Beyond Lightning: The Mystery of "Sprites"

Red sprites -- mostly red, but sometimes including blue stalks or tendrils -- are discharges of high-altitude lightning that appear above the cloud tops of large thunderstorms.

Red sprites shoot tendrils downwards into the stratosphere, then reverse and explode upwards to the edge of the mesosphere. They ignite a few milliseconds after particularly large cloud-to-ground lighting strikes just below them. They typically last for about ten milliseconds, barely long enough to be seen with the naked eye, and are so faint that they're hard to spot. They also register in very low radio frequencies.

Sprites can expand up to 30 miles in diameter, starting at altitudes of ~40 km (~25 miles), shooting tendrils down to ~30 km (~20 miles) and bursts up to 90 km (~56 miles).

Sometimes a sprite is accompanied by a ghostly red halo just above it: this is not an elve (see below), but a weak form of the sprite without sufficient power to create columns, tendrils, "carrots" or "jellyfish" shapes.

Red sprite sightings have been reported for over two hundred years, but were often dismissed as optical illusions or the imaginings of fatigued airline pilots.

Scientists are still debating their exact cause and mechanism, but I find this fascinating: according to NASA, they usually happen when a lightning bolt below them gets confused and discharges a positive charge instead of a negative one.¹ Although some scientists argue that sprites tend to be associated with positive lightning strikes simply because they tend to come from more energetic, powerful storms.²

^{Right: Red sprite captured on video by International Space Station. Below, a thundercloud is illuminated by regular lightning. I've tweaked the photo slightly to make the sprite brighter.}

Blue jets erupt out of cloud tops and shoot upwards in a narrow cone like spray from a firehose. They rise up to about 30 miles, five to six miles across at the top and a mile across at their base. They last about a hundred milliseconds, but are less common than sprites.

Unlike sprites, blue jets do not appear in close association with regular lightning discharges. Instead, there are some hints that jets tend to form over storms with heavy concentrations of hail.⁴.

Blue jets had been mentioned by puzzled airline pilots, but their existence was not verified until the 1994 film by the University of Alaska (the last video, above), when a meteorologist team set out to document sprites and discovered blue jets along the way.

Photos of blue jets are extremely rare, but there's a fantastic blue jet photo at the top of this storm photo gallery by Patrice Huet.

"Elves" stands for Emissions of Light and VLF [very low frequency perturbations] from EMP [electromagnetic pulse] Sources". Yes, the scientists were stretching the words to fit the "sprites" theme. 

Even fainter than sprites, and much larger, elves are huge horizontal rings of light that appear on the border between the ionosphere and the stratosphere. They expand up to 300 km (200 miles) across so quickly that they appear to be moving faster than light!

In fact, elves do not violate the laws of physics, since the nitrogen molecules within the rings are not actually moving, but simply colliding; electron collisions cause them to glow like the gas in neon tubes.⁴

Elves only last for one millisecond, so they cannot be seen by the naked eye.

^{Right: Image of Space Shuttle taken by Space Station (NASA). I've added notes and Elve-shape. Space shuttle is actually in ionosphere, but at this angle appears to be lower.}