Hunting mystery giant lightning from space
before TLES were caught on camera, people who spotted them had been reporting ‘rocket lightning’ or ‘upward lightning’.
Now in need of names, the phenomena were christened sprites and elves because of their fleeting, mysterious nature.
Yet despite their diminutive monikers, these features are anything but small, and extend tens of kilometers into the atmosphere.
Sprites, elves and jets
So, what’s causing these events? Neubert said, “They are slightly different to lightning. It’s a pulse of the electric field that travels up. For the sprite — when the atmosphere gets thin, the field can get a discharge.”
Sprites appear milliseconds after a powerful cloud-to-ground lightning strike.
Elves, on the other hand, are caused by the electromagnetic pulse the strike produces.
A brief, aurora-like expanding halo in the ionosphere, they occur too quickly to be spotted by the human eye and last less than a millisecond.
Dr. Martin Fullekrug from the University of Bath, Somerset, the UK, said, “Although they are more elusive, elves are incredibly well understood.”
They are the most common TLE, thought to occur twice as often as sprites.
Blue jets — upward electrical discharges from cloud tops — are the least well known.
Fullekrug added, “The jets are not very well studied because they’re very faint. They’re mainly blue. Also they’re not necessarily associated with lightning. They pop up now and again and they’re very mysterious.”
While elves are mainly spotted over warm ocean waters, sprites tend to occur over land.
North America, the Democratic Republic of the Congo and South Africa are all good places to see them. But it is possible to spot sprites elsewhere.
A normal summer thunderstorm in the UK is about 10km wide. Sprites appear above mesoscale convective systems — storm complexes about 10 times larger.
Fullekrug said, “In Britain we also have [these storms] from time to time.
“We’re conducting research on one that happened in May last year. It produced a wonderful sequence of sprites [over Cornwall].”
The sprites were spotted by meteor observers, who had cameras trained on the sky to follow the trails of shooting stars.
Stormchasing from space
ASIM’S main goals are to study the physics of TLES, and the characteristics of thunderstorms that produce them.
The payload includes two cameras, which can capture 12 frames per second, plus X-ray and gamma ray detectors.
This will allow the international team of researchers, for many of whom this is the culmination of decades of work, to determine where in the cloud sprites or jets originate.
With the aid of the European Space Agency, ASIM’S minimum mission length is two years.
During this period, it is expected to observe a minimum of one TLE per day, although it is thought that they occur at least every minute, somewhere in the world.
For Neubert, this will be an incredibly exciting time.
He said, “We don’t really know what’s inside lighting. It happens so fast and it’s so dangerous... it’s hard to get to the real inside physics.”
In the thin upper atmosphere, TLES are larger and easier to measure.
He added, “To me, they represent a window to the inside of lightning.”