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[♪ INTRO]

For better or worse, World War I saw tons of technological advancements.

And one of the especially effective ones was a method of early, remote wiretapping.

During the war, militaries ran telephone lines from their command centers to the battlefield,

and the opposition did their best to intercept those signals.

To do this, they stuck two prongs into the ground hundreds of meters away,

then ran attached cables to a device called an amplifier.

The prongs could pick up signals from the phone lines, and from a relatively safe distance,

soldiers would be able to listen in on sensitive conversations.

The goal here was pretty obvious: that sweet, sweet intel.

But occasionally, those soldiers also picked up signals that were a little more alien-sounding.

Things like twangs and hisses and [whistle sounds].

These were sounds that telegraph operators had heard for years,

and no one was quite sure what they were.

But once researchers started studying them more closely,

they were able to turn those noises into one of astronomy's most important tools.

Although soldiers continued to hear these sounds until the end of the war,

we didn't understand what caused them until 1953.

That year, researchers at Cambridge conducted a study tracking when these noises occurred,

and they realized that those sounds, which they called whistlers, are caused by lightning.

Or, more specifically, they're caused by the stuff that makes up lightning: plasmas.

These are extremely hot, electrically-charged gases.

And because they're charged, they can interact with magnetic fields and form waves.

When lightning strikes, these plasma waves travel along Earth's magnetic field lines,

changing speed and pitch as they go.

They vibrate at really low frequencies, and can be heard as radio waves

by field amplifiers, telegraph lines, or any magnetic device.

With this discovery, scientists could finally explain

where the weird telegraph and wiretapping noises came from!

But as they kept researching, they also realized that

whistlers weren't the only plasma phenomenon like this.

Instead, plasma could make a bunch of sounds depending on how it behaved.

And together, these noises were named whistler-mode instabilities or whistler-mode waves.

Today, we know they aren't just caused by lightning.

Auroras can make whistler-mode waves, too, which sound equally bizarre.

But regardless of where they come from, these noises are more than just neat:

They're also really useful tools.

Since each sound is produced by a distinct phenomenon,

we can use whistler-mode waves to understand what's happening in Earth's atmosphere,

on the Sun, and around other planets; really anywhere we find charged gases.

And there's a lot we've learned from them!

Take what we discovered with the Van Allen Probes mission,

which wrapped up operations in July 2019.

It used whistler-mode waves to study the Van Allen belts,

two big lobes of plasma trapped in Earth's magnetic field.

And it discovered that the shape of the belts changes all the time in response to conditions on the Sun.

The Van Allen belts are a major point where particles from the Sun interact with Earth's magnetic field.

And since those interactions can affect things like electronics, and satellites,

and astronauts on the Space Station, they're pretty important to understand!

Whistler-mode waves have also helped us understand the gas giants in our neighborhood.

For example, the 1970s Voyager probes heard whistlers while flying by Jupiter and Neptune.

And in 2006, the Cassini spacecraft heard whistlers on Saturn.

That allowed researchers to infer that those planets have lightning storms,

which opened up new areas of research.

The Voyagers also gathered data on other whistler-mode waves,

and that gave us insight into how Jupiter and Saturn interact with their moons.

For example, Jupiter's moon Io emits tons of gases,

which form an electrically-charged disk that orbits the planet.

This disk is pretty important to know about when you're planning a mission to Jupiter,

at least, if you want to keep your spacecraft functional.

And whistler-mode waves have helped us figure out how thick it is.

Meanwhile, on Saturn, Voyager 1 heard specific types of

whistler-mode instabilities called hiss and chorus waves.

And analyzing them produced evidence that

Saturn was pulling plasma from the atmosphere of its moon Titan.

It was siphoning the gases up along its magnetic field lines.

And although we're not quite sure what the implications of that are,

it's a really cool observation, because the Earth and our Moon are totally different.

Since these discoveries, probes have continued to monitor whistler-mode waves

to better understand these planets' moons, magnetic fields, and atmospheres.

And since gas giants have a lot of atmosphere to study, we're really just scratching the surface.

So early telegraph operators, and WWI spies were listening to plasma,

and from that we could learn about things like gas giants! Science is so cool!

Thanks for learning about this with me,

and thanks to Ridge for sponsoring this episode of SciShow Space!

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[♪ OUTRO]