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The Earth’s Magnetic Field. An Invisible Shield.

Have you watched the movie “The Core”? If so the you will remember that a lot of bad things happen when the earth’s core stops rotating, consequently leading to a collapse of earth’s magnetic field. Rest assured that this is pure Hollywood and although the earth’s magnetic field can reverse itself as it has done numerous times in the past, it can never stop existing as long as the core spins in any direction. So what happens in the movie as a result of the loss of the magnetic field? Well, a lot. Unfortunately the movie gets most of these wrong. No boiling of the seas by the “deadly” microwaves can happen. This is because first of all microwaves are not affected by the presence of a magnetic field and second of all the intensity of the microwaves that “hammer” the earth every day is really small. If it was higher then they would “overshadow” all of today’s radio and mobile communications (these are all based on microwaves).  The truth of the matter is that the earth’s magnetic field can only protect us from charged particles (like high energy protons), which compose the solar wind. These charged particles pose a risk to electronic equipment as NASA found out when the first launch the space telescope Hubble.

 

               

Simple model schematic vs actual magnetic field of earth

 

So how exactly does this field act as a shield to the solar wind? It is quite simple actually. The magnetic field forms a protective and invisible bubble around the earth called a magnetosphere. Keep in mind that the magnetosphere is not actually a sphere as it is affected by the solar wind resulting in a shape which can best be described as a teardrop with a tail extending away from the sun.

 

 

 When a charged particle enters the magnetosphere it changes trajectory. This is because a force is applied on the particle by the magnetic field making it follow a spiral line with a direction parallel to the field’s lines. And since all the field lines meet at the poles, the particles eventually end up there where in our case they interact with the atmosphere producing the beautiful aurora lights. You can see how a beam of charged particles bends in the presence of a magnet by watching the clip below.

 

 

The direction of the applied force on the particle can be found by simply remembering the following right hand rule (doesn’t have to be Fleming’s, it can be your own hand).

 

Image source: Wikimedia Commons, Jfmelero

 

This rule tells us the following. If a particle is moving vertically to the magnetic field lines then the applied force (F) on the particle will be vertical to the plane generated by the B and I vectors. As such, when the particle enters a magnetic field in a not perfect 90 degree angle then it will be caught is a spiral loop with a direction parallel to the field's lines as seen below.

 

 

 Have a go with the following simulation from the King's Centre of Visualization in Science and see if you can replicate the spiral motion. Simulation is titled. "Motion of charged particle in a magnetic field".

 

 

The same principle is used in the Large Hadron Collider as a method for preventing the plasma (which in essence is composed of charged particles) from touching the walls of the containers it runs through. A field is applied which lines are parallel to the desired direction we want the plasma to take…and that is it. The plasma will follow the lines maintaining a spiral motion.

 

 

Igloo Lit Up at Night under Northern Lights Northwest Territories, Canada March 2007
Igloo Lit Up at Night under Northern Lights Northwest Territories, Canada March 2007
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Hope you enjoyed this brief article. If you are interested in finding out a bit more about magnetism, the earth's magnetic field and the problems NASA had when they first launched Hubble then please follow the links below.

 

The Earth's Magnetic Field

Auroras and Radiation Belts

Bad Astronomy: The Core

The South Atlantic Anomaly and NASA's Hubble Problems

NASA on Earth's Magnetic Field