The Space Science division of the South African National Space Agency (Sansa) is focusing its attention on space research in the South Polar region and in the upper atmosphere, with particular interest in phenomena called Transient Luminous Events (TLEs), but more informally known to scientists as Sprites. Although predicted in 1925 by Professor CTR Wilson (1869–1959), who won the Nobel Prize for Physics in 1927, the first scientific observation of a TLE/Sprite only happened in 1989 and that was entirely by chance.

“Sprites are blue-sky research at this time,” points out Sansa Space Science chief scientist Professor Michael Kosch. “At the moment, our Sprite programme is very much in its infancy, but I have been involved in a Sprite observation campaign in Europe. Worldwide, this is a very young field of science.”

Sprites, or TLEs, occur in the part of the atmosphere that is least known to science – the upper middle zone, or mesosphere, that is too high for aircraft or balloons to reach but too low for satellites to operate in and directly analyse. This region of the atmosphere is located between 50 km and 85 km above the surface of the earth. The mesosphere is directly above the stratosphere and directly below the thermosphere (which is the second-last layer of the atmosphere, the last being the exosphere). Most meteors which enter the earth’s atmosphere burn up in the mesosphere and some of their material persists there. Currently, the only way to directly analyse this region is by using sounding (suborbital) rockets, which can carry scientific payloads into the mesosphere: but such missions last only about ten minutes at best and are rather expensive (around $1-million a shot).

“Transient Luminous Events are very short – between one thousandth and one hundredth of a second – gas discharges,” explains Kosch. “TLEs require the local electrical field to exceed the dielectric breakdown threshold of the atmosphere [that is, the threshold at which the atmosphere becomes electrically conductive]. The higher you go, the easier it is to get gas discharges. The threshold can be as low as 100 V/m, even just 10 V/m, if you go high enough.”

Such electrical fields are generated by thunderstorms. The electrical field of a thunderstorm extends upwards as well as downwards, and certain thunderstorms can project upwards electrical fields that are strong enough to trigger a gas discharge – a Sprite. What is striking is that thunderstorms, like nearly all weather, occur in the lowest layer of the atmosphere, the troposphere, which extends from ground level to an altitude of about 20 km. Thus, Sprites occur tens of kilometres above the storms that create them.

Globally, some 40 to 50 lightning strikes happen every second, 70% of them over land and in the tropics. “For TLEs to occur, you need large, convective thunderstorms,” he explains. “Most of the lightning will occur inside the cloud; a lot will be from the bottom of the cloud to the ground. Both are useless for creating TLEs.” Less than 5% of lightning is from the top of the cloud and this triggers Sprites.

But Sprites do not propagate upwards: they start high and propagate downwards. Large-scale TLEs, which can be 10 km across, can start as high as 90 km and descend to about 50 km at speeds of around 100 000 km/s. Streamers They are very bright and are characterised by vertical plasma filaments, or streamers. Yet they are rarely reported, probably because they are so brief and people tend to seek shelter during storms. “Sprites, to date, have never been observed in South Africa,” he points out. “But they clearly happen!”

Observation will require equipment. “We know in general terms what equipment we need, but a Sprite programme is a process of discovery and only as the programme develops will we establish exactly what questions we want to ask and only then determine exactly what equipment we will need.” Meanwhile, an optical TLE observatory is being set up at the South African Astronomical Observatory site, at Sutherland, in the Karoo. It will be able to see Sprites hundreds of kilometres away, above distant thunderstorms, and is scheduled to be ready for December 2015 and the 2015/16 ‘Sprite season’. A second observatory will also be established.

Another valuable capability is over-the-horizon radar. Sansa Space Science already has one of these – called Super DARN – at the South African National Antarctic Expedition’s Sanae IV base, in Antarctica. This will also be used for TLE research. “They’re expensive, but we’d like another one in South Africa, because we would have better access to it and could also use it to study the South Atlantic Magnetic Anomaly.” It is hoped that cooperation with researchers in the US will allow the installation of instruments in South Africa and Namibia, and on Marion Island, Gough Island and Antarctica.

A concern is personnel. “We have a respectable number of people but we don’t have as many PhDs as we’d like,” notes Kosch. “Finding and retaining qualified people is difficult. With our equipment, we also need engineers and technicians. We have enough people to get by, but it would be nice to have more.” Sansa Space Science does have active bursary, training and capacity building programmes.