What are red sprites, blue jets, and other Transient Luminous events?
What are red sprites?:
Red sprites are large-scale electric discharges that occur in the mesosphere, high above very active thunderstorms.
They are the result of the electrical excitation and subsequent photon release of gases in a strong but extremely short-lived electric field caused by lightning below.
Red Sprites are a member of the family of upper atmospheric lightning-related phenomena known as Transient Luminous Events.
Estimates from the ISUAL satellite readings suggest that there are approximately 500,000 red sprite occurrences per year. Or about one per minute worldwide on average.
They materialize as a range of intricate visual shapes flickering in the night sky, so fast that they are often difficult to catch on camera or to discern with the naked eye.
Sprites get their characteristic red color from excitation of nitrogen in the low pressure environment of the upper mesosphere. At such low pressures quenching by atomic oxygen is much faster than that of nitrogen, allowing for nitrogen emissions to dominate despite no difference in composition. As the atmospheric pressure increases in the lower atmosphere, the red emissions are quenched and blue emissions from atmospheric nitrogen excitation dominate, as seen in this video at the 2:42 time marker. Video of Blue colors in red sprites
The science of Red Sprites:
Sprites occur near the top of the mesosphere at about 80 km altitude in response to the electric field generated by strong lightning strokes in an underlying thunderstorm.
99% of Red Sprites are triggered by the discharges of massive positive lightning strikes between an underlying thundercloud and the ground.
Rarely, large negative lightning strokes can generate Red Sprites also. When a sufficiently large positive lightning strike carries charges to the ground, the cloud top is left with a strongly negative net charge. Being as the overlying ionosphere has a net positive charge, a strong but short-lived electric field is generated in the region above the thunderstorm.
In the low pressure of the upper mesosphere the breakdown voltage is drastically reduced, allowing for an electron avalanche to occur and a bi-direction excitation of the atmosphere occurs.
The Science of Red Sprites video.
Appearance of Red Sprites:
Sprites appear as luminous red-orange flashes on camera and video. To the naked eye, and especially at a distance, they usually appear as a quick white flash. They occur above the troposphere at an altitude range of 50–90 km (31–56 mi), and can range in shape from relatively simple and small mile long columns, to massively intricate structures with thousands of miles worth of atmosphere affected.
Some of the most commonly formed shapes include:
1) Jellyfish sprites. The kings of the night sky. Hugely complex and stunning structures with a very little understood etiology. They are also some of the largest and quickest of sprites to occur, sometimes lasting for mere milliseconds but occupying a volume of many tens of miles squared. The Jellyfish 'head' seems to usually be at the highest altitude and is a diffuse glowing, dome-shaped body of excitation. From it, tendrils accelerate at 1/10th the speed of light towards the cloud tops. Many positive and negative streamers in the electric field cause complex zig-zagging shapes, forming the body of the jellyfish.
2) Carrot sprites. Classically beautiful and complex in nature. The carrot sprite usually occurs in clusters of two or more. They generally have bright bodies, reaching tens of miles in length with intricate positive and negative streamers and tufted carrot tops firing high into the atmosphere. Long tendrils are usually present and beading can be seen where it is believed that pockets of gases are excited for longer than the rest of the channel.
3) Column sprites. The most commonly found and simple of the sprites. Though sometimes only a single column sprite can be registered, some column sprite clusters can have dozens of individual elements. Column sprites can have some of the brightest and longest-lived components. Therefore , they are some of the easiest sprites to see with the unaided eyes.
Due to the high speeds of the related excitation, we usually see red sprites as structured streaks of light in images, when they are in fact the lines traced by high speed balls of ionization waves. This can be seen in rarely captured high speed footage, where the the processes tracing out the complex shapes can be better visualized.