Magnetic East and West?

The four points of the magnetic compass have been used for centuries, to help us navigate around the world. A compass is a simple device: the needle is magnetised, and aligns itself with the magnetic field lines between the North and South Pole.  

But there is no West or East pole, so what do East and West mean in terms of magnetic fields?

You can think of the Earth as a gigantic bar magnet:

If I walk due North, I am travelling parallel to the Earth’s magnetic field lines, towards magnetic North.  This is roughly similar to following a line of longitude.  Eventually, I will arrive at the North Pole (both the magnetic poles can be seen as final destinations).

If I walk West however, I am travelling perpendicular to the Earth’s magnetic field lines.  This is roughly similar to following a line of latitude.  There is of course no West Pole; I can continue walking West forever. In fact when I travel West, I am rotating around the axis of the Earth’s magnetic field.  

It turns out there is some special significance to this. If a charged particle passes through a magnetic field, it is induced to rotate around the field’s axis: http://www.youtube.com/watch?v=a2_wUDBl-g8 The charged particles are induced to move West or East (in a circle around the magnetic field lines). This phenomenon is one way the Earth’s magnetic field protects us from the dangerous charged particles emitted by the Sun.  These harmful particles are either deflected, or trapped into moving in a circle around field lines (see the pink/yellow arrows at the poles):

The same principle is used in the Large Hadron Collider, to steer particles in a circle.  Large magnetic fields are used to keep particles moving in a doughnut-shaped path, so they can be accelerated to very high speeds.

So in summary, movement to the North or South can be interpreted as travelling parallel to field lines, towards a pole. Conversely, movement to the West or East can be interpreted as travelling perpendicular to field lines, or rotating around the axis of the magnetic dipole.