Lightning can be beautiful, scary and destructive, some five times hotter than the surface of the sun and carrying a 300 million-volt shock. Now, atmospheric scientists are learning more about superbolts, a rare breed of extreme lightning bolt that can stretch for hundreds of miles and be a thousand times brighter than an ordinary lightning stroke.
Earlier this year, researchers confirmed a pair of ultra-long-distance lightning strikes in South America that spanned up to 442 miles and lasted for nearly 17 seconds. Ongoing research has turned to how much power these fierce discharges contain, as well as their relative rarity.
A new paper published in the American Geophysical Union’s Journal of Geophysical Research: Atmospheres found that roughly one third of 1 percent, or 1 in every 300 lightning strikes, could be classified as a superbolt.
A superbolt is any flash of lightning that is 100-times brighter than average.
The study was led by Michael Peterson, a scientist at Los Alamos National Laboratory in Los Alamos, N.M. His team examined two years’ worth of data from the GOES weather satellites, which peer down on North and South America with ultra high-resolution. The satellites have a device known as the “Geostationary Lightning Mapper,” which maps lightning from above.
The apparatus is about the size of a person and can detect intra-cloud and cloud-to-cloud flashes, many of which are not picked up by most ground-based lightning detection networks.
“We want to see what the boundaries really are,” Peterson said. “It’s about how big and how bright they can get.”
Peterson explained that a bolt’s size and luminosity are two sides of the same coin but present different research challenges. The conditions that give rise to expansive “megaflashes” are also conducive to extremely bright and powerful superbolts.
A megaflash is any lightning discharge greater than 62 miles across. Megaflashes oftentimes are or contain superbolts, since the optical power, or brightness, of a bolt is a product of its size and current.
Peterson said researchers care about the brightness of lightning because the more luminous flashes, from a satellite perspective, are usually the most powerful.
“There is a correlation between peak light power and peak energy,” he explained.
Deducing exactly how powerful a stroke of lightning is based on its luminosity isn’t easy. Lightning happens fast.
“You have these events that are very instantaneous, tens to hundreds of microseconds, and then they’re over,” explained Peterson.
A microsecond is a one millionth of a second.
The Geostationary Lightning Mapper however, can only capture flashes which are longer, or 2,000 millionths of a second, meaning more fleeting peaks in luminosity are often missed.
Peterson said that European Space Agency is planning on launching a satellite that captures images twice as often.
Still, Peterson said that lightning scientists have been privy to a “fire hose” of data since the first Geostationary Lightning Mapper was launched via satellite in 2016. Peterson and his colleagues reviewed GLM data from January of 2018 to January 2020, which featured more than 600 million lightning strikes, and found about 2 million strikes that were considered superbolts. That’s about 0.32 percent of the total discharges.
Not all detected superbolts were actually that super, however. Most flashes observed by the satellite-mounted lightning mapper are “seen” by sensors through a thick layer of cloud, which reduces their brightness. But a few ordinary bolts at the periphery of storms may protrude into clear air in direct view of the sensor, tricking the satellite into thinking an exceptionally bright flash has occurred.
Peterson’s work describes ordinary superbolts as “ubiquitous,” and notes that they can occur with just about any strong thunderstorm in North or South America. But he identified an elite tier of superbolts 1,000 times more powerful than an average lightning stroke.
Those are found in only the most electrified storms in a few parts of the word — namely the central United States and parts of South America’s La Plata Basin, including Paraguay, northern Argentina, and southeast Brazil.
The secret to storms there is their size. Hulking “mesoscale convective systems,” often hundreds of miles across, can bring thunder, lightning and rain for hours.
“There’s a subset of lightning in these larger storms ... where charge in the [anvil cloud] layer is conducive to flashes with a long horizontal extent,” said Peterson.
That allows for extreme discharges that are often both megaflashes and superbolts. A single event can even have dozens of channels that strike the ground, bringing large amounts of current to the surface and posing a serious danger to land dwellers.
Peterson’s team has been working to learn more about those events by comparing satellite-based lightning flash data to surface strike observations collected from a network of ground-based sensors.
“[The satellites] will tell you there is something bright there,” explained Peterson. “Ground networks come in and ... see mostly cloud to ground.”
Other lightning researchers, like Joseph Dwyer at Penn State, say that Peterson’s work clarifies uncertainties in superbolt theory dating back decades.
“So called ‘superbolts’ were first identified in the late ’70s by optical instruments on the Vela satellites,” Dwyer explained in an email. “One unresolved question, however, was whether these superbolts were a different kind of lightning, or if they were just normal lightning seen directly by the spacecraft.”
Now, thanks to work like Peterson’s, it’s looking like superbolts may be super after all.
“These recent studies are beginning to shed light on the topic, showing that at least some superbolts do appear to be much brighter than normal lightning,” said Dwyer. “These studies are also beginning to identify what kinds of lightning are involved in superbolts and where they happen.”
Peterson continues to redouble his efforts at unlocking the secrets behind superbolts. His biggest question: Why is there a superbolt hot spot in the northwest Pacific Ocean between the Korea Peninsula and Japan?
“The sweet spot looking for these things has not really been well-observed,” said Peterson. “It would be fantastic if the Japanese Meteorological Agency was to launch a [lightning mapping] instrument.”
Storms over the ocean produce less lightning than their terrestrial counterparts. Because of that, it’s easier to build up more charge in the storm and generate a stronger electric field. “This can lead to these superbolt cases,” Peterson said.