29.05.2018

Red sprites pop above a thunderstorm over southeastern Germany just before midnight on August 18, 2017. At the time, the storm was about 315 miles (507 km) away. The display lasted a fraction of a second and looked like a white smudge with the naked eye. Ondrej Králik

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In mythology, sprites are supernatural, fairy-like creatures of the air, forest and water. About the size of large insects, they come down from the trees every morning to bathe in dew and pester butterflies. Sprites have been part of folklore for hundreds if not thousands of years.

When it came time to name a strange, newly recognized atmospheric phenomenon that seemed more spirit than real, sprites seemed the perfect choice. What’s more, these sprites are real. They look like clustered tentacles of ruby-red light that flash for the briefest slice of time above powerful thunderstorms. We’ve had anecdotal evidence for them since the late 1880s from folks who saw them out of the corner of their eye and later from airplane pilots trying to steer clear of big storms.

The scientific community remained skeptical of sprites’ existence until the night of July 6, 1989, when the late Prof. John R. Winkler, an auroral physicist at the University of Minnesota, snapped their picture. That evening he was testing a low-light TV camera by taking images of the night sky from his observatory northeast of the Minneapolis–St. Paul metro area. He pointed it toward a thunderstorm along the northern horizon and captured twin flashes of pink light lasting about 0.03 second and reaching upward from the cloud tops to about 12.4 miles (20 km) high.

Phenomenal sprites over Australia

Winkler called it cloud-to-ionosphere lightning, but the flashes were later given the whimsical name red sprites, in part because we really didn’t know what they were. Lightning they are not. Although a light phenomenon and associated with thunderstorms, red sprites are a “cold” electric discharge very different from the lightning bolts that rake a thunderhead. A lightning stroke can heat the air to around 50,000°F (27,760°C) or five times hotter than the surface of the Sun.

youtube.com/watch?v=HOLeZZVKpLU
Sprites are major discharges but without the heat, more like the glow of a fluorescent light — bright but cool. Individual sprites in a cluster span between 33 to 328 feet (10 to 100 m) across and flash into view from 31 and 60 miles (19 and 37 km) high, far above the tops of thunderclouds in a layer of the atmosphere called the ionosphere, the same region that hosts the aurora.

Using video cameras, lightning mappers and low-frequency radio receivers (sprites produce bursts of radio noise), scientists around the globe are learning more what produces red sprites. In the process, they’ve also uncovered other TLEs or transient luminous events including even rarer blue jets, gnomes, trolls, pixies and elves.

Fine video showing a vivid red sprite burst (toward the end)

We now know that red sprites are produced by positive lighting, not the far more common negative variety. In a thunderhead, colliding ice particles whipped around by powerful updrafts can be stripped of electrons and become positively charged or gain electrons and become negatively charged. The positives generally rise to the top of the cloud, while the negatives collect in the cloud’s base. Exactly how and why they separate isn’t fully understood, but the air acts as an insulator and prevents them from connecting and discharging.

But air only insulates to a point. As the charges build in intensity, positive and negative desperately seek each other out. Eventually, the difference in charges is enough to overcome air resistance, and the two connect and discharge in a great bolt of lightning. About 75% of the lightning in a thunderstorm occurs inside or between clouds and 20% between cloud and ground.

Nebraska sprites

In most cases, the flow of electricity is from the negative bottom to the positive top or down to a spot of positively charged ground. That’s negative lightning. Only 5% of lightning runs the other way from positive to negative, what meteorologists call positivelightning. It occurs during the peak and decaying stages of a massive thunderstorm, when a positive strike exits from the top of a thundercloud, sidesteps the cloud bottom and directly strikes the negatively charged Earth.

Positive lighting is five times hotter and more powerful than the negative version, lasts longer and can strike from a distance of several miles—the classic “bolt from the blue.” It’s also the trigger for red sprites and other TLEs.

I’ve been out on more than a dozen occasions when thunderstorms flickered in the distance under a starry sky, but never saw red sprites simply out of ignorance. I just didn’t know how and where to look. The bizarre phenomenon has only come to light in recent years as more and more people have become aware of their presence. Digital still and video cameras have been a great boon in recording the flashes that are now shared around the planet on social media.

Sprites are visible with the naked eye, but you’ll have to invest a little time to spot one. On summer nights, I now make a point to be out when storms are either moving out of the region or happening so far away that only the occasional flash of silent lightning betrays their presence. Not that you need a camera to “see” a sprite, but having one means that you might freeze the elusive 3-millisecond moment that’s all of a red sprite’s life forever. In comparison, a blink of the eye lasts 100 times as long.

Sprites pop up virtually everywhere and anywhere strong thunderstorms occur in North America, South and Central America, Africa and Australia. For obvious reasons, summer is the best time to hunt for them. Lightning storms are frequent especially east of the Rocky Mountains. With their see-forever views, the Central and Northern Plains states are best.

You’ll need a nighttime thunderstorm in your region but not one that covers the sky. That’s too close (and dangerous), and the clouds will block any potential sprites. Best is a clear, starry sky with a line of thunderstorms crackling away along a distant horizon. That way you have a line-of-sight view across the cloud tops.

The next time you notice flashes of lightning in an otherwise cloudless night sky, see what direction they’re coming from and drive to a spot with an open view down to the horizon in that direction. The best sprite viewing occurs when thunderstorms are located between 100 and 200 miles (160 and 480 km) away.

To improve your chances, avoid observing during twilight and in bright moonlight. You need dark skies and dark-adapted eyes to bag a sprite. Tempting as it is, don’t stare at the line of storms but rather just above them while ignoring the lightning flashes. Sprites pop into view roughly once for every 200 lightning strokes.

If you have a moderate to high-end camera or a low-light video camera, you can try to record the elusive tentacled fire. With both video and still, use a lens with a focal length of 24-50mm and set it to its widest opening, anywhere from f/1.4 to f/2.8. If your lens only opens to 3.5 or 4, it’s probably not suitable for recording these fast, faint flashes.

Fix the camera to a tripod and (with a still camera), focus it on the stars using the live view feature on the camera back. Set the ISO to 3200 to 6400 and expose from 5-20 seconds. Take one photo after another, and if you’re lucky, you’ll spot a sprite and take home a photographic souvenir.

Quelle: Astro Bob