The Babcock Model of the Sun's Magnetic Cycle

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  • 0:02 The Magnetic-Dynamo Model
  • 1:07 Differential Rotation
  • 2:22 Opposite Magnetic Polarities
  • 4:45 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will cover a model called the Babcock model or Magnetic-Dynamo model, which helps to explain how many of the features of the solar and sunspot cycle occur.

The Magnetic-Dynamo Model

In 1960, astronomer Horace Babcock proposed what's known as the Babcock Model or magnetic-dynamo model. It is a model that explains many features of the 22-year solar cycle. The solar cycle is the 22-year magnetic cycle of the sun.

The ionized gas of our sun is a good conductor of electricity, like the wires in your homes. This means two things for our lesson.

First, ionized gas rotates around the sun and is stirred, like a hot soup, in the sun by the process of convection, resulting in energy stemming from these processes being converted into a magnetic field. Generation of a magnetic field by a rotating and convecting body of electrically conducting matter, ionized gas in this case, is known as the dynamo effect.

Secondly, if the electrically conducting gas in the sun moves, then the electrical currents and magnetic field have to move with this gas. Thus, the gas is like a hook pulling a rope, the magnetic field, around the sun as it itself moves around the sun.

Differential Rotation

Different regions of gas in the sun move by way of differential rotation, a kind of rotation where the rotation period of a body differs with latitude. The sun's rotation period is shorter at the equator and lengthens as you move away from the equator, towards the poles, meaning the sun rotates faster at its equator.

Differential Rotation
differential rotation

As you can tell by the image on the screen above, this differential rotation drags the rope-like magnetic field around the sun. Had the sun, like a rigid planet, not experienced differential rotation, this would not have occurred.

Here's why. Picture you and two friends holding a rope. You guys are like the electrically conducting gas of the sun that made the rope. The two friends hold either end, representing latitudes farther away from the equator, and you hold the middle, representing the equator. Differential rotation means the middle moves faster than the ends. If you move faster than your friends move with you, the middle will bend and bend towards the direction you are moving in. Had you all moved at the same speed, the rope would remain straight.

Opposite Magnetic Polarities

As the magnetic field in the photosphere (the visible surface of the sun) wraps around the sun, as the hook pulls the rope more and more around the sphere, it strengthens. At the same time, the rising and sinking convection currents cause kinks in the rope to occur. This causes twisted loops of the magnetic field to pop out through the solar surface, first farther away from the equator and, with time, closer to it, and as the lessons on sunspots and the solar cycle explain, this is where sunspots will form.

When the sunspots appear, they tend to do so in pairs or groups. The 'front' end of the pair or group, known as the preceding member, will have the same polarity (north or south) as the magnetic pole in that hemisphere of the sun. The 'back' end of the pair, the following member, will have the opposite polarity. So, it's basically like a bar magnet floating around on the sun, that's all.

With time, differential rotation actually untwists the magnetic field it twisted up before. This causes something interesting to happen. The preceding members of sunspot groups will move towards the sun's equator, and the following members will drift towards their respective poles.

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