What on Earth was that sound? Jungle birds? Monkeys? Sirens?
Combined with the movie of swirling lights you might even have guessed some kind of spacecraft launch or radio tower.
In fact, both the sound and image are of completely natural origin. The movie shows what’s known as a pulsating aurora – a very common, but hard to see, weak aurora that blinks on and off up to 12 times per minute in the night sky.
The sound is of something no one knew was connected to these auroras until recently: a special kind of electromagnetic wave some 40,000 km higher in Earth’s magnetosphere called a chorus wave, since it sounds like birds chirping when played through a speaker.
How the pulsating auroras form has long been a mystery. Stable auroras form when electrons and ionized particles from the solar wind travel down magnetic field lines towards Earth. These collide with nitrogen and oxygen particles in the ionosphere, some100 km above Earth, and the collisions send out blue, green, and red photons to create the colorful light shows of the aurora.
But no one knew what could cause an aurora to turn into a strobe light until scientists at UCLA looked at data from NASA’s THEMIS spacecraft. They discovered that the auroras pulsed in sync to chorus waves far above Earth’s atmosphere. The chorus waves apparently drive the light-inducing solar wind particles down to Earth following its own unique beat.
Linking the two phenomena does more than explain the origins of the pulsating aurora. Using the electromagnetic waves and the aurora to define end points of magnetic field lines gives scientists a new tool to physically map Earth’s constantly changing magnetic field. Knowing the way that the magnetic field moves, in turn, is crucial for understanding space weather and phenomena that can threaten Earth-observing satellites.
Top Image: Pulsating aurora image taken on Oct 30, 2008 in Laukvik, Lofoten Islands, Norway. Courtesy of Jan Koeman. Middle Image: A snapshot of the pulsating aurora taken by a ground-based camera. The black square in the middle is the THEMIS spacecraft. Bottom Image: Schematic diagram showing aurora over North America and spacecraft in space (magenta) embedded in the energetic plasma source (blue cloud). These two regions are connected by the Earth’s magnetic field line. Energetic plasma interacts with waves (red) and precipitate into the upper atmosphere (blue arrows) and generate aurora. The geometry of the plasma cloud determines the aurora shape. Courtesy of Toshi Nishimura