As powerful impulses, the Earth's magnetopause hits the abrupt boundary between the planet's magnetosphere and the surrounding plasma, ripples move along the surface, which is then reflected back as they approach the magnetic poles. The interference of the original and reflected waves leads to a standing wave pattern where specific points appear to stand still while others vibrate back and forth. A drum resonates like this when it is switched in exactly the same way. A new study published in the journal Nature Communications describes the first time this effect has been observed after it was theoretically proposed 45 years ago.
Within Earth's magnetosphere, planetary scientists have long listened to space sounds created by various electromagnetic waves.
This veritable orchestra of waves can be heard as sound when properly processed, and they even display similar behavior to certain musical instruments.
So-called magnetosonic waves pulse through plasma in the same way as the sound jumps through wind instruments.
Another type of wave, known as an Alfvén wave, resonates along magnetic field lines, as well as string instruments' vibrating strings.
While both types of waves can travel anywhere in space, the newly discovered waves are a form of surface waves – waves that require a kind of boundary to travel along.
In this case, the magnetopause functioned as the limit. When a plasma beam – drumstick – hits the magnetopause, the surface waves form a standing wave pattern – where the ends appear to stand still while other points vibrate back and forth – like a drum head.
The fixed points of the wave, which are the rim or edge of the rim, are near the earth's magnetic poles; the waves vibrate the surface of the magnetopause between them.
Although the wave itself remains on the surface, the vibrations ultimately work in the magnetosphere and trigger other types of waves.
"The waves probably penetrate far into the inner magnetosphere, which causes ultra-frequency waves that affect things such as beams, the aurora and even the ionosphere, "says the author Dr. Martin Archer, Space Physicist at Queen Mary University, UK.
Dr. Archer and colleagues used observations from five NASA's time history of events and macro-scale interactions under satellites (THEMIS), as they were ideally positioned as a strong isolated plasma beam embedded in the magnetopause.
The probes were able to detect the oscillations of the border and the resulting sounds within the Earth's magnetic shield, which agreed with the theory, and allowed the scientists to exclude all other possible explanations.
"Given the lack of evidence during the 45 years that they were proposed, there had been speculation that these drum-like vibrations might not even occur," Dr. Archer.
"Now we see that the waves on the surface of the magnetopause reflect between two points near the magnetic bars – much like a drum."  _____
MO Archer et al. . 2019. Direct observations of a surface self-mode of dayide magnetopause. Nature Communications 10, article number: 615; doi: 10.1038 / s41467-018-08134-5