Sprites and Blue Jets: Lightning Above the Storms

What you would actually see

Ordinary lightning is a hot, jagged thread of plasma at roughly 30,000 K — five times the surface temperature of the Sun — carved between cloud and ground. The transient luminous events (TLEs) that live above the storm look and behave nothing like it. They are dim, silent, cold, and they point the wrong way: up.

A red sprite is the largest and most famous. In a single video frame it appears as a cluster of luminous red tendrils, often shaped like a jellyfish, a carrot, or a bundle of glowing columns, spanning tens of kilometres of sky between about 50 and 90 km altitude. The body glows red-orange, with faint bluish filaments dangling below. It flickers into and out of existence in a few milliseconds — far too fast and too faint for the dark-adapted eye to be sure it saw anything, which is exactly why it took a camera to prove they were real.

A blue jet is different in shape and origin: a narrow blue cone that launches from the very top of a thundercloud and climbs at roughly 100 km/s to about 40–50 km before fading. An elve is a flattened, expanding ring of red light near 90–100 km that blooms and vanishes in under a millisecond — you would never resolve it by eye; high-speed cameras reveal it as a doughnut sweeping outward. And the rarest, the gigantic jet, is a tree of light that bridges the whole gap, connecting a cloud top to the ionosphere near 90 km in a single stroke lasting up to several hundred milliseconds.

• Silent: they happen too high and too fast to be heard.
• Cold: sprite plasma glows more like a neon sign than a lightning bolt.
• Fleeting: most last milliseconds; blink and it is gone.

The mechanism: draining charge into a near-vacuum

To understand sprites, forget the lightning channel and think about the electric field it leaves behind. A thundercloud is a giant battery: negative charge collects low, positive charge high. Most cloud-to-ground strokes are negative — they dump negative charge onto the surface. But a minority, perhaps 10%, are positive cloud-to-ground (+CG) strokes that drain a large reservoir of positive charge from the upper cloud to the ground in a single flash.

When a big +CG stroke fires, it suddenly removes charge that had been quietly balancing the field above the cloud. In an instant, an enormous, unbalanced electric field appears in the region between the cloud top and the conductive ionosphere. The strength of this pull is captured by the charge moment change — charge moved multiplied by the height it dropped. Sprites typically need a charge moment change above roughly 100–300 C·km, and the biggest +CG strokes deliver several hundred coulomb-kilometres.

Here the thin air does the rest. Air pressure falls by roughly a factor of ten for every 16 km of altitude, so at 70 km the atmosphere is under a hundred-thousandth of sea-level density (roughly 1/20,000). In that near-vacuum the voltage needed to break air down — to strip electrons loose and start a glow discharge — collapses. The leftover field, harmless near the ground, now easily ignites the tenuous mesosphere. Electrons avalanche, slam into nitrogen molecules, and the excited nitrogen relaxes by emitting the characteristic red light. It is the same physics as a fluorescent tube: a weak field exciting a rarefied gas, not a searing arc.

Blue jets work by a related but distinct route — a genuine electrical leader escaping upward from the charged cloud top rather than a field ignited from above. Their blue colour comes from ionised nitrogen (N₂⁺) emission, which dominates lower down where the air is still relatively dense; the red of sprites comes from neutral nitrogen at higher, thinner altitudes.

Why red on top and blue below — the numbers of altitude

The colour of a TLE is essentially an altimeter. Two facts set the palette: which molecules the electrons excite, and whether those excited molecules have time to glow before a collision robs them of their energy — a process called quenching.

Below roughly 40 km, the air is dense enough that collisions are frequent. There, discharges favour emission from ionised nitrogen (N₂⁺), which radiates in the blue, giving blue jets and blue starters their cool hue. Higher up, above ~50 km, collisions become rare, the longer-lived red emission bands of neutral molecular nitrogen (N₂) can shine before being quenched, and the discharge turns red — the signature of sprites. Near 90–100 km, at the base of the ionosphere, atomic oxygen and nitrogen can add faint greenish tones.

The scale is genuinely hard to grasp. A single well-developed sprite can be:

• Taller than the cruising altitude of an airliner stacked 7 times — its top near 90 km, higher than any aircraft or weather balloon flies.
• Wider than a large city, tens of kilometres across.
• Cold — the gas kinetic temperature stays close to the ambient mesosphere, far below lightning's tens of thousands of kelvin.
• Weak — total optical output is modest, which is why they are nearly invisible against a bright sky and easiest to catch from the dark of space or a distant mountaintop.

Compare an elve: it is not a discharge climbing the sky but the atmosphere ringing like a bell struck by lightning's electromagnetic pulse. The pulse races outward at the speed of light, heating a thin shell of the lower ionosphere near 90 km, and the excited ring can spread to 300–400 km across in under a millisecond — the fastest and highest of the common TLEs.

A worked comparison: sprite versus lightning bolt

It helps to put a sprite and its parent lightning stroke side by side, because they are almost opposites despite being electrically linked.

An ordinary lightning return stroke is dense-air physics: a channel a few centimetres wide, heated to roughly 30,000 K, carrying tens of thousands of amperes for tens of microseconds, radiating a broadband white flash and the shock wave we hear as thunder. It happens at pressures near one atmosphere, and every joule is crammed into a hair-thin filament.

A sprite is thin-air physics. It spans tens of kilometres in each dimension, glows in a specific red band rather than white, stays near the local air temperature, and produces no thunder — the air is far too thin to carry an audible shock to the ground. Its energy per unit volume is minuscule; what makes it visible is sheer size and the eye-catching efficiency of nitrogen's red emission in vacuum-like conditions. A rough way to hold it in mind:

• Lightning: hot, loud, white, thread-thin, ~1 atmosphere, tens of thousands of kelvin.
• Sprite: cold, silent, red, city-sized, ~10⁻³ atmosphere, near ambient temperature.

The link between them is one-directional and probabilistic. Not every +CG stroke makes a sprite, and the sprite lags the stroke by a few milliseconds — the time it takes the field to build and the mesosphere to break down. In very active storm systems, especially the large mesoscale convective systems that sprawl across the U.S. Great Plains in summer, hundreds of sprites can dance above a single storm in one night.

Limits, misconceptions, and open questions

They are not 'red lightning' in the sky. The most common misconception is that a sprite is just a bolt that went the wrong way. It is not a channel at all — it is a cold, diffuse glow discharge, closer to an aurora or a neon sign in physics than to a lightning arc. It does not conduct a large current to the ground and it produces no thunder.

Not caused only by positive lightning. The great majority of sprites follow +CG strokes, but well-documented cases follow strong negative strokes too, so 'positive lightning' is a strong tendency, not an ironclad rule. And a big +CG stroke does not guarantee a sprite: the charge moment change has to clear the threshold.

Do they matter, or are they just pretty? This is genuinely open. TLEs deposit energy and drive chemistry — production of nitrogen oxides (NOₓ) and other species — in a region of the atmosphere that is otherwise hard to reach, and they represent a real electrical coupling between the troposphere and the ionosphere. Whether their global chemical or electrical effect is significant is still debated; current estimates suggest the direct chemical impact is small compared with other sources, but the coupling to the ionosphere is an active research topic.

Are they unique to Earth? Almost certainly not. Lightning is confirmed at Jupiter, Saturn, and likely elsewhere, and the physics of high-altitude breakdown should operate in any thick, weather-driven atmosphere. Sprites have been searched for above Saturn's and Jupiter's storms; the case is suggestive but not yet a clean, imaged detection of a sprite off Earth. It remains one of the field's tantalising loose ends.

History and how they are observed

The idea came before the pictures. In 1925, Nobel laureate C.T.R. Wilson — the same physicist who built the cloud chamber — argued on purely theoretical grounds that the electric field above a thundercloud should be able to break down the thin upper air. For decades, pilots and mountaineers reported flames and glows leaping above storms, but with no instrument to freeze the moment, the reports were filed as folklore.

The accidental proof arrived on July 6, 1989. A team from the University of Minnesota led by auroral physicist John R. Winckler was testing a sensitive low-light television camera for a rocket campaign. Reviewing the tape, they found two bright columns towering above distant thunderheads — the first photographic capture of what would be named the sprite, after the mischievous, elusive air spirits, precisely because the phenomenon was so fleeting and hard to pin down. Follow-up campaigns in the early 1990s, and colour imaging from aircraft, quickly established their red colour and enormous vertical extent.

Observing them is a game of darkness, distance, and speed. The best vantage points are:

• From the ground: a dark site looking sideways at a storm 100–300 km away, so the sprite is silhouetted against black sky above the visible lightning; long-exposure or high-sensitivity video is essential.
• From aircraft: above the haze, with a clear line of sight over the cloud tops.
• From space: the International Space Station and dedicated instruments look down and to the horizon. ESA's ASIM (Atmosphere-Space Interactions Monitor), installed on the ISS in 2018, was built specifically to study TLEs and terrestrial gamma-ray flashes, and citizen photographers and astronauts have since captured spectacular sprite imagery from orbit.

What was folklore for a century, then a lucky video frame in 1989, is now a mapped and monitored family of phenomena — a reminder that even the sky directly above a summer storm held a whole electrical menagerie hiding in plain, fast, faint sight.

The four main transient luminous events, ordered by altitude and character

Event	Altitude	Color & shape	Trigger
Blue jet	Cloud top up to ~40–50 km	Blue narrow cone shooting upward	Charge buildup at cloud top
Red sprite	50–90 km	Red jellyfish/carrot with blue tendrils	Powerful +CG lightning stroke
Elve	~90–100 km	Expanding red doughnut ring, <1 ms	EMP from any strong stroke
Gigantic jet	Cloud top to ~90 km	Blue base, red top bridging to ionosphere	Intracloud leader breaking upward