Mesospheric Bore

Nov. 29, 2016: This month, a lot is happening in the mesosphere. The mesosphere is a layer of Earth’s atmosphere above the stratosphere; it is the realm of sprites, noctilucent clouds (NLCs), and airglow. Starting on Nov. 17th, NASA’s AIM spacecraft spotted bright noctilucent clouds forming in the mesosphere above Antarctica. Then, in an apparently unrelated development on Nov. 24th, the normal dome of airglow over China split in two. Xiao Shuai photographed the event from Mount Balang in Sichuan:

This is called a “mesospheric bore”–and not because it’s dull.  A bore is a type of atmospheric wave with deep ripples at its leading edge.  Indeed, you can see the ripples in Shuai’s photo separating the zone of airglow from clear sky.

Bores fall into the category of “gravity waves”—so called because gravity acts as the restoring force essential to wave motion. Analogy: Boats in water. When a boat goes tearing across a lake, water in front of the boat is pushed upward. Gravity pulls the water back down again and this sets up a wave.

In this case, instead of water, rarefied air is the medium through which the wave propagates.  The sudden boundary in the airglow layer is probably akin to a hydraulic jump.  But what created the disturbance in the first place?  (What is the ‘boat’?) No one knows.

“There may be updates in the coming days as scientists from NASA and the Chinese Academy of Science check data from satellites to learn more about this event,” says Jeff Dai, who has been helping Xiao Shuai process and communicate his extraordinary images. “Also, we encourage other photographers from Thailand, Myanmar, Bangladesh and India to submit their images of the wave.”

Realtime Space Weather Photo Gallery

Sprites above Hurricane Matthew

by Dr. Tony Phillips (Spaceweather.com)

Oct. 2, 2016: On Oct. 1st, Earth weather met space weather above Hurricane Matthew.  As the giant storm system was approaching the Greater Antilles, Frankie Lucena of Puerto Rico photographed red sprites shooting up from the thunderclouds:

Sprites are a strange and beautiful form of lightning that shoot up from the tops of electrical storms. They reach all the way up to the edge of space alongside meteors, auroras, and noctilucent clouds. Some researchers believe cosmic rays help trigger sprites, making them a  true space weather phenomenon.

Seeing sprites above a hurricane is rare. Many hurricanes don’t even have regular lightning because the storms lack a key ingredient for electrical activity: vertical winds. (For more information read the Science@NASA article “Electric Hurricanes.”) But Matthew is not a typical hurricane.  It’s one of the most powerful in recent years, briefly reaching Category 5 at about the time Lucena photographed the sprites.  Perhaps extra-strong winds in the vicinity of the storm set the stage for upward-reaching bolts.

Sprite photographers across the Caribbean and the southeastern USA should be alert for more as the storm system approaches the mainland: observing tips.

Realtime Sprite Photo Gallery

Space Lightning Over China

On Aug. 13th in China, photographer Phebe Pan was photographing the night sky, hoping to catch a Perseid meteor. Instead, he witnessed a spectacular bolt of “space lightning.” Working atop Shi Keng Kong, the highest mountain peak in the Guangdong province, “I was using a fisheye lens to capture as much of the sky as possible,” says Pan. “Suddenly we saw a flash of blue and purple ejected from the top of a nearby thundercloud. It just looked like a tree with branches, and grew up very fast. So awesome!”

“It just looked like a tree with branches, and grew up very fast,” says Pan. “It lasted just less than one second. So awesome!”

Oscar van der Velde, a member of the Lightning Research Group at the Universitat Politècnica de Catalunya, explains what Pan saw: “This is a very lucky capture of a gigantic jet. It’s the first time I’ve seen one captured using a fisheye lens!”

Think of them as sprites on steroids: Gigantic jets are lightning-like discharges that spring from the tops of thunderstorms, reaching all the way to the ionosphere more than 50 miles overhead. They’re enormous and powerful.

“Gigantic jets are much more rare than sprites,” says van der Velde. “While sprites were discovered in 1989 and have since been photographed by the thousands, it was not until 2001-2002 that gigantic jets were first recorded from Puerto Rico and Taiwan.” Only a few dozen gigantic jets have ever been seen.

Like their cousins the sprites, gigantic jets reach all the way up to the edge of space alongside meteors, noctilucent clouds, and some auroras. This means they are a true space weather phenomenon. Indeed, some researchers believe cosmic rays help trigger these exotic forms of lightning, but the link is controversial.

Realtime Sprite Photo Gallery

Rare Blue Starter

by Dr. Tony Phillips (this article originally appeared on Spaceweather.com)

We all know what comes out of the bottom of thunderstorms: lightning bolts. But on Oct. 20th, Thomas Ashcraft of New Mexico saw something coming out of the top. “I captured a form of a transient luminous event called a ‘blue starter’ shooting up from the top of a thunderstorm cloud,” he says. “Blue starters are rarely captured from ground level and there are hardly any specimens on the internet.”

Lightning scientist Oscar van der Velde explains this phenomenon: “A blue starter is an electric streamer discharge coming out of the top of a thundercloud, fanning out and reaching up to the stratosphere as high as 26 km altitude. First reported by UAF scientists Wescott and Sentman in 1995/1996, they were found to be different from blue jets, which reach 35-40 km height.”

“Since then, there have been very few reports of blue starters,” continues van der Velde. “It seems that unusual physical circumstances may be required to produce them. Also, geometry can prevent people from seeing blue starters when a cloud is nearby because the underbody of the cloud can block their view. At larger distances the blue/violet light does not make it to the observer due to scattering.”

Blue starters and blue jets are cousins of sprites, another form of exotic lightning that shoots up instead of down. Sprites, however, are more frequently observed. Check them out in the realtime photo gallery:

Realtime Sprite Photo Gallery

Electric Hurricanes

by Dr. Tony Phillips (this article originally appeared in Science@NASA)

January 9, 2006: The boom of thunder and crackle of lightning generally mean one thing: a storm is coming. Curiously, though, the biggest storms of all, hurricanes, are notoriously lacking in lightning. Hurricanes blow, they rain, they flood, but seldom do they crackle.

Surprise: During the record-setting hurricane season of 2005 three of the most powerful storms–Rita, Katrina, and Emily–did have lightning, lots of it. And researchers would like to know why.

https://smd-prod.s3.amazonaws.com/science-green/s3fs-public/mnt/medialibrary/2006/01/09/09jan_electrichurricanes_resources/flightpath.jpg
Above:
An infrared GOES 11 satellite image of Hurricane Emily. Yellow + and – symbols mark lightning bolts detected by the North American Lightning Detection Network. The green line traces the path of the ER-2. Click to view electric fields measured by the aircraft during the flight.

Richard Blakeslee of the Global Hydrology and Climate Center (GHCC) in Huntsville, Alabama, was one of a team of scientists who explored Hurricane Emily using NASA’s ER-2 aircraft, a research version of the famous U-2 spy plane. Flying high above the storm, they noted frequent lightning in the cylindrical wall of clouds surrounding the hurricane’s eye. Both cloud-to-cloud and cloud-to-ground lightning were present, “a few flashes per minute,” says Blakeslee.

“Generally there’s not a lot of lightning in the eye-wall region,” he says. “So when people see lightning there, they perk up — they say, okay, something’s happening.”

Indeed, the electric fields above Emily were among the strongest ever measured by the aircraft’s sensors over any storm. “We observed steady fields in excess of 8 kilovolts per meter,” says Blakeslee. “That is huge–comparable to the strongest fields we would expect to find over a large land-based ‘mesoscale’ thunderstorm.”

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Above:
The ER-2 en route to a hurricane. [More]

The flight over Emily was part of a 30-day science data-gathering campaign in July 2005 organized and sponsored by NASA headquarters to improve scientists’ understanding of hurricanes. Blakeslee and others from NASA, NOAA and 10 U.S. universities traveled to Costa Rica for the campaign, which is called “Tropical Cloud Systems and Processes.” From the international airport near San Jose, the capital of Costa Rica, they could fly the ER-2 to storms in both the Caribbean and the eastern Pacific Ocean. They combined ER-2 data with data from satellites and ground-based sensors to get a comprehensive view of each storm.

Rita and Katrina were not part of the campaign. Lightning in those storms was detected by means of long-distance sensors on the ground, not the ER-2, so less is known about their electric fields.

Nevertheless, it is possible to note some similarities: (1) all three storms were powerful: Emily was a Category 4 storm, Rita and Katrina were Category 5; (2) all three were over water when their lightning was detected; and (3) in each case, the lightning was located around the eye-wall.

What does it all mean? The answer could teach scientists something new about the inner workings of hurricanes.

Actually, says Blakeslee, the reason most hurricanes don’t have lightning is understood. “They’re missing a key ingredient: vertical winds.”

Within thunderclouds, vertical winds cause ice crystals and water droplets (called “hydrometeors”) to bump together. This “rubbing” causes the hydrometeors to become charged. Think of rubbing your socked feet across wool carpet–zap! It’s the same principle. For reasons not fully understood, positive electric charge accumulates on smaller particles while negative charge clings to the larger ones. Winds and gravity separate the charged hydrometeors, producing an enormous electric field within the storm. This is the source of lightning.

A hurricane’s winds are mostly horizontal, not vertical. So the vertical churning that leads to lightning doesn’t normally happen.

Lightning has been seen in hurricanes before. During a field campaign in 1998 called CAMEX-3, scientists detected lightning in the eye of hurricane Georges as it plowed over the Caribbean island of Hispaniola. The lightning probably was due to air forced upward — called “orographic forcing” — when the hurricane hit the mountains.

“Hurricanes are most likely to produce lightning when they’re making landfall,” says Blakeslee. But there were no mountains beneath the “electric hurricanes” of 2005—only flat water.

It’s tempting to think that, because Emily, Rita and Katrina were all exceptionally powerful, their sheer violence somehow explains their lightning. But Blakeslee says that this explanation is too simple. “Other storms have been equally intense and did not produce much lightning,” he says. “There must be something else at work.”

It’s too soon to say for certain what that missing factor is. Scientists will need months to digest reams of data gathered in this year’s campaign before they can hope to have an answer.

Says Blakeslee, “We still have a lot to learn about hurricanes.”