The Geomagnetic Field and its Variations
The Earth’s magnetic field extends from its inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun. It approximates the field of a magnetic dipole tilted at an angle of about 11o22’ 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, the Earth's field changes over time because it is really generated by the motion of molten iron alloys in the Earth's outer core. 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 harmful ultraviolet radiation and cosmic rays and charged particles from the sun by deflecting most of them. If it were no for this protection, the charged particles would strip away the ozone layer which protects the Earth’s surface from harmful ultraviolet rays. 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.
The record of the Earth's field as it varied over the geological past is preserved in rocks. Reversals of the field have left a series of stripes on the seafloor that made it possible to time seafloor spreading. Paleomagnetists have been able to track the motion of continents in the past. Reversals also provide the basis for magnetostratigraphy, a way of dating rocks and sediments. Magnetic anomalies can be used to search for ores.
Characteristics of the Earth’s Magnetic Field
Near 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 11° with respect to the rotational axis. The geomagnetic south pole points towards the geographic north pole, as is evident from the fact that when an ordinary bar magnet is suspended at the Earth’s surface, its north pole points towards the geographic north. This may seem surprising, but the north pole of a magnet is so defined because it is attracted towards the south pole of another magnet. This dipolar field accounts for 80–90% of the total geomagnetic field strength in most locations on the earth’s surface.
At any location, the Earth's magnetic field can be represented by a three-dimensional vector. If a compass needle is suspended from a string such that it can rotate in any direction, then the direction it points in is the direction of the Earth's field, with the north pole of the compass pointing roughly north, its inclination or dip is the deviation from the vertical, while the declination is the angle the needle would make with true north if it were constrained to lie in a horizontal plane (as in an ordinary compass).
Intensity: The intensity of Earth’s magnetic field is reported in nanoteslas (nT) or gauss, with 1 gauss = 100,000 nT. The geomagnetic field is greatest near the poles and weaker near the Equator. It ranges from about 25,000–65,000 nT, or 0.25–0.65 gauss. By comparison, a strong bar magnet has a field of about 100 gauss. The minimum strength of the Earth’s field occurs over South America while there are maxima over northern Canada, Siberia, and the coast of Antarctica south of Australia. The intensity of the field is also reported in gammas, where 1 gamma = 1 nT or 10-5 gauss.
Inclination: The inclination is given by an angle that can assume values between 90°(up) to 90°(down). In the northern hemisphere, the vector of the geomagnetic 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.
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 true or geographic north. 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).
Magnetic Poles: The positions of the magnetic poles can be defined in at least two ways. Often, a magnetic pole is viewed as a point on the Earth's surface where the magnetic field is entirely vertical. Another way of saying this is that 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 do not occupy directly opposite positions on the globe. They can migrate rapidly – movements of up to 40 km per year have been observed for the north magnetic pole. 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 and magnetic dip poles would coincide and compasses would point towards them. However, the Earth's field has a significant contribution from non-dipolar influences, so the poles do not coincide and compasses do not generally point at either.
Magnetosphere: The Earth’s magnetosphere is the region surrounding the earth where its magnetic field dominates. In spite of its low density, the solar wind, and its accompanying magnet field, is strong enough to interact with the magnetosphere and distort it in different ways. The shape of the Earth's magnetosphere is the direct result of being blasted by solar wind. The sunward side is compressed to a distance of only 6 to 10 times the radius of the Earth, while the night-side magnetosphere is dragged out to possibly 1000 times Earth's radius (its exact length is not known). Many other planets in our solar system have magnetospheres of similar, solar wind-influenced shapes.
Because the ions in the solar plasma are charged, they interact with the magnetosphere and the particles are swept around. Life on Earth has developed under the protection of the magnetosphere.
Fluctuations in speed, density, direction, and associated magnetic field of the solar winds strongly affect Earth's local space environment. For example, the levels of ionizing radiation and radio interference can vary by factors of hundreds to thousands; and its compression upwind can expose geosynchronous satellites to solar wind. These phenomena are collectively called space weather. Our nearest planetary neighbors, Mars and Venus, have no oceans or lakes or rivers. Some researchers have speculated that they were blown dry by the solar wind, and that our Earth escaped this fate because its strong magnetic field deflects the solar wind. Variations in the magnetic field strength have been correlated to rainfall variation within the tropics. 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 maneuvers must be made to compensate.
Variations of Earth’s Magnetic Field
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 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.
Frequently, the Earth's magnetosphere is hit by solar flares causing geomagnetic storms, provoking displays of auroras.
Secular variations: Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation. Magnetic declination is observed to vary by tens of degrees over hundreds of years. Even the intensity of the dipole changes over time. 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. However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.
A prominent feature in the non-dipolar part of the secular variation is a westward drift. It has been noticed that the largr-scale anomalies of the geomagnetic field move westwards in a latitudinal direction at a rate of about 0.2 degrees per year. 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.
Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as paleomagnetic 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.
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 South geomagnetic 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.
What can be Learned from observing the Earth’s Magnetic Field
Composition and configuration of the crust: The relief of metamorphic or igneous terrain buried under kilometers of sediments can be mapped from magnetic anomalies, exploiting the knowledge that sedimentary rocks are generally non-magnetic. Paleomagnetism gives clues to the past rate and direction of continental drift.
Dynamics of the inner Earth: The configuration of the field and its secular change, along with paleomagnetic data, builds our understanding of the colossal forces at work in the deep Earth.
Solar activity: Magnetic observation data of solar events is one basis for the formulation of theories of solar processes.
Commercial Mining: Magnetic anomalies betray ferromagnetic ores such as iron, nickel and cobalt; or diamond deposits associated with kimberlite minerals (magnesium rich ilmenite, olivine, chrome diopside and pyrope garnets); as well as precious metals.
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Department of Geology
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