Transient Luminous Events (TLEs) are electrical discharges that occur in the upper atmosphere. Their blue and red colors arise from electron impact excitation of nitrogen and oxygen, with blue emission below 50 km and red above 50 km.
As previously mentioned in the Cloud Flash section, Blue Jets and Gigantic Jets are extension of cloud flashes in which one end of the bidirectional leader network propagates upward beyond the thunderstorm. Sprite Halos, Sprites, and Elves initiate and develop spatially separate from the thunderstorm in response to the electric field change caused by a ground flash in the underlying storm. The triggering ground flashes are dominantly positive and tend to span large horizontal distances. This results in a large movement of electrons upward and outward horizontally which causes a large electric field change. This field change is translated to the upper atmosphere initiating spatially separate ionization.
A Sprite Halo is a faint short duration expanding illumination at the typical initial breakdown altitude of a Sprite, but fails to develop tendrils (leaders) like a Sprite. Therefore, a Sprite Halo can be thought of as a failed Sprite. Halos are more common than Sprites as lower energy ground flashes can initiate the illumination but not achieve the charge transfer to cause the development of a Sprite.
A Sprite is the bidirectional breakdown of the rarified air at approximately 60 km. Sprites are typically triggered by an underlying horizontally extensive +CG flash that transfers a large amount of charge to ground. Since negative charge is raised during a +CG flash, the positive end of a Sprites bidirectional development travels downward to altitudes near 20 km, whereas the negative breakdown travels upward to 90-100 km.
ELVES (Emission of Light and VLF perturbations due to EMP Sources) are an expanding ring of light omission at the bottom of the ionosphere. They only last a tenth of a millisecond and can expand to a diameter of 1,000 km. They originate in response to an electromagnetic pulse from a preceding ground flash that reaches and interacts with the conductive ionosphere.
The following is a high-speed video that shows both an ELVE and a Sprite. The ELVE expands horizontally prior to the development of tendrils associated with the Sprite.
The following is a high-speed video of a triple sprite. The brightness from the triggering positive ground flash is visible preceding the development of the sprites.
The following low-light standard speed video shows a combination of sprites and a gigantic jet. The gigantic jet is further from the camera than the preceding sprites.
Digital still cameras are able to capture sprites and reveal their red hue.