Elves: Upper-Atmospheric Rings of Power

The large formation of an ELVES recorded by the International Space Station from 250 miles (400 km) above.
The European Space Agency - Elves seen from space
Ksenija Brakočević
Ksenija Brakočević
5 min

Elves are one of the transient luminous events observable by low light camera in the visible wavelength. They last less then 1 ms, which had made them difficult to detect and observe up to previous decade. They appear at the altitude of about 90 km, and they form a ring above their corresponding CG (cloud to ground) lightning which expands from 50 km to 350 km horizontally. Even though it was believed for some time, that cause of elves lies in positive discharges, it appears that they are caused by both negative and positive discharges if they are strong enough. In 2015 it has been discovered that elves can be caused by intracloud discharges as well.

Luminous effect of the elves is caused by heating of ionospheric electrons and subsequent excitation of N2 molecules caused by strong electromagnetic pulses from strong lightning discharge. In recent years it has been theorised that elves can be caused by terrestrial gamma rays (TGF), and in 2019 Atmosphere-Space Interaction Monitor (ASIM) detected elves caused by TGF. Lower lightning current limit for producing elves is a matter of an ongoing speculation and research, values vary from 38 kA to 130 kA. Type of equipment and sensitivity of the sensors have major impact on conclusions regarding this matter. Even though value of current necessary for producing elves is the most curious question, current is not the only parameter that is responsible for creation of elves. Other parameters of the return strike, neutral density of the background, electron density, altitude, pressure etc. play a role in creation of elves.

Elves cause changes in ionisation which can be detected as perturbation of sub-ionospheric propagation of narrowband VLF signals emitted by powerful transmitters for communication with submarines. This occurrence is known as long-recovery early event LORE. The name comes from the fact that it takes several minutes for electron density in lower ionosphere to recover after any strong lightning strike. LORE helps detect elves.

A century ago Wilson predicted possibility of upper atmospheric discharges explaining “while the electric force due to the thundercloud falls off rapidly with height, the electric force required to cause sparking (which for a given composition of the air is proportional to its density) falls off still more rapidly. Thus, if the electric moment of a cloud is not too small, there will be a height above which the electric force due to the cloud exceeds the sparking limit, with the development of cameras and other equipment different types of upper atmospheric discharges were discovered. The most famous ones – sprites, were described in the previous article. It is important to note the difference between sprites and elves. Not only their appearance differ, as well as the altitude where they occur and overall dimensions, they have different causes. Sprites are caused by quasi-static electric field which appear during strong thunderstorm, whereas elves are caused by electromagnetic pulses which are caused by strong lightning discharge.

This article is a suiting addition to the previous stories and has the goal of further explaining interesting phenomena that occur in the skies above, as you could have learned it is quite colourful and dynamic up there.

References

  • I.Kolmašová, O.Santolík, (2020). First observations of elves and their causative very strong lightning discharges in an unusual small-scale continental spring-time thunderstorm, JGR Atmospheres, 126(1), https://doi.org/10.1029/2020JD032825
  • Inan, U. S. (2002). Lightning effects at high altitudes: sprites, elves, and terrestrial gamma ray flashes. Comptes Rendus Physique, 3(10), 1411–1421. doi:10.1016/s1631-0705(02)01418-4
  • Fernsler, R. F., & Rowland, H. L. (1996). Models of lightning-produced sprites and elves. Journal of Geophysical Research: Atmospheres, 101(D23), 29653–29662. doi:10.1029/96jd02159
  • Nagano, I., Yagitani, S., Miyamura, K., & Makino, S. (2003). Full-wave analysis of elves created by lightning-generated electromagnetic pulses. Journal of Atmospheric and Solar-Terrestrial Physics, 65(5), 615–625. doi:10.1016/s1364-6826(02)00324-3
  • M. Tomicic, O. Chanrion, (2023). Observations of Elves and Radio Wave Perturbations by Intense Lightning, 128(10), https://doi.org/10.1029/2022JD036541
  • C. P. Barrington-Leighand, U. S. Inan, (1999), Elves triggered by positive and negative lightning discharges, 26(6), 683-684, https://doi.org/10.1029/1999GL900059