Definition
Earth's magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole and the other magnetic field N pole near the Earth's geographic south pole.
A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. Earth is surrounded by a magnetosphere, as are the magnetized planets Jupiter, Saturn, Uranus and Neptune.
Topics of Interest
The cause of the Earth's magnetic field is probably explained by dynamo theory. The magnetic field extends several tens of thousands of kilometres into space as the magnetosphere. An imaginary line joining the magnetic poles would be inclined by approximately 11.3° from the planet's axis of rotation.
The locations of the magnetic poles are not static but wander as much as 15km every year (Dr. David P. Stern, emeritus Goddard Space Flight Center, NASA). The pole position is usually not that indicated on many charts and many magnetic pole marking brings a confusion as to what is being located at the given positions. The Geomagnetic Pole positions are usually not close to the position that commercial cartographers place "Magnetic Poles." "Geomagnetic Dipole Poles", "IGRF Model Dip Poles", and "Magnetic Dip Poles" are variously used to denote the magnetic poles.
The Earth's field is changing in size and position. The two poles wander independently of each other and are not at directly opposite positions on the globe. Currently the south magnetic pole is farther from the geographic south pole than the north magnetic pole is from the north geographic pole. The fields protect the earth and its inhabitants by repelling the solar wind.
The field is similar to that of a bar magnet, but this similarity is superficial. The magnetic field of a bar magnet, or any other type of permanent magnet, is created by the coordinated motions of electrons (negatively charged particles) within iron atoms. The Earth's core, however, is hotter than 1043 K, the Curie point temperature at which the orientations of electron orbits within iron become randomized. Such randomization tends to cause the substance to lose its magnetic field. Therefore the Earth's magnetic field is caused not by magnetised iron deposits, but mostly by electric currents in the liquid outer core.
Another feature that distinguishes the Earth magnetically from a bar magnet is its magnetosphere. At large distances from the planet, this dominates the surface magnetic field. Electric currents induced in the ionosphere also generate magnetic fields. Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations which can deflect surface magnetic fields by as much as one degree.
The strength of the field at the Earth's surface ranges from less than 30 microteslas (0.3 gauss) in an area including most of South America and South Africa to over 60 microteslas (0.6 gauss) around the magnetic poles in northern Canada and south of Australia, and in part of Siberia.
Magnetometers detect minute deviations in the Earth's magnetic field caused by iron artifacts, kilns, some types of stone structures, and even ditches and middens in geophysical survey. Using the magnetic instruments adapted from airborne devices developed during World War II to detect submarines, the magnetic variations across the ocean floor have been mapped. The basalt -- the iron-rich, volcanic rock making up the ocean floor -- contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. The distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials record the Earth's magnetic field.
In October 2003, the Earth's magnetosphere was hit by a solar flare causing a brief but intense geomagnetic storm, provoking unusual displays of aurorae.
Magnetic field reversals: Based upon the study of lava formations in Hawaii, it has been deduced that the Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years. The last such event, called the Brunhes-Matuyama reversal, occurred some 780,000 years ago.
The mechanism responsible for geomagnetic reversals is not well understood. Some scientists have produced models for the core of the Earth wherein the magnetic field is only quasi-stable and the poles can spontaneously migrate from one orientation to the other over the course of a few hundred to a few thousand years. Other scientists propose that the geodynamo first turns itself off, either spontaneously or through some external action like a comet impact, and then restarts itself with the magnetic "North" pole pointing either North or South. External events are not likely to be routine causes of magnetic field reversals due to the lack of a correlation between the age of impact craters and the timing of reversals. Regardless of the cause, when magnetic "North" reappears in the opposite direction this is a reversal, whereas turning off and returning in the same direction is called a geomagnetic excursion.
Using a magnetic detector (a variant of a compass), scientists have measured the historical direction of the Earth's magnetic field, by studying the layered iron-rich lava rocks. This is possible as each layer has been found to maintain the original magnetic field at its time of cooling. They have found that the poles have shifted a number of times throughout the past.
The earth's magnetic field strength was measured by Carl Friedrich Gauss in 1835 and has been repeatedly measured since then, showing an exponential decay with a half-life of about 1400 years. This could also be stated as a relative decay of about 10% to 15% over the last 150 years.
Magnetic field electrogenerators: Some free-energy enthusiasts claim that the Earth's magnetic field could be used to generate power, but such claims are regarded as pseudoscience by many skeptics. Many designs for using the Earth's electromagnetic field and atmospheric electricity have been researched, but have failed to gain any widespread acknowledgement in the scientific community. There is also some energy stored in the form of separated electrical charges, which can provide low direct currents at high voltages. However, ordinary electric motors cannot use this energy directly as a prime mover. Benjamin Franklin developed several motors that used the Earth's fields. Oleg D. Jefimenko has researched several machine designs for tapping the Earth's electromagnetic field.
The Earth's magnetic field can be used as the starting field for a self-excited electric generator. Cromwell Varley discovered in 1867 that an electric generator did not need to be started with a conventional prime mover. He used the Earth's magnetic field to induce enough field strength in the stator windings to get a generator running.
Electrodynamic tethers (long conducting wires, such as the one deployed from the tether satellite, which can operate on electromagnetic principles as generators) can induce a current by moving through the planet's magnetic field. When the conductive tether is trailed in a planetary or solar magnetosphere (magnetic field), the tether cuts the field, generates a current, and thereby slows the spacecraft into a lower orbit. The tether's end can be left bare, and this is sufficient to make contact with the ionosphere and allow a current to flow through a phantom loop. A cathode tube may also be placed at the end of the tether. The cathode tube will interact with the planet's magnetic field in the vacuum of space. A double-ended cathode tube tether will allow alternating currents.
A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. Earth is surrounded by a magnetosphere, as are the magnetized planets Jupiter, Saturn, Uranus and Neptune.
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede has a small magnetosphere — but it is situated entirely within the magnetosphere of Jupiter, leading to complex interactions. The ionospheres of weakly magnetized planets such as Venus and Mars set up currents that partially deflect the solar wind flow, but do not have magnetospheres, per se.
The term magnetosphere has also been used to describe regions dominated by the magnetic fields of celestial objects, e.g. pulsar magnetospheres.
The Earth's magnetosphere was discovered in 1958 by Explorer 1 during the research performed for the International Geophysical Year. Before this, scientists knew that electric currents existed in space, because solar eruptions sometimes led to "magnetic storm" disturbances. No one knew, however, where those currents were and why, or that the solar wind existed. In August and September of 1958, Project Argus was performed to test a theory about the formation of radiation belts that may have tactical use in war.
The magnetosphere of Earth is a region in space whose shape is determined by the extent of Earth's internal magnetic field, the solar wind plasma, and the interplanetary magnetic field (IMF). In the magnetosphere, a mix of free ions and electrons from both the solar wind and the Earth's ionosphere is confined by electromagnetic forces that are much stronger than gravity and collisions.
In spite of its name, the magnetosphere is distinctly non-spherical. All known planetary magnetospheres in the solar system possess more of an oval tear-drop shape due to the effects of the solar wind.
On the side facing the Sun, the distance to its boundary (which varies with solar wind intensity) is about 70,000 km (10-12 Earth radii or RE, where 1 RE=6371 km; unless otherwise noted, all distances here are from the Earth's center). The boundary of the magnetosphere ("magnetopause") is roughly bullet shaped, about 15 RE abreast of Earth and on the night side (in the "magnetotail" or "geotail") approaching a cylinder with a radius 20-25 RE. The tail region stretches well past 200 RE, and the way it ends is not well-known.
The outer neutral gas envelope of Earth, or geocorona, consists mostly of the lightest atoms, hydrogen and helium, and continues beyond 4-5 RE, with diminishing density. The hot plasma ions of the magnetosphere acquire electrons during collisions with these atoms and create an escaping "glow" of energetic neutral atoms (ENAs) that have been used to image the hot plasma clouds by the IMAGE and TWINS missions.
The upward extension of the ionosphere, known as the plasmasphere, also extends beyond 4-5 RE with diminishing density, beyond which it becomes a flow of light ions called the polar wind that escapes out of the magnetosphere into the solar wind. Energy deposited in the ionosphere by auroras strongly heats the heavier atmospheric components such as oxygen and molecules of oxygen and nitrogen, which would not otherwise escape from Earth's gravity. Owing to this highly variable heating, however, a heavy atmospheric or ionospheric outflow of plasma flows during disturbed periods from the auroral zones into the magnetosphere, extending the region dominated by terrestrial material, known as the fourth or plasma geosphere, at times out to the magnetopause.
Earth’s magnetosphere provides protection, without which life as we know it could not survive. Mars, with little or no magnetic field is thought to have lost much of its former oceans and atmosphere to space in part due to the direct impact of the solar wind. Venus with its thick atmosphere is thought to have lost most of its water to space in large part owing to solar wind ablation.
Due to the size of the Jupiter's magnetosphere there is a possibility of very weak and very brief seasonal head-tail interaction between Earth's magnetosphere and Jupiter's magnetosphere. The magnetospheres of the outer gas planets may weakly interact, although their magnetospheres are much smaller than Jupiter's.
A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft A magnetic sail could also thrust directly against planetary and solar magnetospheres.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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