What are transient luminous events (TLEs)?

It may come as a surprise to many that such a spectacular phenomenon as flashes of light above thunderstorms was not unambiguously discovered until 1989. In fact, sporadic reports of such phenomena have been around for a long time. Their existence was also predicted theoretically by C.T.R. Wilson (Nobel prize laureate for the invention of the cloud chamber) who developed the theory of thunderstorms during the 1920's and 30's.

However, the transient luminous events, as they are called today, are short-lived. A high-speed, sensitive video camera is required to capture them. Such instruments are common today but were not widely available before the 1990's. The first "red sprite" was captured purely accidentally (Franz et al.) while testing an auroral camera.

Several types of transient luminous events (TLEs) are now known to occur in the middle atmosphere above thunderstorms, and those have been given names such as "blue jets", "blue starters", "giant jets", "sprite halos", "red sprites", "elves" and "trolls".

TLEs occur because lightning flashes remove charge from the clouds. This creates an electric field between the clouds and the ionosphere. If this field is strong enough, the air molecules may become ionised (lose some of their electrons), so that an electrical current can flow. This is called "breakdown". According to one theory, there must also be energetic electrons created by cosmic rays available, since the electric fields above thunderstorms are not sufficiently strong to create a normal breakdown.

Elves, on the other hand, are a bit different from the other TLEs. Elves are large rings of light that expand outwards with a speed larger than that of light! Since nothing can move with such a high speed, elves must occur because a wave from below "hits" the lower ionosphere and heats its electrons, much like a water wave hits a beach. That wave is the strong radio-wave pulse that is always emitted by a lightning.

Carl-Fredrik