Red sprites are among the most captivating types of Transient Luminous Events (TLEs). TLEs are upper-atmospheric electrical phenomena associated with thunderstorms. Sprites are large-scale electrical discharges that occur high above thunderstorm clouds, in the mesosphere.
- A red sprite is a type of upper-atmospheric electrical discharge that appears at altitudes between 50–90 km (31–56 mi) in the atmosphere above some thunderstorms.
- Sprites are huge. Some are 50 km (31 mi) in diameter.
- Sometimes a red halo precedes a sprite. The sprite itself usually is reddish orange, with red arching branches above it and tendrils stretching below it that are often blue or blue-green.
Appearance, Location, and Duration
Sprites are reddish-orange hue at higher altitudes, transitioning to a bluish color at lower altitudes. They often have a jellyfish-like shape, with tendrils reaching down and a larger halo-like structure above. Sprites usually appear directly above large thunderstorm systems or cumulonimbus clouds. But, not all thunderstorms have sprites.
Their location is typically in the altitude range of 50 to 90 kilometers above the Earth’s surface. This places them in the mesosphere, well above the altitude of typical lightning in the troposphere.
The duration of a sprite is incredibly brief. These events last for just a few milliseconds, making them challenging to observe and study.
How Sprites Work
Sprites result from the complex interplay between thunderstorm activity and the Earth’s upper atmosphere. When a powerful lightning strike occurs, it creates an electrical imbalance between the thunderstorm and the ionosphere, the Earth’s highest atmospheric layer. This imbalance generates strong electric fields that accelerate electrons to high energies. These energetic electrons collide with nitrogen molecules in the atmosphere and excited them, causing them to emit light. The resulting display is a sprite.
Positive cloud-to-ground lightning strikes are the type most likely to remove large amount of positive charge from the upper atmosphere. The red and blue wavelengths comes from excited N2 and N2+ from the dielectric collapse.
Types of Sprites
There are different types of sprites, based on their visual characteristics and structure. Here are the main types:
- Classic Sprites (C-sprites): These are the most common types of sprites and are characterized by their vertical, columnar shape. They have one or several columns and are typically reddish-orange at the top, fading to blue at lower altitudes.
- Carrot Sprites: These bear a resemblance to the shape of a carrot, with a broad, bright head and a narrower stem. They often branch out into several tendrils as they descend.
- Jellyfish Sprites (A-sprites): Also known as “angel sprites,” they have a distinct top-heavy appearance with tendrils hanging down, similar to the tentacles of a jellyfish or the shape of an angel. The “body” of the sprite is usually brighter and wider, with tendrils extending downward.
- Column Sprites: These are narrow and typically less bright than classic sprites. They are more or less uniform in width and appear singularly or in clusters.
- Basketball Sprites: These rare sprites feature a circular region of brightening, akin to a basketball, with faint, vertical columns below.
Red Sprites vs Lightning
Red sprites and lightning are both plasma-based electrical phenomena, but they differ significantly in several aspects:
- Altitude and Location:
- Lightning typically occurs within the troposphere, the lowest layer of Earth’s atmosphere, where most weather phenomena take place. It occurs within or between clouds, or between a cloud and the ground.
- Red Sprites occur in the upper atmosphere. This is much higher than the altitude at which typical lightning occurs.
- Appearance and Color:
- Lightning features a bright, white-yellow flash that can illuminate the sky. It follows a more-or-less direct path.
- Red Sprites have a reddish-orange hue at higher altitudes, transitioning to blue at lower altitudes. They often appear as a series of vertical streaks or tendrils, sometimes resembling a jellyfish with a bulbous upper end and trailing tentacles.
- Lightning strikes last for a very short time, typically around 30 microseconds to a few milliseconds.
- Red Sprites also have a brief lifespan but are generally longer than typical lightning strikes, lasting up to several milliseconds.
- Formation Mechanism:
- Lightning results from the build-up and discharge of electrical energy between clouds, between a cloud and the air, or between a cloud and the ground. This is often due to the separation of positive and negative charges within a thunderstorm.
- Red Sprites result from the discharge of positive lightning between a thunderstorm cloud and the upper atmosphere. This discharge creates an electric field above the thunderstorm, accelerating electrons and producing light emission at high altitudes.
- Energy and Impact:
- Lightning carries a significant amount of electrical energy and is a major hazard during thunderstorms, posing risks to life and property.
- Red Sprites are a more diffuse plasma discharge. They carry less energy that is directed upwards into the atmosphere, thus posing minimal risk to the Earth’s surface.
- Detection and Observation:
- Lightning is visible with the naked eye and a variety of ground-based and satellite-based technologies.
- Red Sprites are more elusive, often requiring specialized equipment for observation, such as high-speed cameras and low-light video technology.
Discovery of Red Sprites
The earliest known report of red sprites comes from Toynbee and Mackenzie in 1886. In 1925, Nobel laureate C. T. R. Wilson proposed an explanation for the phenomenon, based on an electrical breakdown in the upper atmosphere. Scientists at the University of Minnesota accidentally captured the first image of a sprite in 1989 while testing a low-light video camera. Since then, sprites have been the subject of intense research.
Origin of the Name “Sprite”
The term “sprite” was coined by Dr. Davis Sentman of the University of Alaska Fairbanks. He chose the name for its connotation of a whimsical, fleeting nature, akin to the mythological creatures known as sprites. The name aptly captures the elusive and transient character of these stunning natural phenomena. “Sprite” is also a bacronym for Stratospheric/mesospheric Perturbations Resulting from Intense Thunderstorm Electrification.
Are Red Sprites Dangerous?
Sprites, despite their dramatic appearance and connection to thunderstorms, present minimal risk to aircraft, spacecraft, or ground-based systems. Their impact is primarily limited due to several factors:
- Altitude Range: Sprites occur at high altitudes (50 to 90 kilometers above the Earth’s surface), which is well above the cruising altitude of commercial aircraft (typically around 10 kilometers) and below the orbits of most spacecraft. That being said, the 1989 loss of a NASA stratospheric weather balloon was retroactively attributed to an encounter with a sprite.
- Duration and Energy: Sprites are extremely short-lived phenomena, lasting only a few milliseconds. Additionally, their energy is not typically directed towards the Earth’s surface or into space in a manner that would impact aircraft or spacecraft.
- Location and Frequency: Because of their association with thunderstorms, sprite activity is predictable and avoidable. There’s little evidence suggesting that sprites have any direct physical impact on aircraft or ground-based systems.
Sprites on Other Planets
Other planets with substantial atmospheres and thunderstorm activity likely have sprites and other forms of upper atmospheric lightning. However, the atmospheric composition determines the color of the discharge, so sprites elsewhere in the solar system are not necessarily red.
For example, the Juno spacecraft detected quick (~1.4 millisecond) electrical flashes corresponding to the H2 Lyman band emission spectrum. Scientists believe the phenomena were elves, sprites, and/or sprite halos. Because Jupiter’s atmosphere differs vastly from Earth’s, the sprites could be blue, red, or possibly pink.
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