Between Earth and the Sun, space is not truly empty but is filled with a soup of charged particles called plasma. This plasma comes from the Sun, pumped out in a steady stream called the solar wind and sporadically blasted away in explosive solar eruptions. When this solar plasma strikes Earth, it causes the magnetic field lines and plasma around Earth to vibrate like the plucked strings of a harp, producing ultralow-frequency waves.

In 2007, NASA launched five satellites to fly through Earth’s magnetic “harp” – its magnetosphere – as part of the THEMIS mission (Time History of Events and Macroscale Interactions during Substorms). Since then, THEMIS has been gathering a bounty of information about plasma waves across Earth’s magnetosphere. 

The frequencies of the waves THEMIS measures are too low for our ears to hear, however. So a NASA citizen science project called HARP – or Heliophysics Audified: Resonances in Plasmas – has sped up the waves recorded by THEMIS to convert them to sound waves.

In this clip from March 17-20, 2012, whistle sounds are repeatedly heard over ambient background whooshes and crunches. The whistles change in pitch as a THEMIS spacecraft moves toward and away from Earth on three orbits, with a maximum altitude of about 50,000 miles. These sounds are related to Earth’s magnetic harp, which has longer magnetic strings and lower pitches farther away from Earth. These three orbits happened near the end of a geomagnetic storm when the solar wind was blowing past Earth with speeds up to about 1,300,000 miles per hour, which is fast even by solar wind standards! There’s often more harp activity during fast solar wind conditions and geomagnetic storms. At the same time, many other types of magnetic vibrations occur during geomagnetic storms, most of which sound like background whooshes and crunches.

Multiple Passes through the Magnetospheric Harp on March 17-20, 2012

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