Transient luminous events (TLE): sprites, elves, jets

Transient luminous events (TLE) are brief optical discharges above storm clouds, at altitudes where ordinary lightning does not occur. They form a link between tropospheric storms and the ionosphere (see #68). Among the main types are:

• Sprites (red/reddish discharges): extensive discharges in the mesosphere, roughly at an altitude of ~40-90 km. They tend to be triggered by strong positive cloud-to-ground discharges (see #66).
• Elves (expanding rings): rapidly expanding luminous rings at an altitude of around ~90 km, at the lower edge of the ionosphere, caused by the electromagnetic pulse of lightning.
• Blue jets: blue jets directed upward from the top of the storm cloud into the lower stratosphere.

These phenomena are brief and high above the storm, so their direct optical observation from the ground is difficult and dependent on favorable conditions. This is the main motivation for their indirect detection through accompanying electromagnetic transients — Q-bursts (see #66, #67).