Magneticosphere - Earth's Magnetic Field

Earth's magnetic field (and the surface magnetic field) isapproximately a magnetic dipole, with one pole near the north pole andthe other near the geographic south pole. An imaginary line joining themagnetic poles would be inclined by approximately 11.3° from theplanet's axis of rotation. The cause of the field is probably explainedby dynamo theory. The magnetic field extends several tens of thousandsof kilometers into space as the magnetosphere.

Magnetic poles

Magnetic declination from true north in 2000.The locations ofthe magnetic poles are not static but wander as much as 15km every year(Dr. David P. Stern, emeritus Goddard Space Flight Center, NASA). Thepole position is usually not that indicated on many charts and manymagnetic pole marking brings a confusion as to what is being located atthe given positions.

The Geomagnetic Pole positions are usually not close to the positionthat commercial cartographers place "Magnetic Poles." "GeomagneticDipole Poles", "IGRF Model Dip Poles", and "Magnetic Dip Poles" arevariously used to denote the magnetic poles.

The Earth's field is changing in size and position. The two poleswander independently of each other and are not at directly oppositepositions on the globe. Currently the south magnetic pole is fartherfrom the geographic south pole than the north magnetic pole is from thenorth geographic pole.

Field characteristics

The field is similar to that of a bar magnet, but thissimilarity is superficial. The magnetic field of a bar magnet, or anyother type of permanent magnet, is created by the coordinated motionsof electrons (negatively charged particles) within iron atoms. TheEarth's core, however, is hotter than 1043 K, the Curie pointtemperature at which the orientations of electron orbits within ironbecome randomized. Such randomization tends to cause the substance tolose its magnetic field. Therefore the Earth's magnetic field is causednot by magnetised iron deposits, but mostly by electric currents in theliquid outer core.

Another feature that distinguishes the Earth magnetically froma bar magnet is its magnetosphere. At large distances from the planet,this dominates the surface magnetic field. Electric currents induced inthe ionosphere also generate magnetic fields. Such a field is alwaysgenerated near where the atmosphere is closest to the Sun, causingdaily alterations which can deflect surface magnetic fields by as muchas one degree.

Magnetic field variations

The strength of the field at the Earth's surface ranges fromless than 30 microteslas (0.3 gauss) in an area including most of SouthAmerica and South Africa to over 60 microteslas (0.6 gauss) around themagnetic poles in northern Canada and south of Australia, and in partof Siberia.

Magnetometers detect minute deviations in the Earth's magneticfield caused by iron artifacts, kilns, some types of stone structures,and even ditches and middens in geophysical survey. Using the magneticinstruments adapted from airborne devices developed during World War IIto detect submarines, the magnetic variations across the ocean floorhave been mapped. The basalt - the iron-rich, volcanic rock making upthe ocean floor - contains a strongly magnetic mineral (magnetite) andcan locally distort compass readings. The distortion was recognized byIcelandic mariners as early as the late 18th century. More important,because the presence of magnetite gives the basalt measurable magneticproperties, these magnetic variations have provided another means tostudy the deep ocean floor. When newly formed rock cools, such magneticmaterials record the Earth's magnetic field.

In October 2003, the Earth's magnetosphere was hit by a solarflare causing a brief but intense geomagnetic storm, provoking unusualdisplays of aurorae.

Geomagnetic Reversal- Pole Shifts

A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged. These events, which typically last a few hundred to a few thousands years, often involve an extended decline in field strength followed by a rapid recovery after the new orientation has been established.

Over very long periods, geomagnetic reversals seems to have occurred with a frequency of 1 to 5 events per million years; however, this duration is highly variable. During some periods of geologic time (e.g. Cretaceous long normal), the Earth's magnetic field is observed to maintain a single orientation for tens of millions of years. Other events seem to have occurred very rapidly, with more than one reversal in 50,000 years. The last reversal was the Brunhes-Matuyama reversal approximately 780,000 years ago.

Based upon the study of lava formations in Hawaii, it has beendeduced that the Earth's magnetic field reverses at intervals, rangingfrom tens of thousands to many millions of years, with an averageinterval of approximately 250,000 years. The last such event, calledthe Brunhes-Matuyama reversal, occurred some 780,000 years ago.

The mechanism responsible for geomagnetic reversals is not wellunderstood. Some scientists have produced models for the core of theEarth wherein the magnetic field is only quasi-stable and the poles canspontaneously migrate from one orientation to the other over the courseof a few hundred to a few thousand years. Other scientists propose thatthe geodynamo first turns itself off, either spontaneously or throughsome external action like a comet impact, and then restarts itself withthe magnetic "North" pole pointing either North or South. Externalevents are not likely to be routine causes of magnetic field reversalsdue to the lack of a correlation between the age of impact craters andthe timing of reversals. Regardless of the cause, when magnetic "North"reappears in the opposite direction this is a reversal, whereas turningoff and returning in the same direction is called a geomagneticexcursion.

Using a magnetic detector (a variant of a compass), scientistshave measured the historical direction of the Earth's magnetic field,by studying the layered iron-rich lava rocks. This is possible as eachlayer has been found to maintain the original magnetic field at itstime of cooling. They have found that the poles have shifted a numberof times throughout the past.

Magnetic Field Decay

The earth's magnetic field strength was measured by CarlFriedrich Gauss in 1835 and has been repeatedly measured since then,showing an exponential decay with a half-life of about 1400 years. Thiscould also be stated as a relative decay of about 10% to 15% over thelast 150 years.

Magnetic Field Electro generators

Some free-energy enthusiasts claim that the Earth's magneticfield could be used to generate power[4], but such claims are regardedas pseudoscience by many skeptics. Many designs for using the Earth'selectromagnetic field and atmospheric electricity have been researched,but have failed to gain any widespread acknowledgement in thescientific community. There is also some energy stored in the form ofseparated electrical charges, which can provide low direct currents athigh voltages. However, ordinary electric motors cannot use this energydirectly as a prime mover. Benjamin Franklin developed several motorsthat used the Earth's fields. Oleg D. Jefimenko has researched severalmachine designs for tapping the Earth's electromagnetic field.

The Earth's magnetic field can be used as the starting fieldfor a self-excited electric generator. Cromwell Varley discovered in1867 that an electric generator did not need to be started with aconventional prime mover. He used the Earth's magnetic field to induceenough field strength in the stator windings to get a generatorrunning.

Electrodynamic tethers can induce a current by moving through theplanet's magnetic field. When the conductive tether is trailed in aplanetary or solar magnetosphere (magnetic field), the tether cuts thefield, generates a current, and thereby slows the spacecraft into alower orbit. The tether's end can be left bare, and this is sufficientto make contact with the ionosphere and allow a current to flow througha phantom loop. A cathode tube may also be placed at the end of thetether. The cathode tube will interact with the planet's magnetic fieldin the vacuum of space. A double-ended cathode tube tether will allowalternating currents.