Small satellites unravel the mysteries of Sprites
For many years, reports from pilots of unexplained lights above the clouds were treated sceptically. But since they were first photographed in 1989, scientists have actively tried to unravel the mysterious electrical events that occur above thunderstorms.
Some of these “transient luminous events” (TLE); which are known as red sprites, blue jets, and elves; are now well-documented, but much remains to be discovered about their properties. Satellites have a significant role to play in enhancing our understanding of them.
Red sprites, jet-like features that occur in a layer of the earth’s atmosphere called the mesosphere, are plasma discharges that can take a variety of forms. These are the most frequently observed members of the TLE family. Originating at the tops of thunderclouds, blue jets are more elusive, and appear to be related to other rare discharges known as blue starters and gigantic jets. Elves are short-lived, expanding rings of light that occur above the storms that create them.
What all these phenomena may have in common with conventional lightning is that they generate radio-frequency emissions. The crackle that interrupts a radio transmission during a thunderstorm is broadband electromagnetic energy that is created by the lightning flash. To address the question of whether TLE’s are also radio sources, Los Alamos National Laboratory in the US commissioned SSTL to provide a satellite platform for their CFESat mission. This satellite is equipped with three low-frequency antennas designed specifically to detect and geo-locate the radio sources and so determine whether they originate above or below the clouds.
This is only the first step on the journey to understand TLEs. Scientists are increasingly looking to test the theory that the luminous plasmas are associated with electrons and other particles being accelerated to a significant fraction of the speed of light. The TARANIS satellite, currently under development in France by CNES, (based on the Myriade platform that has been designed by Astrium), will test this hypothesis. Due for launch in 2015, TARANIS is equipped with a number of instruments designed to detect and characterise high-energy particles.
As satellite and detector capabilities improve, it is expected that sprite investigation will become increasingly dependent on micro and nano-satellite missions. In particular, formations of two or more satellites will allow the temporal and spatial aspects of the events in the atmosphere to be measured.
An Anglo-French consortium has recently been formed to take this sprite research further. SSTL’s Dr Stuart Eves is contributing information on small satellite platforms that could be used to carry future detectors and sensors, and it is hoped that SSTL will ultimately have the opportunity to help the UK contribute to this fascinating area of research.
