Transient Luminous Events (TLEs) are electrical discharges that occur in the upper atmosphere. Their blue and red colors arise from electron impact excitation of molecular nitrogen 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 and Sprites 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 negative charge in the form of the +CG return stroke and subsequent negative leader growth upward and outward horizontally which causes a large electric field change. This field change is translated to the upper atmosphere initiating spatially separate streamer formation (and emission) in the form of a Sprite. Due to the rarified air of the upper atmosphere, the bidirectional breakdown of a Sprite is from streamer activity and not thermalized leaders. The figure below shows the physical mechanism for Jets and Sprites and the red and blue coloring represents the polarity of the breakdown that occurs (red for positive and blue for negative) and not the color of the emissions resulting from the TLE.
A Sprite Halo is a faint short duration expanding illumination at the upper initial breakdown altitude of a Sprite, but fails to develop tendrils (streamers) 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.
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.
Green GHOSTS (Green Emissions from Excited Oxygen in Sprite Tops) were first observed by Paul Smith and Hank Schyma in 2019. These can occur at the top of Sprites and are a blend of emissions with the primary emission from atomic oxygen at 557.7 nm. However, there are other emissions from gaseous neutral iron and nickel as well as molecular nitrogen and ionized oxygen that contribute to the total emission. Below is an image sequence from video of a Sprite and lingering Green Ghost.
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.
Secondary Jets sometimes referred to as Trolls are secondary events that follow a Sprite. They occur in the lower portion of the upper atmosphere and it is believed they may be generated after and as a result of the lowering of positive charge from the preceding Sprite. Therefore the Secondary Jet may be another breakdown in the upper atmosphere similar to a regular Sprite just at lower altitude or it may be due to the subsequent additional lightning leader activity in the underlying storm that propagates upward out of the storm similar to the Gigantic Jet driven by the charge arrangement change from the preceding Sprite or possibly a combination of both as multiple Secondary Jets have been recorded following a preceding Sprite.
Next section: Five Ways Lightning Can Injure of Kill You
Previous section: Upward Lightning from Tall Objects