Lightning sprites are a form of electrical discharge that occurs high above the clouds of active thunderstorms. These events are not conventional lightning, but rather a large-scale, brief electrical phenomenon in the upper atmosphere. Sprites were first scientifically confirmed in 1989. They are now categorized as Transient Luminous Events (TLEs), which are distinct from the hot plasma channel of a typical lightning bolt.
Appearance and Altitude
Sprites span vertical distances far exceeding the height of a typical thunderstorm cloud. They occur primarily within the mesosphere, the layer of atmosphere extending from approximately 40 kilometers to 90 kilometers above the Earth’s surface. Clusters of sprites can reach a horizontal width of up to 50 kilometers, making them one of the largest atmospheric electrical structures observed.
The distinct reddish-orange color of the sprite is caused by the excitation of neutral nitrogen molecules in the low-pressure atmosphere at these heights. The upper portion is typically red, transitioning to a bluish hue in the lower hanging tendrils. They are brief, often lasting for only a few tens of milliseconds, though the duration can range up to about 300 milliseconds.
These events display a variety of shapes, which scientists often categorize based on their structure. The most common forms are column sprites, which appear as vertical pillars of light, and carrot sprites, named for their bright, compact head with long, trailing tendrils below. The visual characteristics also include a faint, pancake-shaped glow known as a sprite halo, which sometimes precedes the main discharge. Sprites are rarely seen with the unaided eye because of their short duration and faint light compared to ground lightning, making sensitive cameras necessary for observation.
The Electrical Mechanism of Formation
The formation of a sprite is linked to the preceding discharge of a powerful, high-current lightning strike between the thundercloud and the ground. Sprites are typically triggered by positive cloud-to-ground (+CG) lightning, which transfers a net positive charge from the cloud to the earth. This type of lightning is less common but carries a much higher current than the more frequent negative strikes.
When the positive charge is rapidly removed from the storm cloud base, the cloud top is momentarily left with a strong residual negative charge. This sudden change in charge distribution creates a quasi-electrostatic field that extends rapidly upward into the mesosphere above the cloud. The upper atmosphere at mesospheric altitudes has a lower electrical breakdown voltage due to the reduced air density.
The intense electric field then accelerates free electrons in the upper atmosphere. These electrons collide with nitrogen and oxygen molecules, transferring energy and causing the gas to glow, a process known as an electron avalanche. The resulting light is a cold plasma discharge, similar to the phenomenon seen in a fluorescent tube, which is why sprites do not have the extreme heat of a regular lightning channel. The entire process initiates at the top of the mesosphere and propagates downwards, creating the characteristic red glow and tendril structures.
This rapid electrical imbalance and subsequent atmospheric discharge create the sprite. The sprite acts to equalize the strong electric field created by the intense lightning, completing an electrical circuit between the storm cloud and the ionosphere.
Related Transient Luminous Events (TLEs)
Sprites are one of several related high-altitude phenomena known as Transient Luminous Events (TLEs). Other TLEs are differentiated from sprites by their altitude, color, and formation mechanism.
Blue Jets are another type of TLE that shoot upward from the core of the thunderstorm cloud, propagating into the stratosphere up to altitudes of about 40 to 50 kilometers. As their name suggests, they are distinctly blue, and their formation is not directly linked to a specific cloud-to-ground lightning strike, unlike sprites.
A third category is ELVES, which is an acronym for Emissions of Light and Very Low-Frequency Perturbations due to Electromagnetic Pulse Sources. ELVES appear as a rapidly expanding, flattened ring of light that can span hundreds of kilometers in diameter, near the boundary of the ionosphere at about 90 to 100 kilometers. They are triggered by the powerful electromagnetic pulse (EMP) created by an intense lightning flash, causing the thin layer of atmosphere to glow briefly.