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TOPIC: TERRESTRIAL MAGNETISM

•	MAGNETIC FIELD OF EARTH

Magnetic field

A magnetic field may be represented by a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The magnetic field is most commonly defined in terms of the Lorentz force it exerts on moving electric charges. There are two separate but closely related fields to which the name 'magnetic field' can refer: a magnetic B field and a magnetic H field. Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. In special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons. Magnetic fields have had many uses in ancient and modern society. The Earth produces its own magnetic field, which is important in navigation. Rotating magnetic fields are utilized in both electric motors and generators. Magnetic forces give information about the charge carriers in a material through the Hall effect. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits.

Dynamo theory

In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales. When William Gilbert published de Magnete in 1600, he concluded that the Earth is magnetic and proposed the first theory for the origin of this magnetism: permanent magnetism such as that found in lodestone. In 1919, Joseph Larmor proposed that a dynamo might be generating the field.[1][2] However, even after he advanced his theory, some prominent scientists advanced alternate theories. Einstein believed that there might be an asymmetry between the charges of the electron and proton so that the Earth's magnetic field would be produced by the entire Earth. The Nobel Prize winner Patrick Blackett did a series of experiments looking for a fundamental relation between angular momentum and magnetic moment, but found none.[3][4]

Earth's magnetic field

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. It is approximately the field of a magnetic dipole tilted at an angle of 11 degrees with respect to the rotational axis—as 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, fortunately slowly enough that the compass is 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.s

Magnetic field produced by earth. Importance Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century.[3]

The 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.[4] 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.[5]

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.[6] Reversals also provide the basis formagnetostratigraphy, a way of dating rocks and sediments.[7] The field also magnetizes the crust; magnetic anomalies can be used to search for ores.[8] Main characteristics

Description 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]

Intensity The intensity of the field is greatest near the poles and weaker near the Equator. It is generally reported in nanoteslas (nT) or gausses (G), with 1 G = 100,000 nT. It ranges between approximately 25,000 and 65,000 nT (0.25–0.65 G).[10][11] By comparison, a strong refrigerator magnet has a field of about 100 G.[12] 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. Inclination

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.

•	Angle of Dip at the Poles The magnetic lines of force at the poles of Earth are vertical due to which the                                              magnetic needle becomes vertical. The angle of dip at the magnetic poles of Earth is 90 o. •	Angle of Dip at the Equator The lines of force around the magnetic equator of the Earth are perfectly horizontal. So the magnetic needle will become horizontal there. Thus, the angle of dip at the magnetic equator of the Earth will be 0 o. The angle of dip varies from place to place.

Declination

Declination 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).

Dipolar approximation The variation between magnetic north (Nm) and "true" north (Ng).

Near the surface of the Earth, its magnetic field can be closely approximated by the field of amagnetic 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. The dipole is roughly equivalent to a powerful barmagnet, 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 80–90% of the field in most locations.[9] Magnetic poles The movement of Earth's North Magnetic Pole across the Canadian arctic, 1831–2001. The positions of the magnetic poles can be defined in at least two ways.[13] A magnetic dip pole is a point on the Earth's surface where the magnetic field is entirely vertical.[14] 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.[15] 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. Time dependence

Short-term variations The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere (ionospheric dynamo region) andmagnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the Earth's interior, particularly the iron-rich core.[9] Frequently, the Earth's magnetosphere is hit by solar flares causing geomagnetic storms, provoking displays of aurorae. The short-term instability of the magnetic field is measured with the K-index. Data from THEMIS show that the magnetic field, which interacts with the solar wind, is reduced when the magnetic orientation is aligned between Sun and Earth - opposite to the previous hypothesis. During forthcoming solar storms, this could result in blackouts and disruptions in artificial satellites.[18] Secular variation Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation. Over hundreds of years, magnetic declination is observed to vary over tens of degrees.[9] A movie on the right shows how global declinations have changed over the last few centuries.[19] The direction and intensity of the dipole change over time. Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century.[9] At this rate of decrease, the field would reach zero in about 1600 years.[20] However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.[21] A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2 degrees per year.[20] This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD.[22] Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to aspaleomagnetic secular variation or paleosecular variation (PSV). The records typically include long periods of small change with occasional large changes reflecting geomagnetic excursions and geomagnetic reversals.[23] Magnetic field reversals

Although the Earth's field is generally well approximated by a magnetic dipole with its axis near the rotational axis, there are occasional dramatic events where the North and Southgeomagnetic poles trade places. These events are called geomagnetic reversals. Evidence for these events can be found worldwide in basalts, sediment cores taken from the ocean floors, and seafloor magnetic anomalies. Reversals occur at apparently random intervals ranging from less than 0.1 million years to as much as 50 million years. The most recent such event, called the Brunhes–Matuyama reversal, occurred about 780,000 years ago.[24][25]

The past magnetic field is recorded mostly by iron oxides, such as magnetite, that have some form of ferrimagnetism or other magnetic ordering that allows the Earth's field to magnetize them. This remanent magnetization, or remanence, can be acquired in more than one way. In lava flows, the direction of the field is "frozen" in small magnetic particles as they cool, giving rise to a thermoremanent magnetization. In sediments, the orientation of magnetic particles acquires a slight bias towards the magnetic field as they are deposited on an ocean floor or lake bottom. This is called detrital remanent magnetization.[6] Thermoremanent magnetization is the form of remanence that gives rise to the magnetic anomalies around ocean ridges. As the seafloor spreads, magma wells up from the mantle and cools to form new basaltic crust. During the cooling, the basalt records the direction of the Earth's field. This new basalt forms on both sides of the ridge and moves away from it. When the Earth's field reverses, new basalt records the reversed direction. The result is a series of stripes that are symmetric about the ridge. A ship towing a magnetometer on the surface of the ocean can detect these stripes and infer the age of the ocean floor below. This provides information on the rate at which seafloor has spread in the past.[6] Radiometric dating of lava flows has been used to establish a geomagnetic polarity time scale, part of which is shown in the image. This forms the basis of magnetostratigraphy, a geophysical correlation technique that can be used to date both sedimentary and volcanic sequences as well as the seafloor magnetic anomalies.[6] Studies of lava flows on Steens Mountain, Oregon, indicate that the magnetic field could have shifted at a rate of up to 6 degrees per day at some time in Earth's history, which significantly challenges the popular understanding of how the Earth's magnetic field works.[26] Temporary dipole tilt variations that take the dipole axis across the equator and then back to the original polarity are known as excursions Geomagnetic secular variation

Geomagnetic secular variation refers to changes in the Earth's magnetic field on time scales of about a year or more. These changes mostly reflect changes in the Earth's interior, while more rapid changes mostly originate in the ionosphere or magnetosphere.[1] The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the Earth's interior, particularly the iron-rich core. These changes are referred to as secular variation.[1]

Secular variation can be observed in measurements at magnetic observatories, some of which have been around for hundreds of years (the Kew Observatory, for example). Over such a time scale, magnetic declination is observed to vary over tens of degrees.[1] A movie on the right shows how global declinations have changed over the last few centuries.[2] To analyze global patterns of change in the geomagnetic field, geophysicists fit the field data to a spherical harmonicexpansion (see International Geomagnetic Reference Field). The terms in this expansion can be divided into a dipolar part, like the field around a bar magnet, and a nondipolar part. The dipolar part dominates the geomagnetic field and determines the direction of the geomagnetic poles. The direction and intensity of the dipole change over time.[1] Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century. At this rate of decrease, the field would reach zero in about 1600 years.[3] However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.[4] A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2 degrees per year.[3] This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD. The Earth's magnetic field as both a tool and a        hazard in the modern world

Navigation 11th century A.D. There is evidence of compasses being used in Europe about 100 years later but the first observations of declination were not recorded until the 16thcentury. In 1700 the first magnetic chart, covering the Atlantic Ocean, was produced by Halley. Contemporary values of declination, or for some maps the difference between grid north and magnetic north, are depicted on the majority of modern sea charts, topographic maps and aeronautical charts. The earliest writings about compass navigation are credited to the Chinese and date from these values must be kept up-to-date by regular revision of the models from which they are derived. This is an example of the Earth's magnetic field being a tool. Directional drilling A specialist form of navigation is directional drilling in the petroleum industry. There are two main methods of navigation available for drilling deviated wells towards often small targets in the oil reservoirs. One method makes use of gyro tools but this can be expensive. The other method makes use of magnetic tools. As accuracy is critical for economic reasons and to avoid well collisions, the accuracy requirements on the Earth's magnetic field values, used to correct the direction of the well from magnetic bearings to true bearings, are typically 0.1° in direction and 50 nT in total intensity. To attain these accuracies account must be taken of the crustal field, daily variations and magnetic storms. This application is an example of the Earth's magnetic field being a tool in the modern world but it can also be a hazard if no account is taken of these other sources.

Geomagnetically induced currents The most severe magnetic storm in recent times occurred in March 1989 and this had a number of serious impacts on technological systems by generating damaging geomagnetically induced currents. In particular, the power transmission system in Quebec, Canada, was shut down over 9 hours. Other effects such as increased corrosion in pipelines are also likely. This is an example of the Earth's magnetic field being a hazard in the modern world.

Satellite operations When a magnetic storm is underway the Earth's atmosphere expands because of heating, and increases the atmospheric drag on satellites at altitudes below about 1000 km. The orbit of the satellite can be changed and sometimes expensive manoeuvres must be made to compensate. Other effects on satellites are caused by radiation hits which can interfere with onboard computers. The prediction of magnetic activity, as a monitor of conditions at satellite altitude, is therefore of great interest to satellite operators. This is another example of the Earth's magnetic field being a hazard in the modern world.

Exploration geophysics Total intensity traverses and surveys over an area can aid understanding of the underlying geology and in the case of iron ore deposits, can indicate very clearly their locations. This is an example of the Earth's magnetic field being a tool but it may turn into a hazard, especially at high latitudes, if care is not taken to remove the daily variations and magnetic storm effects from the data before interpretation. CONCLUSION •	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. It is approximately the field of a magnetic dipole tilted at an angle of 11 degrees with respect to the rotational axis—as if there were a bar magnet placed at that angle at the center of the Earth. •	 Main characters of the earth’s magnetic field are intensity, inclination, declination etc. •	A magnetic dip pole is a point on the Earth's surface where the magnetic field is entirely vertical. •	 The Earth's magnetic field as both a tool and a  hazard in the modern world