Earth s Magnetic Field

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GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. between the horizontal plane and the field vector measured positive downwards and. declination or magnetic variation D the horizontal angle between true north and the. field vector measured positive eastwards Declination inclination and total intensity. can be computed from the orthogonal components using the following equations. D arctan I arctan F H2 Z2,where H is given by, The International System of Units SI unit of magnetic field intensity strictly flux. density most commonly used in geomagnetism is the Tesla At Earth s surface the total. intensity varies from 24 000 nanotesla nT to 66 000 nT Other units likely to be. encountered are the Gauss 1 Gauss 100 000 nT the gamma 1 gamma 1 nT and. the rsted 1 rsted 103 4 A m 1,2 2 Observatories, A geomagnetic observatory is a location where absolute vector observations of Earth s. magnetic field are recorded accurately and continuously with a time resolution of one. minute or less over a long period The site of the observatory must be magnetically. clean and remain so for the foreseeable future The earliest magnetic observatories. where continuous vector observations were made began operation in the 1840s. There are two main categories of instruments at an observatory The first category. comprises variometers that make continuous measurements of elements of the. geomagnetic field vector but in arbitrary units for example millimeters of photographic. paper in the case of photographic systems or electrical voltage in the case of fluxgates. A fluxgate sensor comprises a core of easily saturable material with high permeability. Around the core there are two windings an excitation coil and a pick up coil If an. alternating current is fed into the excitation coil so that saturation occurs and if there is a. component of the external magnetic field along the fluxgate element the pick up coil. outputs a signal not only with the excitation frequency but also other harmonics related. to the intensity of the external field component Both analog and digital variometers. require temperature controlled environments and installation on extremely stable. platforms though some modern systems are suspended and therefore compensate for. platform tilt Even with these precautions they can still be subject to drift They operate. with minimal manual intervention and the resulting data are not absolute. The second category comprises absolute instruments that can make measurements of the. magnetic field in terms of absolute physical basic units or universal physical constants. The most common types of absolute instruments are the fluxgate theodolite for. measuring D and I and the proton precession magnetometer for measuring F In the. former the basic unit is an angle The fluxgate sensor mounted on the telescope of a. nonmagnetic theodolite is used to detect when it is perpendicular to the magnetic field. vector Collimation errors between the fluxgate sensor and the optical axis of the. Encyclopedia of Life Support Systems EOLSS, GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. theodolite and within the theodolite are minimized by taking readings from four. telescope positions With the fluxgate sensor operating in null field mode the stability. and sensitivity of the sensor and its electronics are maximized True north is determined. by reference to a fixed mark of known azimuth This can be determined astronomically. or by using a gyro attachment In a proton precession magnetometer the universal. physical constant is the gyromagnetic ratio of the proton and the basic unit is time. frequency Measurements with a fluxgate theodolite can only be made manually. whereas a proton magnetometer can operate automatically. Figure 1 Locations of currently operating geomagnetic observatories. The locations of currently operating magnetic observatories are shown in Figure 1 It. can be seen that the spatial distribution of the observatories is rather uneven with a. concentration in Europe and a dearth elsewhere in the world particularly in the ocean. 2 3 Satellites, Since the 1960s Earth s magnetic field has been observed intermittently by satellites. The first satellites measured only the strength of the magnetic field sometimes using. only a nonabsolute instrument but beginning in 1999 there have been a number of. satellite missions attempting to measure the full field vector using star cameras to. establish the direction of a triaxial fluxgate sensor An absolute intensity instrument is. normally also carried and both magnetic instruments are kept remote from the main. body of the satellite by mounting them at the end of a nonmagnetic boom Satellites. provide an excellent global distribution of data but generally only last for a short period. i e months to a few years,Encyclopedia of Life Support Systems EOLSS.
GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. Satellites which have provided valuable vector data for geomagnetic field modelling are. Magsat which flew in 1979 and 1980 and Orsted and CHAMP which were launched in. 1999 and 2000 respectively and are still returning data in 2006 The Orsted satellite. takes just over 2 years to sample all local times and flies in the altitude range 640 850. km The CHAMP satellite samples all local times in 4 5 months and flies in the altitude. range 350 450 km,2 4 Other Direct Observations, Earth s magnetic field is observed in a number of other ways These are repeat stations. and surveys made on land from aircraft and ships Repeat stations are permanently. marked sites where high quality vector observations of Earth s magnetic field are made. for a few hours or sometimes a few days every few years Their main purpose is to. track changes in the core generated magnetic field. Most aeromagnetic surveys are designed to map the crustal field As a result they are. flown at altitudes lower than 300 m and they cover small areas generally once only. with very high spatial resolution Because of the difficulty in making accurately. oriented measurements of the magnetic field on a moving platform these kinds of. aeromagnetic surveys generally comprise total intensity data only However between. 1953 and 1994 the Project MAGNET program collected high level three component. aeromagnetic data specifically for modeling the core generated field The surveys were. mainly over the ocean areas of Earth at middle to low latitudes A variety of platforms and. instrumentation were used the most recent set up included a fluxgate vector magnetometer. mounted on a rigid beam in the magnetically clean rear part of the aircraft a ring laser gyro. fixed at the other end of the beam and a scalar magnetometer located in a stinger. extending some distance behind the aircraft s tail section. Modern marine magnetic surveys are also invariably designed to map the crustal field. but with careful processing it is possible to obtain information about the core generated. field from the data In a marine magnetic survey a scalar magnetometer is towed some. distance behind a ship usually along with other geophysical equipment as it makes. either a systematic survey of an area or traverses an ocean. Prior to the establishment of observatories and an absolute method of measuring. magnetic intensity by Gauss in the 1830s magnetic observations were madeby mariners. engaged in merchant and naval shipping as well as by others These measurements. were mainly of declination and serve to extend the global historic data set back to the. beginning of the seventeenth century,2 5 Indirect Observations. Prior to the seventeenth century indirect observations of Earth s magnetic field are. possible from archaeological remains and rocks using paleomagnetic techniques The. subject of rock magnetism and paleomagnetism is covered elsewhere in this. encyclopedia see Rock Magnetism and Paleomagnetism. Encyclopedia of Life Support Systems EOLSS, GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. 3 Characteristics of Earth s Magnetic Field,3 1 Reversals. When a rock is formed it usually acquires a magnetization parallel to the ambient. magnetic field i e the core generated field From careful analyses of directions and. intensities of rock magnetization from many sites around the world it has been. established that the polarity of the axial dipole has changed many times in the past with. each polarity interval lasting several thousand years These reversals occur slowly and. irregularly and for a period of roughly 30 million years centered approximately 100. million years before present there were no reversals at all In addition to full reversals. there have been many aborted reversals when the magnetic poles are observed to move. toward the equator for a while but then move back and align closely with Earth s spin. axis The solid inner metal core is thought to play an important role in inhibiting. reversals At the present time we are seeing a 6 decline in the dipole moment per. century Whether this is a sign of an imminent reversal is impossible to say. 3 2 The Present Magnetic Field, In a source free region near the surface of Earth the magnetic field is the negative.
gradient of a scalar potential that satisfies Laplace s equation A solution to Laplace s. equation in spherical coordinates is called a spherical harmonic expansion and its. parameters are called Gauss coefficients There are internal or external coefficients. modeling the field generated inside or outside the planet respectively A separation of. the core and crustal fields both internal is not perfect The internal field is often called. the main field, The main field coefficients change with time as the core generated field changes and in. commonly used spherical harmonic models for example the International Geomagnetic. Reference Field IGRF and the World Magnetic Model this secular variation is. assumed to be constant over five year intervals It has only been possible to accurately. determine the small but persistent field generated outside Earth largely by the ring. current since satellite data have become available. At the Earth s surface the main field can be approximated by a dipole placed at the. Earth s centre and tilted to the axis of rotation by about 11 However significant. deviations from a dipole field exist Figures 2 6 show maps of declination inclination. horizontal intensity vertical intensity and total intensity at 2005 0 and their predicted. secular variations for the period 2005 0 to 2010 0 derived from the 10th Generation. IGRF model,Encyclopedia of Life Support Systems EOLSS. GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. Figure 2 Contour maps of declination degrees at 2005 0 and secular variation of. declination arc minutes year for 2005 0 2010 0,Encyclopedia of Life Support Systems EOLSS. GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. Figure 3 Contour maps of inclination degrees at 2005 0 and secular variation of. inclination arc minutes year for 2005 0 2010 0,Encyclopedia of Life Support Systems EOLSS. GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. Figure 4 Contour maps of total intensity nT at 2005 0 and secular variation of total. intensity nT year for 2005 0 2010 0,Encyclopedia of Life Support Systems EOLSS.
GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. Figure 5 Contour maps of horizontal intensity nT at 2005 0 and secular variation of. horizontal intensity nT year for 2005 0 2010 0,Encyclopedia of Life Support Systems EOLSS. GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. Figure 6 Contour maps of vertical intensity nT at 2005 0 and secular variation of. vertical intensity nT year for 2005 0 2010 0,3 3 Westward Drift. Using direct observations of the magnetic field over the past 400 y the pattern of. declination seen at Earth s surface appears to be moving slowly westward This is. particularly apparent in the Atlantic hemisphere at mid and equatorial latitudes This. may be related to the motion of fluid at the core surface slowly westward dragging with. it the magnetic field lines,Encyclopedia of Life Support Systems EOLSS. GEOPHYSICS AND GEOCHEMISTRY Earth s Magnetic Field Susan Macmillan. 3 4 Geomagnetic Jerks, The rate of change of declination at Lerwick Eskdalemuir and Greenwich Abinger. Hartland observatory series in the UK is shown in Figure 7 It can be seen from this plot. that there have been a number of changes in the general trend of secular variation in the. past in particular at about 1925 1969 1978 and 1992 These sudden changes are. known as jerks or impulses and at the present time are not well understood and are. certainly not predictable Some researchers have found evidence for a correlation with. length of day changes, Figure 7 Rate of change of declination at Greenwich GRW Abinger ABN Hartland.
HAD Eskdalemuir ESK and Lerwick LER observatories 1900 2005. 3 5 Crustal Magnetic Field, The field arising from magnetic materials in Earth s crust varies on all spatial scales and. is often referred to as the anomaly field Knowledge of the crustal magnetic field is. often very valuable as a geophysical exploration tool for determining the local geology. The anomalies seen at midocean spreading ridges are of particular interest At these. locations molten mantle comes to the surface and solidifies to form new oceanic crust. of Earth the crust ionosphere magnetosphere all the way to the Sun Observing Earth s magnetic field provides valuable insights on all these processes These observations are made by geomagnetic observatories extensive surveys made on land at sea and from aircraft and satellites and from magnetic properties of rocks 1 Introduction Earth s magnetic field is generated in the fluid

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