Saturday, 26 March 2011

CASE 249 - Earths magnetic field

Earth's magnetic field (and the surface magnetic field) is approximately a magnetic dipole, with the magnetic field South pole near the Earth's geographic north pole (see Magnetic North Pole) and the other magnetic field N pole near the Earth's geographic south pole (see Magnetic South Pole). This makes the compass usable for navigation. The cause of the field can be explained by dynamo theory. A magnetic field extends infinitely, though it weakens with distance from its source. The Earth's magnetic field, also called the geomagnetic field, which effectively extends several tens of thousands of kilometres into space, forms the Earth's magnetosphere. A paleomagnetic study of Australian red dacite and pillow basalt has estimated the magnetic field to be at least 3.5 billion years old.

Magnetic poles and magnetic dipole

North Magnetic Pole and South Magnetic Pole

Magnetic declination from true north in 1700
The positions of the magnetic poles can be defined in at least two ways.
Often, a magnetic (dip) pole is viewed as a point on the Earth's surface where the magnetic field is entirely vertical. Another way of saying this is that the inclination of the Earth's field is 90° at the North Magnetic Pole and -90° at the South Magnetic Pole. At a magnetic pole, a compass held in the horizontal plane points randomly, while otherwise it points nearly to the North Magnetic Pole or away from the South Magnetic Pole, though local deviations exist. The two poles wander independently of each other and are not at directly opposite positions on the globe. Magnetic dip pole can migrate rapidly: movements of up to 40 km per year have been observed for the North Magnetic Pole.
The Earth's magnetic field can be closely approximated by the field of a magnetic dipole positioned near the centre of the Earth. A dipole's orientation is defined by an axis. The two positions where the axis of the dipole that best fits the geomagnetic field intersect the Earth's surface are called the North and South geomagnetic poles. For best fit the dipole representing the geomagnetic field should be placed about 500 km off the center of the Earth. This causes the inner radiation belt to skim lower in Southern Atlantic ocean, where the surface field is the weakest, creating what is called the South Atlantic Anomaly.
If the Earth's magnetic field were perfectly dipolar, the geomagnetic and magnetic dip poles would coincide. However, significant non-dipolar terms in an accurate description of the geomagnetic field cause the position of the two pole types to be in different places.

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