Blue jet stabs high into stormy sky

Q: One night I was watching a storm and the lightning turned blue. How did that happen? Even my science teacher wants to know. (Hannah, Shady Point, Oklahoma)

The blue jet that Pasko and his colleagues captured on video — using a low-light monochromatic camera. The image is false-color enhanced to show the blue color that the researchers saw but the camera could not record. [© Pasko, Pennsylvania State University, used with permission]

A: Lucky you! You might be one of few who has seen a blue jet — an electrical flash that strikes up into the ionosphere like blue forked fire. This is a high-altitude phenomenon (starting 10 miles (16 km) high and reaching another 40 miles (64 km) up) but so tremendous that it’s clearly visible from the ground.

Why does it happen? The ionosphere sits at a 300,000-volt higher potential than the lower atmosphere. Blue jets apparently "ground" the ionosphere by discharging this difference like ordinary lightning does for potentials between clouds and between clouds and the ground.

On the other hand, blue jets may help charge the ionosphere. We don’t know yet which way the current flows. Blue jets push science frontiers.

What causes the blue light? The electric field that exists between cloud top and ionosphere speeds up free electrons bopping about in the neighborhood. The sped-up electrons smack into nearby oxygen and nitrogen molecules. The collisions excite electrons bound by the molecules into higher states. When the excited electrons drop back to their normal states, they release energy in the form of a bluish-green light. That causes the glow.

Also, the hit molecules break down into electrons and positively charged ions. This creates more free electrons and the process snowballs. More about this in a moment.

So that’s how it happens.

Only in the past 10 years or so — once in 1994 and once in 2001 — has anybody recorded a color movie or video of the phenomenon. In 1994, a University of Alaska weather team flew over stormy Arkansas, looking for a different phenomenon, called "sprites". They found themselves battered by intense hail inside a thunderstorm and witnessing not sprites — but blue jets.

In just 22 minutes, Davis Sentman’s team spotted 56 blue jets — about 3 a minute. Eugene Westcott, one of the researchers, named them "blue jets."

In 2001, deep in the jungles of Puerto Rico, Victor Pasko an electrical engineering professor at Pennsylvania State University and colleagues were on the ground looking for sprites. Instead, they found the most spectacular blue jet ever recorded.

September 14, 2001 at the Arecibo Observatory. Hunched on a wooden hatch cover on top of a flat concrete roof, Victor Pasko glances around. Not much to see in the gloom of the tropical night. Grey granite cliffs hem the observatory in two directions. Rolling hills dense with rounded trees darken the horizon. His laptop glows in front of him, dimly lighting a video camera perched atop its tripod. The low-light-sensitive camera gazes fixedly up, ready. The jungle murmurs a cacophony of insect calls like static, buzzing in the background.

The sky view holds his attention as it has for the past 20 nights of fruitless searches. No sprites. No blue jets. Occasional sheet lighting illuminates the sky in the distance. Pasko, a patient hunter, pecks a couple of commands into his laptop to gain a higher vantage. The GOES 8 Weather Satellite, high above Earth, spills its data over his screen.

At 11:20 pm, GOES reports a sprawling system about 120 miles (200 km) to the west. Pasko and fellow investigator Mark Stanley see lightning and dark ominous clouds that way. They aim the camera toward the top of the distant clouds. The tops soar about 10 miles (16 km) above ground.

Then it happens. Video.  From the top of the anvil-shaped cloud, the marvel unfolds slowly. A broad-base bluish flickering rises from the cloud. Then, bright enough to see with the naked eye, the light suddenly shoots another 40 (64 km) miles straight up into the ionosphere. The blue bolt branches as it rises into two channels. Each channel separates into many streamers and stabs into the ionosphere at 4 million miles per hour (7 million km/h).

Pasko and Stanley stare at a gigantic electrical event glowing a blue that they had never seen before and cannot describe. They scream wildly.

Now, to better understand how it works, Pasko and his students model the phenomenon. It’s like an electron avalanche, he says, that can flood up toward the ionosphere or slide earthward, depending on the electric field direction. Intense hail may trigger the avalanche, says Pasko.

The field accelerates the electrons and slams them into air molecules. The molecules breakdown into ions and free electrons and emit light. The newly generated electrons also accelerate. The process runs away — avalanches.

These bizarre blue wonders may happen daily over the globe but only a very few people have seen them. That’s why I say: Hannah, lucky you!

Further Reading:

Ball lightning:  I was wondering if Ball Lightning exists. What do you know about it? Where does it happen? When and why?
Cause:  What causes lightning?
Heat:  Lighting is supposed be three times hotter than the sun. Since the lighting flashes are closer than the sun how come we don’t feel the heat when it flashes?
How wide & long:  My kids were wondering how wide and long lighting can be.
MountEverest: Does lightning strike Mount Everest?
Ocean strikes:  If lightning strikes the ocean, do the marine animals get hurt or killed?
Where it hits:  Where in the world do the most lightning strikes occur?

Penn State University: Researchers capture unusual sprite-like blue jet

news@nature.com Giant blue jet caught on film

University of Wisconsin, Madison: Anyone can access the GOES images. A great site.

(Answered May 27, 2005; updated Sep. 11, 2007)