Earth’s magnetic field ‘sings’ during energetic solar storms

News release from the American Geophysical Union

Data from ESA’s Cluster mission has provided a recording of the eerie ‘song’ that Earth sings when it is hit by a solar storm. The new findings were published today in the AGU journal Geophysical Research Letters.

The song comes from waves that are generated in the Earth’s magnetic field by the collision of the storm. The storm itself is the eruption of electrically charged particles from the Sun’s atmosphere.

A team led by Lucile Turc, a former ESA research fellow who is now based at the University of Helsinki, Finland, made the discovery after analysing data from the Cluster Science Archive. The archive provides access to all data obtained during Cluster’s ongoing mission over almost two decades.

Cluster consists of four spacecraft that orbit Earth in formation, investigating our planet’s magnetic environment and its interaction with the solar wind – a constant flow of particles released by the Sun into the Solar System.

As part of their orbits, the Cluster spacecraft repeatedly fly through the foreshock, which is the first region that particles encounter when a solar storm hits our planet. The team found that in the early part of the mission, from 2001 to 2005, the spacecraft flew through six such collisions, recording the waves that were generated.

The new analysis shows that, during the collision, the foreshock is driven to release magnetic waves that are much more complex than first thought.

“Our study reveals that solar storms profoundly modify the foreshock region,” said Turc, who is the lead author of the new study.

Earth magnetic field surrounds the planet in a protective bubble that is distorted by the solar wind. In this image, Earth can be seen as the small dot at left with the magnetic “bow shock” extending around it in a large arc. The swirls to the right are part of the foreshock region where charged particles from the sun encounter particles reflected from the bow shock. The image was generated by the Vlasiator computer simulation developed at the University of Helsinki. Image: Vlasiator team, University of Helsinki

When the frequencies of these magnetic waves are transformed into audible signals, they give rise to an uncanny song that is more reminiscent of the sound effects from a science fiction movie than from a natural phenomenon.

In quiet times, when no solar storm is striking the Earth, the song is lower in pitch and less complex, with one single frequency dominating the oscillation. When a solar storm hits, the frequency of the wave is roughly doubled, with the precise frequency of the resulting waves being dependent on the strength of the magnetic field in the storm.

“It’s like the storm is changing the tuning of the foreshock,” Turc said.

And it doesn’t stop there. Not only does the frequency of the wave change but it also becomes much more complicated than the single frequency present in quiet times. Once the storm hits the foreshock, the wave breaks into a complex network of different, higher frequencies.

Computer simulations of the foreshock, performed using a model called Vlasiator, which is being developed at the University of Helsinki, demonstrate the intricate wave pattern that appears during solar storms.