Universities in the US have long wrangled over who owns the world's largest drum. Unsubstantiated claims to the title included the "Purdue Big Bass Drum" and "Big Bertha", which was interestingly named after the German World War I cannon and ended up becoming radioactive during the Manhattan Project
Unfortunately for the Americans, however, the Guinness Book of World Records says a traditional Korean "CheonGo" drum holds the true title. This is over 5.5 meters in diameter, some six feet tall (18 by 20 feet) and weighs over seven tonnes.
But my latest scientific results, just published in Nature Communications have fired all of the contenders away. That's because the world's largest drum is actually several tens of times bigger than our planet ̵
You may think this is nonsense. But the magnetic field (magnetosphere) that surrounds Earth, protecting us by diverting the solar wind around the planet, is a gigantic and complicated musical instrument
We have known for 50 years or so that poor magnetic types of sound waves can bounce around and resonate within this environment, forming well defined notes in exactly the same way wind and stringed instruments do
But these notes form at frequencies of thousands of times lower than we can hear with our ears. And this drum-like instrument within our magnetosphere has long eluded us – until now.
Massive magnetic membranes
The main feature of a drum is its surface – technically referred to as a membrane (drums are also known as membranophones). When you hit this surface, the ripples can spread across it and get reflected back to the fixed edges.
The original and reflected waves can interfere by reinforcing or canceling each other out. This leads to the "standing wave patterns," in which specific points appear to be standing still while others vibrate back and forth
The specific patterns and their associated frequencies are determined entirely by the shape of the drum surface. In fact, the question "Can one hear the shape of a drum?" has intrigued mathematicians from the 1960s to today.
The outer boundary of Earth's magnetosphere, known as magnetopause, behaves very much like an elastic membrane. It grows or shrinks depending on the varying strength of the solar wind, and these changes often trigger ripples or surface waves to spread out across the boundary
While scientists have often focused on how these waves travel down the sides of the magnetosphere, they would also travel to the magnetic poles
Physicists often take complicated problems and simplify them considerably to gain insight. This approach helped theorists 45 years ago first demonstrate that these surface waves might indeed get reflected back, making the magnetosphere vibrate just like a drum.
But it was not clear whether removing some of the simplifications in theory could stop the drum from being possible
It also turned out to be very difficult to find compelling observational evidence for this theory from satellite data. In space physics, unlike say astronomy, we are usually dealing with the totally invisible
. We can not just take a picture of what's going on everywhere, we have to send satellites out and measure it.
The conundrum is often if the satellites are in the right place at the right time to find what you are looking for
Over the past we have been developing the theory of this magnetic drum to give us testable signatures to search for our data
We were able to come up with some strict criteria that we thought could provide evidence for these oscillations.
Thankfully, NASA's THEMIS mission did not give us four but five satellites to play with. (19659003)
The event in question was a jet of high speed particles (plasma) impulsively slamming into the magnetopause. Once we had that, everything fell into place almost perfectly. We have even recreated what the drum actually sounds like
This research really goes to show how tricky science can be in reality. Something that sounds relatively straightforward has taken us 45 years to prove
And this journey is far from over, there's plenty more work to do to find out how often these drum-like vibrations occur (both here at Earth and potentially at other planets, too) and what their consequences on our space environment are.
This will ultimately help us unravel what kind of rhythm the magnetosphere produces over time. As a former DJ, I can not wait – I love a good beat.
Martin Archer, Space Plasma Physicist, Queen Mary University of London
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