Earth Magnetic Field

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Earth's magnetic fieldFrom Wikipedia, the free encyclopediaJump to: navigation, search

Computer simulation of the Earth's field in a normal period between reversals.[1] The tubes represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of the Earth is centered and vertical. The dense clusters of lines are within the Earth's core.[2]Earth's magnetic field (also known as the geomagnetic field) is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun. Its magnitude at the Earth's surface ranges from 25,000 to 65,000 nanoteslas (0.25 to 0.65 gauss). It is approximately the field of a magnetic dipole tilted at an angle of 11 degrees with respect to the rotational axisas if there were a bar magnet placed at that angle at the center of the Earth. However, unlike the field of a bar magnet, Earth's field changes over time because it is generated by the motion of molten iron alloys in the Earth's outer core (the geodynamo).The Magnetic North Pole wanders, but slowly enough that a simple compass remains useful for navigation. At random intervals (averaging several hundred thousand years) the Earth's field reverses (the north and south geomagnetic poles change places with each other). These reversals leave a record in rocks that allow paleomagnetists to calculate past motions of continents and ocean floors as a result of plate tectonics.The region above the ionosphere, and extending several tens of thousands of kilometers into space, is called the magnetosphere. This region protects the Earth from cosmic rays that would strip away the upper atmosphere, including the ozone layer that protects the earth from harmful ultraviolet radiation.Contents[hide] 1 Importance 2 Main characteristics 2.1 Description 2.1.1 Intensity 2.1.2 Inclination 2.1.3 Declination 2.2 Dipolar approximation 2.3 Magnetic poles 3 Magnetosphere 4 Time dependence 4.1 Short-term variations 4.2 Secular variation 4.3 Magnetic field reversals 4.4 Earliest appearance 4.5 Future 5 Physical origin 5.1 Earth's core and the geodynamo 5.1.1 Numerical models 5.2 Currents in the ionosphere and magnetosphere 5.3 Crustal magnetic anomalies 6 Measurement and analysis 6.1 Detection 6.2 Statistical models 6.2.1 Spherical harmonics Radial dependence 6.2.2 Global models 7 Biomagnetism 8 See also 9 References and Bibliography 10 Further reading 11 External links

[edit] ImportanceThe Earth is largely protected from the solar wind, a stream of energetic charged particles emanating from the Sun, by its magnetic field, which deflects most of the charged particles. These particles would strip away the ozone layer, which protects the Earth from harmful ultraviolet rays.[3] Calculations of the loss of carbon dioxide from the atmosphere of Mars, resulting from scavenging of ions by the solar wind, are consistent with a near-total loss of its atmosphere since the magnetic field of Mars turned off.[4]The polarity of the Earth's magnetic field is recorded in sedimentary rocks. Reversals of the field are detectable as "stripes" centered on mid-ocean ridges where the sea floor is spreading, while the stability of the geomagnetic poles between reversals allows paleomagnetists to track the past motion of continents.[5] Reversals also provide the basis for magnetostratigraphy, a way of dating rocks and sediments.[6] The field also magnetizes the crust; magnetic anomalies can be used to search for ores.[7]Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century.[8][edit] Main characteristics[edit] Description

Common coordinate systems used for representing the Earth's magnetic field.At any location, the Earth's magnetic field can be represented by a three-dimensional vector (see figure). A typical procedure for measuring its direction is to use a compass to determine the direction of magnetic North. Its angle relative to true North is the declination (D) or variation. Facing magnetic North, the angle the field makes with the horizontal is the inclination (I) or dip. The intensity (F) of the field is proportional to the force it exerts on a magnet. Another common representation is in X (North), Y (East) and Z (Down) coordinates.[9][edit] IntensityThe intensity of the field is greatest near the poles and weaker near the Equator. It is often measured in gausses (G) but is generally reported in nanoteslas (nT), with 1 G = 100,000 nT. A nanotesla is also referred to as a gamma ().[10][11][12] The field ranges between approximately 25,000 and 65,000 nT (0.250.65 G). By comparison, a strong refrigerator magnet has a field of about 100 G.[13]

Intensity of the Earth's magnetic field from the World Magnetic Model for 2010.A map of intensity contours is called an isodynamic chart. An isodynamic chart for the Earth's magnetic field is shown to the left. A minimum intensity occurs over South America while there are maxima over northern Canada, Siberia, and the coast of Antarctica south of Australia.[edit] InclinationMain article: Magnetic dip

Inclination of the Earth's magnetic field from the World Magnetic Model for 2010.The inclination is given by an angle that can assume values between -90 (up) to 90 (down). In the northern hemisphere, the field points down. It is straight down at the North Magnetic Pole and rotates upwards as the latitude decreases until it is horizontal (0) at the magnetic equator. It continues to rotate upwards until it is straight up at the South Magnetic Pole. Inclination can be measured with a dip circle.An isoclinic chart (map of inclination contours) for the Earth's magnetic field is shown on the right.[edit] DeclinationMain article: Magnetic declinationDeclination is positive for an eastward deviation of the field relative to true north. It can be estimated by comparing the magnetic north/south heading on a compass with the direction of a celestial pole. Maps typically include information on the declination as an angle or a small diagram showing the relationship between magnetic north and true north. Information on declination for a region can be represented by a chart with isogonic lines (contour lines with each line representing a fixed declination).

Declination of the Earth's magnetic field from the World Magnetic Model for 2010. Isogonic lines give the declination in signed degrees.An isogonic chart (map of declination contours) for the Earth's magnetic field is shown on the left.[edit] Dipolar approximation

The variation between magnetic north (Nm) and "true" north (Ng).See also: Dipole model of the Earth's magnetic fieldNear the surface of the Earth, its magnetic field can be closely approximated by the field of a magnetic dipole positioned at the center of the Earth and tilted at an angle of about 10 with respect to the rotational axis of the Earth.[11] The dipole is roughly equivalent to a powerful bar magnet, with its south pole pointing towards the geomagnetic North Pole. This may seem surprising, but the north pole of a magnet is so defined because it is attracted towards the Earth's north pole. Since the north pole of a magnet attracts the south poles of other magnets and repels the north poles, it must be attracted to the south pole of Earth's magnet. The dipolar field accounts for 8090% of the field in most locations.[9][edit] Magnetic poles

The movement of Earth's North Magnetic Pole across the Canadian arctic, 18312001.The positions of the magnetic poles can be defined in at least two ways.[14]A magnetic dip pole is a point on the Earth's surface where the magnetic field is entirely vertical.[15]The inclination of the Earth's field is 90 at the North Magnetic Pole and -90 at the South Magnetic Pole. The two poles wander independently of each other and are not directly opposite each other on the globe. They can migrate rapidly: movements of up to 40 km per year have been observed for the North Magnetic Pole. Over the last 180 years, the North Magnetic Pole has been migrating northwestward, from Cape Adelaide in the Boothia peninsula in 1831 to 600 km from Resolute Bay in 2001.[16] The magnetic equator is the line where the inclination is zero (the magnetic field is horizontal).If a line is drawn parallel to the moment of the best-fitting magnetic dipole, the two positions where it intersects the Earth's surface are called the North and South geomagnetic poles. If the Earth's magnetic field were perfectly dipolar, the geomagnetic poles and magnetic dip poles would coincide and compasses would point towards them. However, the Earth's field has a significant contribution from non-dipolar terms, so the poles do not coincide and compasses do not generally point at either.[edit] MagnetosphereMain article: MagnetosphereSome of the charged particles from the solar wind are trapped in the Van Allen radiation belt. A smaller number of particles from the solar wind manage to travel, as though on an electromagnetic energy transmission line, to the Earth's upper atmosphere and ionosphere in the auroral zones. The only time the solar wind is observable on the Earth is when it is strong enough to produce phenomena such as the aurora and geomagnetic storms. Bright auroras strongly heat the ionosphere, causing its plasma to expand into the magnetosphere, increasing the size of the plasma geosphere, and causing escape of atmospheric matter into the solar wind. Geomagnetic storms result when the pressure of plasmas contained inside the magnetosphere is sufficiently large to inflate and thereby distort the geomagnetic field.

Simulation of the interaction between Earth's magnetic field and the interplanetary magnetic field. The magnetosphere is compressed on the day (Sun) side due to the force of the arriving particles, and extended on the night side.The solar wind is responsible for