Magnetosphere: Definition & Facts

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  • 0:02 The Magnetosphere
  • 0:32 Van Allen Belts
  • 1:36 Magnetopause, -tail & Bow Sock
  • 3:00 The Aurorae
  • 5:08 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
In this lesson, you'll learn about how the northern lights are formed thanks to the magnetosphere and solar wind. Of course, you'll learn a lot more, including what the Van Allen belts, magnetopause, and magnetotail are.


I think Earth has a magnetic personality. It's beautiful and warm. I even hear this personality is so magnetic that people who see Earth from afar - astronauts - begin to love and nurture Earth even more as they see how beautiful and fragile it is. But enough of the saccharine stuff. Earth really does have some magnetic properties in the astronomical sense. It even has a magnetic tail!

This lesson is all about the outermost part of the Earth's or any other planet's atmosphere, the magnetosphere.

The Van Allen Belts

The magnetosphere is filled with just a tiny bit of plasma, but it's dominated by the Earth's magnetic field. Plasma is ionized gas made up of positively-charged ions and negatively-charged electrons. Some of these ionized gases come from Earth's upper atmosphere and others from solar wind particles. The solar wind is a stream of plasma emitted by the sun.

The magnetosphere loves to trap energetic charged particles in regions called Van Allen belts. They are two doughnut-shaped areas centered on the equator where, like a magnet traps metal, the Earth's magnetic field traps charged particles. The outer Van Allen belt, lying about 15,000 kilometers above the surface of the Earth, contains electrons. The inner belt, lying approximately 2,500 kilometers above the Earth's surface, contains protons.

While the diagram on the screen makes it seem like these belts are separate, in reality, they gradually merge with one another. This means the inner belt's energetic protons decrease in number as it merges with the outer belt.

Diagram of Earth

Magnetopause, Magnetotail, and Bow Shock

Now, while the magnetosphere has some plasma, there's another kind of plasma I mentioned before, that of the solar wind, which interacts with the magnetosphere. You see, the solar wind rushes from the sun and towards our planet like a stream of water may rush towards an object, like a tree during a flood. As this water rushes towards the tree, you can clearly tell how this sort of bowing effect takes place in front of it.

Well, as the solar wind comes close to the magnetosphere, it begins to slow down, like water slows down in the space in front of the tree. The area of space where the solar wind slows down as it approaches the Earth's magnetosphere is called the bow shock.

Internal to the bow shock is the magnetopause, the outer boundary of the magnetosphere. It's a roughly 100 to 200 kilometer thick boundary between the magnetosphere and solar wind.

The solar wind will flow around the Earth's magnetosphere on the sunward side, the side the solar wind is coming from. This flow will actually compress the magnetosphere on the sunward side.

However, on the night side of Earth, a magnetotail will form. This is part of the magnetosphere located behind a planet that is stretched by the solar wind. It sort of looks like that tail that extends behind the tree, opposite the side from where the water is crashing into it.

Knowing all of this, another way to think of the magnetosphere is as a teardrop-shaped cavity or void in the solar wind, as the solar wind's particles are inconveniently forced to flow around the Earth.

The Aurorae

Energy that's carried by this solar wind gets transferred to the charged particles that make up the magnetosphere itself. Like eddies and currents form right behind a tree as water flows past it, it is mainly in the magnetotail where the solar wind creates currents of electrons, that this energy transfer occurs. These currents of high energy electrons make their way into Earth's upper atmosphere close to the Earth's magnetic poles.

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