Our planet’s magnetic field is roughly the same shape as that of a bar magnet, and has an average magnetic flux density of about 50 μT (micro tesla). The field varies from 24–66 µT at the Earth’s surface, according to the .
A magnetic compass will point towards the nearest magnetic pole, rather than the geographic pole which is the point where the Earth’s axis of rotation passes through its surface. The horizontal angle between the magnetic and geographic poles is called the declination. If a compass is suspended vertically, it will also point downwards by an angle related to the distance from the pole and this vertical angle is known as the inclination. In fact, a compass suspended above a magnetic pole would point directly downwards!
By convention, the North-pointing end of a compass is regarded as a ‘true’ magnetic North pole, and since opposite poles attract, if follows that the Earth’s Magnetic North Pole is actually a South pole! The above image shows the Earth’s internal magnetic field represented by an equivalent bar magnet, with its true North pole (shown in red) pointing downwards. Lines of magnetic flux are considered to have a ‘direction’ or ‘orientation’ and, by convention, leave the North pole of the magnet. A compass aligns itself to the field, with its North pole pointing towards the Earth’s Magnetic North Pole – which is really a South pole.
The position of the Earth’s magnetic poles moves over time, currently travelling at a rate of about 40 km per year, according to . (A graphical representation of this movement is available from the .) The NASA report also outlines recent research into the origin of the Earth’s magnetic field, which is thought to be caused by a dynamo effect. In this theoretical model, the Earth’s solid inner iron-nickel core is surrounded by a molten metal outer core, and spinning vortices in this metallic ocean give rise to circulating electric currents, which in turn produce the Earth’s magnetic field.
Geologists have identified numerous reversals of this magnetic field by examining the magnetic field ‘locked into’ volcanic rocks at their time of creation. Evidence of regular magnetic reversals are seen for example in newly created rocks at the junction of mid-ocean ridges associated with the junction of tectonic plates. The reports that, on average, the Earth’s magnetic field has reversed every 200-250,000 years over the last 10 million years – although the last reversal occurred almost 800,000 years ago.
The Earth’s own magnetic field interacts with the much larger one produced by the Sun, with the resulting teardrop-shaped field being known as the magnetosphere. This field is continually bombarded by a stream of negatively charged electrons and positively charged ions (mostly protons or hydrogen nuclei) emitted by the Sun, and known as the solar wind.
These charged particles are deflected by the Earth’s magnetosphere and travel towards the magnetic poles, where they enter the upper atmosphere. Collisions between these energetic particles and gases in the atmosphere (primarily nitrogen) cause the characteristic green glow of the aurora, which may be seen on suitable dark nights at higher latitudes.
The aurora is known as the Northern Lights (in latin, aurora borealis) in the Northern hemisphere, and as the Southern Lights (aurora australis) in the South.