The ancients believed that the Earth was surrounded by celestial spheres, which produced divine music when they moved. We lived, so to speak, in a huge musical instrument. This may sound silly but modern science has proved them right to a certain extent. Satellites recording sound waves resonating with the Earth’s magnetosphere – the magnetic bubble that protects us from space radiation – show that we are indeed living inside a massive, magnetic musical instrument.
There are two key things which control how the notes of musical instruments sound: the size and shape of the instrument and the speed of sound throughout it. These determine the pitch of the notes and the timbre, the character or quality of the sound, via the standing waves or resonances that are excited within the instrument as sound waves bounce around it. It’s elegantly simple, yet explains the rich variety of musical sounds that are possible.
The same is true within Earth’s protective magnetosphere, which is carved out by the solar wind. There are always a few sound waves – oscillations in pressure which travel through the medium that they’re in – travelling around in space.
Well, they aren’t exactly the same type of sound waves that we get on Earth. Space is filled with plasma rather than normal gas: a different state of matter made of charged particles which can generate and be affected by electric and magnetic fields. These kinds of interactions can give rise to the plasma-equivalent of sound waves: magnetosonic waves. These too are pressure waves, but with some added magnetism.
Such “magnetosonic” waves can bounce around within the magnetosphere and often set up “resonances”, where the frequency is just right so that these waves grow and grow in energy rather than fizzling out quickly.
Most musical instruments support just one type of resonance – be that the vibrations of a string such as in a guitar, surface waves on a membrane like on a drum, or sound within a cavity like in a flute. However, the magnetosphere has analogues of all three of these types of resonance going on at once.