The Earth Magnetic Field
Picture comes from: http://en.wikipedia.org/wiki/Image:Geomagnetisme.svg#filelinks
There are lots of planets that have a magnetic field. Some of them have very complex magnetic field. But the Earth's and Jupiter's magnetic fields can be simplified as a dipole magnetic field, which can be written:
Where
is Permeability constant, 
is unit vector in the direction 
. 
is magnetic moment which for Earth is 
.
Reference
Margaret G. Kivelson and Christopher T. Russell, Introduction to Space Physics, Cambridge University Press 1995.