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The Solar Corona & Solar Wind

The Solar Corona & Solar Wind
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  • 0:01 The Solar Corona &…
  • 1:03 Temperature and Density
  • 2:47 The Solar Wind
  • 3:39 Why the Corona Looks…
  • 4:31 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will define the solar corona and go over how it's involved with the formation of the solar wind and why it's hotter than the photosphere despite being the outermost layer of the sun's atmosphere.

The Solar Corona & Coronagraphs

They're on her royal highness's head, they're in your mouth, they're part of plants, the names of ships, currencies, cars, bands, awards, and more. They are crowns, and the sun's got one too. It's called a solar corona, technically, where 'corona' comes from the Greek for crown.

The solar corona is the outermost part of the sun's atmosphere, beginning right at the top of the chromosphere and moving out into space for millions of kilometers, and while it derives much of its light from the photosphere, it's only one-millionth as bright as the photosphere. That's no brighter than a full moon. Therefore, you can only see the solar corona with specialized equipment or during a total solar eclipse. This is because during the normal daytime, the photosphere's bright light outshines the corona, like it does the chromosphere, but telescopes specially designed to take photographs of the inner corona, called coronagraphs, can block out the light of the photosphere and help astronomers study its properties a bit better.

Temperature & Density

Like the chromosphere, the corona's temperature rises with altitude. Right where it's nearest the chromosphere, the temperature is around 500,000 Kelvin, but it rises to over 2 million Kelvin in the outer corona.

All of this doesn't make sense normally because heat should flow from hot areas to cool areas, not vice versa. We should therefore expect the sun to get continuously cooler and cooler towards its outer layers as a result. This is just like if you were to walk away from a heat lamp, it would get colder with increasing distance away from it.

Instead, the photosphere, the layer closest to the hotter inner parts of the sun, has a temperature of 5,800 Kelvin, and the outer corona has one of 2 million Kelvin! Astronomers aren't fully sure of why this odd temperature gradient occurs here, but it seems that magnetic fields extending from the photosphere and into the corona are at least partially responsible. This network of looped magnetic fields covering the solar surface is called a magnetic carpet.

These magnetic fields and the temperature variations are tied in with the density of the solar corona. The ionized coronal gas, like the chromosphere, has a very low density. Actually the coronal gas's density is 10 trillion times less dense than the air we breathe in its inner reaches, and even less dense than the best vacuums humans have ever created on Earth in the outer reaches.

Because of this low density, the molecules in the chromosphere and solar corona are easily moved about and agitated by the magnetic fields and waves. Just imagine a loop of rope whipping your arm, that's going to burn and make your arm feel all warm. So, just imagine these magnetic loops whipping the gas about and making it hotter.

The Solar Wind

Furthermore, looped magnetic fields, like a lasso on a cow, trap the ionized low density gas in the solar corona, but like a poorly tied lasso can unravel and let a cow run away, a magnetic field that unravels into space allows for gas to move away from the sun in the solar wind.

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