The movement of metal fluids like iron and nickel in the outer core of the Earth is believed to generate the Earth's magnetic field according to the Dynamo Theory.
One can imagine a massive bar magnet or dipole running through the earth creating the magnetic North and South poles.
However the magnetic poles do not coincide with the geographic poles.
The magnetic North Pole, for example, is inclined about 11.5 degrees from the geographic North Pole. The magnetic North Pole is located near Ellesmere Island to the west of northwest Greenland.
Magnetic poles is offset from geographic pole by 11.5 degrees (Source: http://www.fas.org/irp/imint/docs/rst/Intro/Part2_1a.html)
The angle between the geographic and magnetic poles is the primary cause of magnetic declination, the difference between a magnetic compass reading for North and the true direction of geographic North.
The line where magnetic declination is zero is known as the agonic line, as explained in the previous blog entry. And again, because of magnetic anomalies this line is not easy to predict.
The agonic line can be thought of as a sort of magnetic meridian. A medieval Chinese text discusses such a central meridian where magnetic declination is zero.
The center of the Earth's magnetic dipole, or "bar magnet," also deviates from the center of the Earth's core from which it is offset by about 700 kilometers toward Southeast Asia.
The reason for this offset of the magnetic poles and center can only be guessed at, but the inequality results in differing strengths of the Earth's magnetic field.
Because the center of the dipole is closer to Southeast Asia, the magnetic field in Southeast Asia is stronger than anywhere else in the world. This is known as the Southeast-Asian Anomaly and it is the area of the Earth's surface closest to the dipole's center. The magnetic field is strongest along the magnetic equator in the Southeast-Asian Anomaly.
In the Western Hemisphere we find the South Atlantic Anomaly (SAA), which is the area of the Earth's surface furthest from the center of the dipole. Here the magnetic field is weaker than in other areas of the globe.
Because the Southeast-Asian Anomaly represents a stronger field it is known as a positive anomaly while the SAA is a negative anomaly.
The known practical effects of these anomalies is mainly found in space flight where they interfere with communications in the ionosphere. All communications are disrupted in the Southeast-Asian Anomaly and high frequency communications in the South Atlantic Anomaly.
The Southeast-Asian Anomaly and South Atlantic Anomaly (SAA) from Earth's Magnetic Field.
Biomagnetism and Magnetoreception
Research has clearly shown that many species are able to orient themselves and navigate using magnetoreception to detect the Earth's magnetic field.
There are two types of known magnetoreception -- one using detection of the poles, or polarity; and the other detects differences in magnetic inclination or "dip."
While magnetoreception is widely recognized in birds, bacteria and other species, the evidence for human magnetoreception has been more controversial with some political overtones.
Robin Baker first claimed to have found evidence for human magnetic orientation in tests he conducted in 1987. His methods though were questioned. In 1989, however, R. G. Murphy found evidence of human orientation that was disturbed when a magnet, but not a piece of brass, was placed on the head of the subject. This suggested that the orientation was magnetic in nature.
While Baker's research with other species was acclaimed, when he turned toward human study, he encountered resistance to his efforts. Helen Saul writes:
Sadly, Baker's research stopped before it got much further. When he shifted his interest from animals, birds, and fish to humans, it became so contentious that he lost the general support of the scientific community..."The stark contrast between positive results and negative interpretations remains an unexplained feature of the literature on human magnetoreception," he [Baker] wrote in 1987. He eventually left his post at Manchester University, and since then, the work has fallen by the wayside.
Henry H. Bauer writes in Science Or Pseudoscience: magnetic healing, psychic phenomena, and other heterodoxies:
Baker's heresy has been to study human sensing of magnetic fields, which attracts opposition for no obvious reason: “Every animal seriously tested has been found to have a magnetic sense, and it now seems that the final search will be for an animal that is magnetically blind. It would be more surprising to discover that Man just happened to be that animal than to discover he was not”...the “atmosphere surrounding the study of human magnetoreception has, both publicly and less publicly, been just a little unpleasant.”
University of Hawai'i anthropologist Ben Finney picked up on Baker's research by discussing the subject with modern navigators trained in traditional Pacific Islander (Micronesian) navigation.
Finney came to the conclusion that magnetoreception might be employed by navigators on an intuitive basis:
Taken together with experimental findings about magnetoreception in various species, these reports suggest that skilled noninstrument navigators may be able to turn to magnetoreception for orientation cues of last resort and that this magnetic sense of direction may also play an unconscious role in dead reckoning. In the light of experimental evidence on the hierarchy of directional senses, it is not surprising that noninstrument navigators would refer the more exact cues available from the passage of the sun and stars across the sky to any feelings they might have about their bearings but be open to following those feelings whenever the situation demands.
Finney suggests, as I have also in my own writings, that magnetoreception may be a latent ability in humans that must be learned much like humans must learn how to swim.
The subjects-typically students-usually employed in magnetoreception experiments have been navigationally naive in comparison with such Pacific island masters as Mau Piailug and Nainoa Thompson. If there are striking differences in performance between young birds without navigational experience and veterans of migrational or homing flight, might not this also be the case for humans?
Paul Kekai Manansala
Baker, Robin. Human navigation and magnetoreception, Manchester:
Manchester University Press, 1989.
Bauer, Henry H. Science Or Pseudoscience: magnetic healing, psychic phenomena, and other heterodoxies, University of Illinois Press, 2001.
Gehmeyr, Michael. Earth's Magnetic Field, Boston University, 2005.
Finney, Ben . "A Role for Magnetoreception in Human Navigation?" Current Anthropology, Vol. 36, No. 3. (Jun., 1995), pp. 500-506.
Murphy, RG. The development of magnetic compass orientation in children. Paper presented at the Royal Institute of Navigation Conference, 1989.
Saul, Helen. Phobias: Fighting the Fear, Arcade Publishing, 2004.