September 24, 2021

I, Science

The science magazine of Imperial College

Will the sky above Boston give a little whistle?

Take yourself into the middle of nowhere and hold a radio receiver to the sky.  Above the snapcracklepop of static, you might just be lucky enough to catch a whistler.

Sounding like weaponry from an early sci-fi film, whistlers are the remnants of lightning strikes that have travelled tens of thousands of miles through space from the other side of the Earth.  Scientists monitor whistlers from other planets to understand their atmospheres and gravitational fields, but recent research from Antarctica is challenging the theories on these audible oddities.

You might be wondering how sound can travel through space.  Whistlers aren’t actually sound waves – they’re pulses of very low frequency (VLF) electromagnetic energy and, like light and radio waves, can pass through a vacuum.  With the right equipment whistlers can be caught and converted into sound – click here to hear some.

All sorts of objects in space give off VLF energy.  When Voyager flew to Jupiter in 1990, it picked up the vibrations released when solar winds interact with the planet’s swirling gases.  Once converted, Jupiter’s song is hauntingly beautiful – see this clip.

Back on Earth, lightning strikes send out intense pulses of  VLF energy.  Some of this energy – the whistler – flies out of the atmosphere at the speed of light.  It hurtles through space and, if the conditions are right, will follow the path of Earth’s magnetic field – re-entering the atmosphere at the same longitude as the lightning strike, but in the opposite hemisphere.

Whistlers heard at the British Antarctic Survey’s research station, Rothera, originate 12,270 km away – near Boston on America’s east coast.  Using an Automated Whistler Detection system, scientists at Rothera caught up to 187,000 whistlers a day, then compared them with Boston storm data.

They found that not every lightning flash produced a whistler.  Despite lightning striking day and night near Boston, whistler intensity was highest when it was daylight at both Boston and Rothera – the exact reverse of previous observations from Hungary.  Also, strikes that occurred over water were much more likely to produce whistlers than those over land.

The findings, published in the Journal of Geophysical Research, have left the international team of researchers at a complete loss for explanations.