What does the Sun sound like? It’s a simple enough question, and one that comes with a simple enough
answer: The Sun is silent, of course! Because sound doesn’t travel in a vacuum, to us the Sun
sounds like nothing at all. But let’s do away with simple answers. What if sound actually
could propagate through space? Without a giant cosmic muffler between us and our closest star,
what would we hear? Would the Sun sing to us, an enchanting melody in our sky? Or would its noise
be such a cataclysmic cacophony, it would instantly be the last thing we ever heard?
Scientists actually have the answer. Through spacecraft like the Parker Probe, which
currently is making skimming orbits around the Sun, they can detect the kinds of pressure waves
and particles emitted by the Sun, and can convert those waves into the sounds we might hear if space
was capable of such propagation. And interestingly enough, not all of the Sun’s sounds are the same.
I’m Alex McColgan, and you’re watching Astrum. Join with me today as we listen to several
different sounds made by the Sun, recreated through clever science. Because it turns out
that these sounds may allow scientists to solve two of the Sun’s greatest mysteries.
In our imaginary scenario where space has suddenly gained the ability to propagate sound,
we can now start listening to the Sun all the way from the Earth. Although the Earth is 151.9
million km away from the Sun on average, it’s perhaps unsurprising to learn that the Sun is
very loud – easily audible from where we are. It would sound something like this:
Just a reminder, there is no sound in space. This is a sound reconstruction,
turning amplitudes and frequencies into sound waves, creating clips of what the
waves might sound like to the human ear if an atmosphere were present.
This soothing buzz has an almost electrical feel to it. It reminds me of generators,
or perhaps even something vaguely radioactive – which is appropriate, given that this is the
Sun we’re talking about. The nuclear fusion going on inside the Sun is equal to 3.85x1026
W every second – about a million times more than what humanity produces in an entire year.
All of this power being created has an unsurprising knock-on effect on the volume
of sound the Sun would produce. To get a sense of what this would be like, take the device that
you’re watching this on, and connect it up to the speakers they use at your next rock concert. Then
play this noise at full blast. I’m sure they won’t mind. At 110 dB, this volume level is
survivable for short periods of time, but likely would end up with the entire population of the
planet going deaf if they weren’t wearing earplugs as it plays in our sky non-stop.
In fact, we can consider it extremely fortunate that we don’t actually have to deal with this.
Species would have probably evolved with no ears as hearing would be pointless. If we get
closer to the surface of the Sun, that sound level grows exponentially, with some experts
rating it at 270 dB. Remember, decibels are a logarithmic scale, so every 10 dB higher it goes,
you have to multiply the sound level by 10. To get a sense of how loud this would be,
270 dB is louder than our atmosphere itself can physically convey. There is a cap on sound at 190
dB in Earth’s atmosphere (different mediums have different volume caps) due to some complicated
quirks of wave amplitude and energy levels, which we won’t go into here. The volume of the Sun would
be 100,000,000 times higher than this cap. The Sun is loud. Which is a shame, as some of the
Sun’s sounds can only be differentiated when we enter that ear-splitting zone.
By the time the noise of the Sun has reached us, many of the Sun’s sounds have blurred into one,
meaning we miss out on some of their fascinating intricacies. To gain a better insight into the
Sun, we would need to get closer, entering into that blisteringly loud zone of noise.
So, let’s ignore the volume of the Sun for now, and instead let’s focus on its sound patterns.
If we were to enter a spacecraft and head towards the Sun, we would soon
begin to hear a difference in the sound’s uniformity. Rather than a constant buzzing,
we would hear something like this: This is the sound of the solar wind.
The Sun emits a constant stream of charged particles that blasts out across the solar system,
and its existence represents one of the Sun’s greatest enigmas to date. You see, it’s tempting
to think of this stream from starting inside the Sun and blasting outward under the incredible
force of all that nuclear fusion. However, by the 1960’s an important discovery had been made
through observation. Although particles within the Sun’s atmosphere – or corona – were moving
around with an average speed of 145m/s, particles outside of the corona were travelling at 618 m/s.
This meant that particles that had enough speed to escape from the Sun’s surface were drifting into
space, and then something in the Sun’s corona was hyper-accelerating them from subsonic to
supersonic speeds, firing them off. What kind of process or mechanism is doing this remains a
mystery to this day. The corona contains a second enigma too. It is far too hot. Although you might
expect the centre of the Sun to be the warmest part of it, and the temperature to gradually go
down the further from the centre you go, this stops being true once you enter the corona.
The surface of the Sun has an average temperature of 5,600°C (9930°F), but within a few kilometres,
the corona suddenly sees the temperature jump up to as high as 2 million °C! Something in the Sun’s
corona is flicking solar wind into space, and something is making things very hot. Scientists do
not know why this is happening either, although it is thought to have something to do with magnetic
fields. It's here that we would meet the third sound of our tour of the Sun. As our spaceship
would draw closer, and we passed over the boundary between the corona and empty space – something
known in science as the Alfven surface – we would begin to hear the following: These
are known as Langmuir waves. They are formed by tiny oscillations of plasma particles vibrated
by light itself as they stream from the Sun through tiny funnels on the Sun’s surface;
a magnetic equivalent to the sound a kettle makes when it boils. We have entered one such
stream. Countless of those streams fire out from the Sun’s surface. The Sun’s
exterior is covered in islands of hot material rising from the star’s centre. Once there,
the material cools and sinks back into the star’s mass along the island’s edges, and something about
this process of rising and falling material, or something going on within the Sun itself, makes
these sinking zones stream out powerful magnetic field lines. Plasma is scooped up by the magnetic
force and is sent streaming out into space – the starting point of the solar wind. Scientists
don’t know what causes this process, but the sound of Langmuir waves is eerily beautiful.
As the winds head out, not all the waves created in this way travel at the same
speed. This has the effect of spreading out sounds as faster parts of the wave run ahead
while slower parts of the wave fall behind, creating what are known as dispersive waves:
Such waves are filled with “chirps” – little eddies of sound,
where fast waves coming from behind meet slow waves already travelling ahead.
Eventually, this evens out into the hum we heard at the beginning. There is one last sound that
might offer an insight into the entire mystery. One that has some intriguing implications.
Here, right within the Sun’s boiling corona, we can finally begin to detect whistler-mode waves.
Whistler waves are the distant echo of high energy events that ring through the plasma
of a celestial body’s magnetosphere. We get them on Earth. Whenever lightning cracks,
you get more than just thunder. A ripple travels through the planet’s magnetosphere
that can be picked up kilometres away, if you have the right detectors. To truly get a sense
of the scale of these waves, here is what they sound like on Earth: Just a few small little
cracks and whistles. Nothing too special. Now here’s what they sound like on the Sun:
To go from sparse, brief clicks to a cacophony of wailing wind indicates that the Sun’s
magnetosphere is not just experiencing the odd crack of lightning. Something is going
on constantly, all over, merging individual moments together into one incessant roar.
Scientists are still trying to figure out what that roaring is being caused by.
But at least one theory for the force behind this sound, which can account
for the dramatic acceleration and superheating of matter within the corona, is a dry-sounding
little event called magnetic reconnection. Magnetic reconnection sounds fairly innocuous,
and yet its innocent, technical-sounding name disguises a cataclysm. The Sun is a tensely
wound-up ball of shifting magnetic field lines, due to all that churning plasma within its heart.
I cannot stress enough how powerful these are. When magnetic field lines travel away
from the Sun, it is sometimes possible for them to merge with other field lines,
causing some of those iconic loops to form that you’ve likely seen in images of the Sun before:
However, at the moment such arcs are formed, all hell breaks loose. When two parallel lines touch,
creating an arc, there’s a massive realignment in the excess field lines’ polarity – everything
above the arc needs to find a new place to go to, to ensure magnetic equilibrium is maintained.
Cut from their tethers, the field lines further out from the sun form an arc of their own,
which then catapults off into space like the releasing of a gargantuan stretched rubber band.
This rapidly accelerating magnetic force is known as a nanoflare,
and it indirectly heats the corona above it to multi millions of degrees in seconds.
Scientists have only recently developed the capability to be able to even see nanoflares,
and aren’t entirely certain the nanoflares they’ve seen are indeed the hypothesised
events they were expecting, although they do seem to be accompanied by the expected super-heating.
This is an explanation that is still being worked out. Much of it is not yet understood. And still,
it’s not completely farfetched that such moments when the Sun gets so wound up it snaps explain
why its corona is as hot as it is, and why it is flinging solar winds out across the solar system
with quite the force we witness. Although, to go from a single crack into the roaring wail that we
hear, and to account for the constant solar wind travelling outwards in all directions from the
Sun, nano-flares – or whatever the force behind this phenomenon is – must be extremely widespread
throughout the entire corona. The Sun continues to hold many mysteries. The Parker Solar Probe,
which collected many of these sounds, continues its journey around the Sun; gathering valuable
data with every pass. The Parker Probe is only 4 years into its seven-year mission, so hopefully,
it will collect plenty more information for scientists to pour over. Perhaps they will
have more sounds for us, each one a clue to the ancient riddle of how the universe works. Because
ironically, just like listening to the ticking of a clock to try and figure out how the clockwork
functions, the secrets to the Sun might one day be solved by what we hear rather than by what we see.
At Astrum, we have some big news. Our scientists have pooled their resources, and have spent
countless hours perfecting the ultimate way to store and consume hot beverages in space (provided
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