Tuesday, June 18, 2013

Of Blue Jets and Elves

Weird flashes of light and electromagnetic radiation above thunderstorms have been mystifying researchers since their existence was first confirmed back in 1989. The best known of these transient luminous events are known as sprites (previously covered on TES athttp://tinyurl.com/nc7lqnghttp://tinyurl.com/p2wc5umand http://tinyurl.com/ly86kua), but less famous examples are known as blue jets (first seen in 1994 and filmed in 2002) and elves (filmed from the space shuttle in 1996). The extremely brief nature of these flashes makes them hard to see by design, luck playing an important part in the game, and they are often hard to spot since they're usually obscured by clouds. Careful observation of distant storm fronts and aeroplane flights are your best chance of glimpsing them.

These phenomena were first reliably reported by pilots in world war 2, though authorities doubted them at the time, and fliers stopped mentioning them for fear of losing their wings, much like early NASA astronauts were reluctant to mention the blue flashes of Cherenkov radiation they saw when cosmic rays struck the aqueous humour in their eyes, for fear of being taken off the flight list. No doubt people have been spotting them throughout history, and they must account for some of the generic reports of weird lights in the sky.

Blue jets are narrow cones of light that shoot upwards 70 Km or so from the electrically active core of thunderstorms, lasting about two tenths of a second. They move fairly slowly, about 110Km/second and are about 15 degrees wide. They are not obviously related to lightning, nor aligned with the local magnetic field, but seem to be connected with intense hail. Red versions also exist, known as trolls. Brighter, lower altitude (up to 20Km) blue flashes are known as blue starters, also forming above areas where large hailstones are falling. Some scientists think these electrical flashes help fix nitrogen, needed by plants for photosynthesis.

Elves (Emissions of Light and Very Low Frequency Perturbations due to Electromagnetic Pulse Sources ) are more elusive, as one would expect. These large (several hundred Km) discs of light last only a few milliseconds, and, like sprites, form above cloud-ground lightning strikes when an electromagnetic pulse shoots up from the clouds towards the ionosphere.

The blue jet shown in this colour enhanced image (taken using a low light, monochrome high speed TV camera) was serendipitously captured above Puerto Rico from near the Arecibo observatory in 2001, as part of a research project into sprites. It was visible to the naked eye as a 'gigantic flash', as Victor Pasko of Pennsylvania State University (who shot the film) described it. The storm was fairly ordinary, implying that jets might be quite common. Hot spots are visible at the top where it impacts the ionosphere. Its EMP fingerprint was monitored by radio equipment as far away as Antarctica.

The exact hows and whys of these phenomena are still poorly constrained. The light is produced by atoms, fluorescing as they are excited by the passing currents, either upwards in the case of jets, or downwards in that of sprites. The blue colour is thought to come from fluorescing nitrogen, air's most common gas. Elves are also thought to be glowing nitrogen, due to collisions between electrons. They are thought to be caused by an EMP propagating in the ionosphere. Their electrical energy could affect atmospheric chemistry by helping gases react with each other.

These discharges may represent the missing link in Earth's electrical circuit, connecting the surface with the ionosphere, and accounting for the 300,000 volt potential difference between them. Since they occur at altitudes too low for satellites (due to atmospheric drag pulling them back down) and too high for balloons or aircraft, only remote studies are currently possible. The European Space Agency is funding a microsatellite in 2010, currently being built by the French CNRS, specifically designed to study these phenomena. Named after Taranis, the Gaulish/Celtic god of thunder, it is slated for launch by Arianespace using a Vega or Soyuz rocket in late 2015. Hopefully our understanding of these wonderful flashes will improve once it's operational. Whatever they are, these mysterious and elusive flashes never cease to wow me.

Image credit: Victor Pasko/Penn State.






A movie: elf.gi.alaska.edu/movies/output.mpg


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