Lightning illuminating a thunderstorm has inspired artists and poets throughout the generations. But even among the most brilliant of lightning displays, one phenomenon stands out: gigantic jets, a type of transient luminous event that can span hundreds of square kilometers and reach from the top of the clouds to the edge of space. Atmospheric scientists don’t know a whole lot about how or why gigantic jets form, but one event recently caught scientists by surprise when it took place somewhere unexpected: in Oklahoma.
“What was weird is there were no lightning flashes in this particular storm region before the jet occurred,” said Levi Boggs, lead author on a study of the Oklahoma jet and a research scientist at Georgia Tech Research Institute’s Severe Storms Research Center in Smyrna. “The gigantic jet was the first flash that initiated in that particular region.… That flash just happened to be in the form of a gigantic jet instead of a cloud-to-ground discharge, and we still don’t really know why that is.”
By sheer luck, the 14 May 2018 storm generated a gigantic jet right above two lightning mapping arrays in southwestern Oklahoma. Data from these arrays as well as satellite observations allowed Boggs and his team to create the first 3D radio and optical map of a gigantic jet, which they hope will expand scientists’ understanding of this rare and powerful phenomenon.
Very Strange, Very Powerful
Gigantic jets are among the rarest lightning phenomena observed on Earth. They form when an electrical charge builds up in a thundercloud and then escapes out the top. The discharge can be as powerful as a cloud-to-ground lightning strike but covers a much larger area and reaches from the cloud top all the way to the lower ionosphere, 100 kilometers in altitude.
“They usually form in tropical storm systems that feature really tall cloud tops,” Boggs said. Although gigantic jets sometimes occur over land, most appear over the ocean near the equator. The strong convection and cold cloud tops in tropical and oceanic storms help build the massive charge a gigantic jet needs and transport it toward the cloud top to be released. Building up enough charge for a gigantic jet also means that the storm will usually discharge an abundance of smaller, more average lightning strikes.
The Oklahoma storm, however, defied these expectations. A thunderstorm that passed through the region had weak convection, warm cloud tops, and no strong rainfall. Nor did it produce any lightning flashes before moving on and settling about 100 kilometers to the southwest.
The scientists speculated that the storm somehow built up a large reservoir of negative charge, could not discharge it, and left the reservoir behind when it moved. Later, a tiny updraft, a “convective perturbation” as the team called it, overlapped with the center of the massive charge buildup and provided just enough energy to initiate a flash.
“Imagine you have this big pile of newspapers with gasoline on them. Nothing’s happening with that, but then this little, tiny match gets thrown on, and the whole thing bursts into flames,” Boggs explained. “That’s what we think happened here.”
The gigantic jet covered an area about 60 × 60 kilometers and transferred about 300 coulombs of charge to the ionosphere, “which is more than double the amount of the previous largest amount by a gigantic jet on record,” according to Boggs. “It’s right up there with the most powerful cloud-to-ground strikes that have ever been recorded.”
Entirely by chance, this particular jet occurred near the center of one cluster of lightning mapping array sensors and very close to another. The two arrays used very high frequency radio waves to map the structure of the discharge up to 45 kilometers in altitude. At the same time, the Geostationary Lightning Mapper (GLM) on NOAA’s Geostationary Operational Environmental Satellites (GOES) 16 and 17 observed the event in optical wavelengths. When combined with the radio mapping data, the GLM observations allowed the scientists to create the first 3D map of the charge and plasma structure of a gigantic jet. The team published these results in Science Advances on 3 August.
“If we would have set up a camera to look for these things, which we’ve done in the past, you just never catch ’em. It’s like Murphy’s law,” Boggs joked. “You set up all these nice fancy instruments, and then you just don’t get anything. These random occurrences, when you see them you have to take advantage of them.”
More Observations from Space to Come
“I find [the study] quite exciting as it applies some new techniques and data sets to the scientific problem of gigantic jets,” said atmospheric scientist Timothy Lang of NASA Marshall Space Flight Center in Huntsville, Ala. Lang, who was not involved with the research, was particularly excited at the use of GLM to learn that the streamer transition zone in gigantic jets can occur much lower in altitude than previously thought.
“One of the present weaknesses of gigantic jet research is the rarity of the phenomenon itself, so it becomes difficult to put all the observing pieces together enough times to paint a coherent picture,” Lang said. They’re so rare we don’t quite know how rare they are: There could be anywhere from a few thousand to more than 50,000 events per year, but only a handful of those are actually observed. This study “demonstrates the diversity of meteorological scenarios that can produce gigantic jets, as [this] storm was much weaker than many previous gigantic jet–producing cases. So the next step is to build out our capabilities of observing gigantic jets consistently, especially from space. GLM is helping with that.”
Boggs and his collogues recently received a grant to mine GLM data with machine learning algorithms to catch more gigantic jets in action. The instrument runs nonstop and can capture a hemisphere at a time, which should allow the scientists to study more of these events in great detail. They hope to catch more gigantic jets like the Oklahoma event that form in diverse meteorological conditions, to pin down the occurrence rate of these extreme events, and to explore how jets of various strengths may affect the ionosphere.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer