A detailed 3D study of a massive electrical discharge that rose 50 miles
into space above an Oklahoma thunderstorm has provided new information about
an elusive atmospheric phenomenon known as gigantic jets. The Oklahoma
discharge was the most powerful gigantic jet studied so far, carrying 100
times as much electrical charge as a typical thunderstorm lightning bolt.
The gigantic jet moved an estimated 300 coulombs of electrical charge into
the ionosphere—the lower edge of space—from the thunderstorm. Typical
lightning bolts carry less than five coulombs between the cloud and ground
or within clouds. The upward discharge included relatively cool
(approximately 400 degrees Fahrenheit) streamers of plasma, as well as
structures called leaders that are very hot—more than 8,000 degrees
Fahrenheit.
"We were able to map this gigantic jet in three dimensions with really
high-quality data," said Levi Boggs, a research scientist at the Georgia
Tech Research Institute (GTRI) and the paper's corresponding author. "We
were able to see very high frequency (VHF) sources above the cloud top,
which had not been seen before with this level of detail. Using satellite
and radar data, we were able to learn where the very hot leader portion of
the discharge was located above the cloud."
Boggs worked with a multi-organization research team, including the
Universities Space Research Association (USRA), Texas Tech University, the
University of New Hampshire, Politecnica de Catalunya, Duke University, the
University of Oklahoma, NOAA's National Severe Storms Laboratory, and the
Los Alamos National Laboratory. The research is reported Aug. 3 in Science
Advances.
Steve Cummer, professor of electrical and computer engineering at Duke, uses
the electromagnetic waves that lightning emits to study the powerful
phenomenon. He operates a research site where sensors resembling
conventional antennas are arrayed in an otherwise empty field, waiting to
pick up signals from locally occurring storms.
"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," Cummer said.
Doug Mach, a co-author of the paper at Universities Space Research
Association (USRA), said the study was unique in determining that the 3D
locations for the lightning's optical emissions were well above the cloud
tops.
"The fact that the gigantic jet was detected by several systems, including
the Lightning Mapping Array and two geostationary optical lightning
instruments, was a unique event and gives us a lot more information on
gigantic jets," Mach said. "More importantly, this is probably the first
time that a gigantic jet has been three-dimensionally mapped above the
clouds with the Geostationary Lightning Mapper (GLM) instrument set."
Gigantic jets have been observed and studied over the past two decades, but
because there's no specific observing system to look for them, detections
have been rare. Boggs learned about the Oklahoma event from a colleague, who
told him about a gigantic jet that had been photographed by a
citizen-scientist who had a low-light camera in operation on May 14, 2018.
Fortuitously, the event took place in a location with a nearby VHF lightning
mapping system, within range of two Next Generation Weather Radar (NEXRAD)
locations and accessible to instruments on satellites from NOAA's
Geostationary Operational Environmental Satellite (GOES) network. Boggs
determined that the data from those systems were available and worked with
colleagues to bring it together for analysis.
"The detailed data showed that those cold streamers start their propagation
right above the cloud top," Boggs explained. "They propagate all the way to
the lower ionosphere to an altitude of 50–60 miles, making a direct
electrical connection between the cloud top and the lower ionosphere, which
is the lower edge of space."
That connection transfers thousands of amperes of current in about a second.
The upward discharge transferred negative charge from the cloud to the
ionosphere, typical of gigantic jets.
The data showed that as the discharge ascended from the cloud top, VHF radio
sources were detected at altitudes of 22 to 45 kilometers (13 to 28 miles),
while optical emissions from the lightning leaders remained near the cloud
top at an altitude of 15 to 20 kilometers (9 to 12 miles). The simultaneous
3D radio and optical data indicate that VHF lightning networks detect
emissions from streamer corona rather than the leader channel, which has
broad implications to lightning physics beyond that of gigantic jets.
Why do the gigantic jets shoot charge into space? Researchers speculate that
something may be blocking the flow of charge downward—or toward other
clouds. Records of the Oklahoma event show little lightning activity from
the storm before it fired the record gigantic jet.
"For whatever reason, there is usually a suppression of cloud-to-ground
discharges," Boggs said. "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."
For now, there are a lot of unanswered questions about gigantic jets, which
are part of a class of mysterious transient luminous events. That's because
observations of them are rare and happen by chance—from pilots or aircraft
passengers happening to see them or ground observers operating
night-scanning cameras.
Estimates for the frequency of gigantic jets range from 1,000 per year up to
50,000 per year. They've been reported more often in tropical regions of the
globe. However, the Oklahoma gigantic jet—which was twice as powerful as the
next strongest one—wasn't part of a tropical storm system.
Beyond their novelty, gigantic jets could have an impact on the operation of
satellites in low-earth orbit, Boggs said. As more of those space vehicles
are launched, signal degradation and performance issues could become more
significant. The gigantic jets could also affect technologies such as
over-the-horizon radars that bounce radio waves off the ionosphere.
Boggs is affiliated with the Severe Storms Research Center, which was
established at GTRI to develop improved technologies for warning of severe
storms, such as tornadoes, that are common in Georgia. The work on gigantic
jets and other atmospheric phenomena is part of that effort.
Reference:
Levi D. Boggs et al, Upward propagation of gigantic jets revealed by 3D
radio and optical mapping, Science Advances (2022).
DOI: 10.1126/sciadv.abl8731
Tags:
Physics