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The Sun's corona is hotter than its surface, but where do scientists think such immense heat comes from?

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[♪ INTRO] If you’ve seen any pictures from the recent solar eclipse, you’ve probably noticed a big, wispy halo shining from behind the moon.

That’s the corona, the outermost layer of the Sun’s atmosphere, and it’s pretty sweet. It’s also home to one of the biggest mysteries in astronomy.

Even though the corona is the top layer of the Sun’s atmosphere, it’s hundreds of times hotter than the surface. A whopping 1 to 4 million degrees Celsius, compared to about 5500 degrees. Astronomers have done enough research to give them some ideas why, but to really figure it out, they’re getting ready to send a probe into the fires of the Sun itself.

The corona is so thin that the rest of the Sun’s light usually outshines it, which is why you can only see it clearly from Earth during an eclipse. It’s also enormous, extending millions of kilometers above the Sun’s surface. Like the rest of the Sun, the corona is made of plasma, which is basically a charged gas that forms when atoms become so hot that their electrons break free from their orbits.

For the corona to be as hot as it is, astronomers think there has to be some totally different kind of heating mechanism happening there, because usually it gets colder the farther you get from a heat source. One popular idea is that it involves wave heating, where strong waves of energy are created by turbulence on the Sun’s surface and then travel out to the corona. These waves move kind of like ocean waves, except that instead of water, there are ions and electrons in the plasma moving back and forth.

And whenever those charged particles move, they also create a magnetic field. The field doesn’t move exactly the same way as the particles, but it can change strength and direction as the wave travels, and the magnetism also gives the wave some extra energy. According to this hypothesis, as the waves move away from the Sun, all that energy eventually turns into heat that makes the corona all nice and cozy.

But that still might not be enough to make the corona as hot as it is. In 2011, scientists studying the Sun’s transition zone, the region of the Sun’s atmosphere just below the corona, found that wave heating could be enough to get the corona to the bottom of its temperature range -- or about a million degrees Celsius. But when there’s a lot of solar activity, the corona can get up to four times hotter than that, so something has to be causing the extra boost.

To help explain it, astronomers have another hypothesis called magnetic reconnection, which happens when pockets of magnetism in the Sun’s plasma connect and release a ton of energy. These pockets are called magnetic domains, and they’re regions where the magnetic fields line up so they’re pointing in the same direction. Here on Earth, this happens all the time when rocks or metals become magnetized, but because those domains are in solid materials, they don’t interact in the same way.

On the Sun, because there’s all that plasma swirling around, magnetic domains can come into contact with each other -- and when they do, things get kinda weird. Usually, when magnetic fields combine, we can predict the outcome. But with magnetic reconnection, there’s all kinds of weird bending and stretching to throw off our calculations.

That’s because there are a lot of other factors involved that we usually don’t have to deal with on Earth, like the fact that the Sun is rotating and plasma is moving around all over the place, which creates a constantly changing system we don’t totally understand yet. One thing we do know is that, when two domains collide that were lined up in opposite directions, they annihilate each other and release a huge amount of heat energy, and that might be enough to boost the corona to those higher temperatures. We’re not positive these two ideas explain everything, or even if that’s really what’s happening in the corona.

Instead, there might be a lot of smaller mechanisms we don’t know about. So to gather more data, NASA is about to fly a probe into the Sun! It’s called the Parker Solar Probe, and it’ll have a bunch of instruments to study things like coronal heating, magnetism, and plasma dynamics.

To get that data, the probe will fly within 6 million kilometers of the sun, which is closer than we’ve ever been before! The probe’s SWEAP instrument, which stands for Solar Wind Electrons Alphas and Protons, will gather information on what’s going on in the corona, which, along with with tons of observations from the ground, will hopefully tell us why it’s so hot! The Parker Solar Probe isn’t scheduled to launch until Summer 2018, so we’ll just have to hang tight until then -- but maybe the next time a solar eclipse comes around, the corona won’t be such a mystery.

Thanks for watching this episode of SciShow Space! If you want to learn more about the Parker Solar Probe, you can watch our earlier episode where we give you all the details. [♪ OUTRO]