Red sprites, often mistaken for “red lightning,” are large-scale, luminous electrical discharges that occur far above the thunderstorms that trigger them. These fleeting, reddish-orange flashes are a specific type of Transient Luminous Event (TLE). Sprites are extremely rare to observe with the naked eye due to their brief duration, typically lasting from a few milliseconds up to 300 milliseconds. They were first captured on film by scientists in 1989.
Distinguishing Sprites from Ground-Level Lightning
Red sprites are fundamentally different from the familiar bright, hot lightning bolts that strike the ground or flash within clouds. Standard lightning, which occurs in the lower atmosphere (troposphere), is a high-temperature electrical arc. Sprites are cold plasma phenomena, meaning they involve electrical breakdown in a low-pressure environment without the intense heating seen in a traditional lightning channel.
These reddish discharges appear high in the mesosphere, spanning altitudes from about 50 to 90 kilometers above the Earth’s surface, which is far above the top of the thundercloud. Unlike cloud-to-ground lightning, sprites do not travel from the cloud to the ground or connect the cloud to the upper atmosphere. They are a form of atmospheric glow that is more similar to the discharge inside a fluorescent tube than a lightning strike.
The Electrical Trigger for High-Altitude Discharges
The formation of a red sprite is directly linked to a massive electrical disturbance created by a specific type of lightning strike below. Sprites are overwhelmingly triggered by exceptionally powerful positive cloud-to-ground (+CG) lightning strokes. While positive CG strikes account for only about ten percent of all lightning, they transfer a significantly larger amount of charge than the more common negative CG strikes.
When this powerful positive lightning bolt discharges to the ground, it rapidly removes a large amount of positive charge from the cloud below. This sudden charge removal leaves a strongly negative residual charge in the atmosphere just above the cloud top. This creates a significant electrical field disturbance that propagates upward into the much thinner air of the mesosphere.
The strong upward-propagating electric field initiates the sprite’s luminous discharge in the upper atmosphere. This field causes an electron avalanche in the low-pressure air, which is the initial phase of sprite formation. This process rapidly couples the powerful electrical event in the lower atmosphere with the high-altitude region.
The Physics Behind the Red Glow
The distinctive red color of the sprite results from the physics and chemistry occurring in the low-pressure environment of the mesosphere. At altitudes between 50 and 90 kilometers, the electrical discharge interacts primarily with the most abundant gas, molecular nitrogen (\(\text{N}_{2}\)). The electrical energy of the discharge excites these nitrogen molecules into a higher energy state.
As these excited nitrogen molecules quickly return to their normal, lower energy state, they release the excess energy in the form of light. The specific wavelengths of light emitted fall predominantly within the red and near-infrared parts of the spectrum. This emission gives the upper parts of the sprite their characteristic reddish-orange hue. Lower down, the color can transition to a bluer shade as the discharge interacts with different atmospheric components at higher pressures.
Other Transient Luminous Events
Red sprites are one member of a family of atmospheric phenomena known collectively as Transient Luminous Events (TLEs). Other TLEs are associated with powerful underlying thunderstorms but are distinct in appearance, altitude, and color.
Two other TLEs are Blue Jets and ELVES.
Blue Jets
Blue Jets are cone-shaped discharges that shoot upward from the top of the thundercloud. They are blue in color and reach altitudes of up to about 50 kilometers, making them lower than sprites.
ELVES
ELVES stands for Emission of Light and Very Low Frequency perturbations due to Electromagnetic Pulse Sources. ELVES appear as a rapidly expanding ring of light that can span hundreds of kilometers across at altitudes near 100 kilometers. They are triggered by the electromagnetic pulse from a lightning flash, causing a brief, diffuse glow in the ionosphere.