Star-gazing sprite spotters aren’t away with the fairies
UCT team is the first to photograph these celestial bursts of electricity from the ground
Ground-based watchers have photographed sprites in the sky above SA for the first time.
But if the image in your mind is of the air spirit Ariel, in Shakespeare’s The Tempest, or similar fairy and elf-like creatures in the pantheon of children’s literature and games, you’re going to be disappointed.
The sprites in question are actually electrical discharges between 50km and 85km above Earth’s surface, each packing about 278 kilowatt hours of electricity, or a gigajoule.
Sprites, discovered in 1989, are the visual evidence above thunderstorms of discharges triggered by large lightning strikes, almost like a puff of smoke left behind as lightning arcs towards Earth.
They have been successfully studied from the International Space Station, but now a team spearheaded by the SpaceLab – part of the electrical engineering department at the University of Cape Town – has reported 100 sightings from the ground.
The scientists, led by Stanislaus Nnadih, set up a low-light camera at the Southern Africa Astronomical Observatory at Sutherland in the Northern Cape, during the peak seasonal lightning period between December 2015 and February 2016.
On two nights out of the 22 on which they conducted observations, they captured 54 video frames containing about 100 sprites in total, Nnadih reports in the September/October edition of the South African Journal of Science.
The observation of sprites is more than academic. Other studies have shown that they can affect the chemistry of the mesosphere – the so-called middle atmosphere above the stratosphere and below the thermosphere – by altering the composition of the oxides of nitrogen and hydrogen.
“These chemical changes could have an impact on the heating or cooling of this region or interfere with long-distance communication through the lower ionosphere,” Nnadih says in his paper.
“It is important to determine the occurrence rate and geographical distribution of sprites and the energy associated with these events in order to verify these assertions.”
Globally, according to Nnadih, sprites are estimated to occur between one and three times a minute. The examples Nnadih’s team observed were generally carrot- and column-shaped, like those seen elsewhere.
“We plan to observe sprites in future using multiple cameras with filters to extract spectral information and estimate the characteristic energy of the electrons within a sprite,” said Nnadih, whose collaborators were Mike Kosch, Peter Martinez and Jozsef Bor.