strikes the ground.
(Michael Peterson/Los Alamos National Laboratory)
Above: A superbolt lasting nearly 7 seconds captured by the Geostationary Lightning Mapper over the southeastern United States in February 2019.
The researchers combed two years of data for lightning strikes that shone 100 times brighter than a typical bolt detected from space, and found about 2 million events intense enough to be called a superbolt – roughly one in every 300 lightning events.
Just keep in mind, it's possible though that some superbolts appeared brighter than other strikes, if they were on the fringes of a storm cloud and the satellite detector had a cloud-free view.
When the researchers raised the bar to lightning events at least 1,000 times brighter than an ordinary lightning strike, they identified key hotspots of energetic superbolt activity.
The most radiant cases were concentrated in the central United States and in the Rió de La Plata Basin, which spans Uruguay, Paraguay, and parts of Argentina and Brazil.
However, the GLM detector might not have captured every single superbolt. Although the satellites are fixated on the Americas, from Alaska in the north to Argentina's southern tip, GLM measures the most energetic lightning bolts, but not necessarily the most powerful flashes, if they happen to be shorter than 2 milliseconds.
"[U]sing total energy to screen for the brightest lightning cases will miss short-duration yet extremely powerful optical pulses," the study authors write in their paper.
There was significant overlap, however, with superbolts identified by Los Alamos researchers in a second study, which classified superbolts by their peak power – the same way these extreme events had first been defined.
In the second study, the researchers analysed 12 years of data from another satellite and counted lighting strikes as superbolts if they produced 100 gigawatts of power. For comparison, that's more power in one bolt than all the solar panels in the US combined.
"One lightning stroke even exceeded 3 terawatts of power – thousands of times stronger than ordinary lightning detected from space," Peterson said.
Combining satellite data with ground-based measurements, the researchers also found that superbolts are indeed a different kind of lightning.
The most powerful superbolts (producing more than 350 gigawatts of power) resulted from rare positively charged cloud-to-ground events, rather than negatively charged cloud-to-ground events, which characterises most lightning strikes.
The results also showed that superbolts often occur over the ocean and tend to spark from megaflashes, which stretch hundreds of miles horizontally from tip to tail.
"Oceanic storm systems, particularly during the winter, and especially those located around Japan are shown to produce these intense superbolts," the researchers explain in the second paper.
This somewhat aligns with results from a 2019 study, which found superbolts mainly formed over the oceans and seas, although that research detected most superbolts in the North Atlantic, west of Europe.
So, the matter is by no means settled. Atmospheric scientists need to keep comparing measurements from different ground-based and orbiting instruments to understand differences between them, and to better characterise extreme lightning events.
"It will be an important undertaking by the atmospheric electricity community to reconcile the top events recorded by the various optical and [ground-based radio-frequency] instruments and then come to a consensus on what is – and what is not – a superbolt," the researchers write.
The two papers were published here and here in the Journal of Geophysical Research: Atmospheres.
#Nature | https://sciencespies.com/nature/scientists-detect-superbolts-1000-times-brighter-than-typical-lightning-strikes/
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