Geomagnetism
1. Introduction to Geomagnetism
Variations in the geomagnetic field have been recorded and studied for a few centuries now, while the understanding of the Earth’s internal magnetic structure has come more recently, within the last 100 years. The discovery of the Earth’s magnetosphere in the space age has provided another huge area of study of the Earth’s magnetic field and its effects on the solar wind, and the more recent concept of “space weather”, involving studies into the effects of solar activity on the geomagnetic field. There are now vast areas of geomagnetism, and so, for the sake of being concise, this work will revolve mainly around the basic concepts of the generation and properties of the Earth’s magnetic field, relatively recent discoveries, and some concepts still being researched.
The Earth’s magnetic field and its effects on the Earth and its surroundings are of interest to many. Geomagnetism is the study of the Earth’s magnetic field, and the science can be dated back to the 13th and 14th centuries. However, although significant on its own to the Earth’s natural state, changes in the Earth’s magnetic field can be linked to many basic geophysical processes and properties. It is rare indeed to find a geophysical experiment that does not seek to provide information that will improve our understanding of the Earth’s magnetic field.
2. Magnetic Field of the Earth
Declination and Inclination: When a compass is placed at a point, it does not point towards the true north pole, i.e., the line joining the geographical north and south poles. This indicates that the direction of the magnetic field at that point is not parallel to the geographical axis. The angle between the geographical and magnetic axis at a place is called the angle of declination of Earth’s magnetic field. In India, the needle normally points towards the northeast direction. This means that the declination is westward. The line joining the points of equal declination is called an isogonic line. The declination changes with time. Inclination is the angle made by the direction of Earth’s magnetic field with the horizontal direction. This means that the vertical component of Earth’s magnetic field is Bv = B sin i, where i is the angle of inclination. In India, the inclination is about 35 to 36, and it is increasing at the rate of 30′ per year. The line joining the places of equal inclination is called an isoclinic line.
Magnetic field of Earth: Let us assume that a bar magnet is placed at the centre of the Earth and is aligned along the polar axis. The south pole of the bar magnet will be near the geographical north pole of the Earth and the north pole of the bar magnet will be near the geographical south pole of the Earth. In this case, the magnetic characteristics of Earth can be explained. If a magnetic needle is freely suspended at the centre of the Earth, then it will lose its directive property, i.e., it will not point in any particular direction. This means that the horizontal component of Earth’s magnetic field will be zero. The field lines will pass normally through the surface of Earth because the needle will be acted upon only by the field due to the magnet, and no force will be experienced due to magnetic induction in the needle.
3. Geomagnetic Poles and Equator
In calculating the radial force acting on a small magnetic element, it was shown above that the force is proportional to BHsinI, where I is declination. On summing forces for all elements making up a bar magnet, the radial force is found to be greatest when the bar is oriented at 90° to the geomagnetic field. Hence, ionospheric currents show preferential flow across magnetic field lines and tend to concentrate at high latitudes. It is the locations of this current concentration which appear as the auroral ovals. By mapping the position where the classical, ring current produced by a magnetic storm is centered, it has been found that this too is located close to the auroral ovals. This is an example of “magnetic mirroring”.
Geomagnetic field lines can pass through all points on the Earth’s surface. However, in a first approximation, results show that they behave as if they pass through a point only above the Earth’s surface. This location is generally in a different place from the equivalent point for the magnetic field. The magnetic poles are defined as the points where the geomagnetic axis meets the Earth’s surface. They could be defined equally as the positions where the inclination is 90°, i.e. the field lines are vertical. Theory predicts that the magnetic poles should coincide with points where the radial component (downward or upward) of the geomagnetic field is maximum.
4. Effects of Geomagnetism
The magnetotail is formed by magnetic reconnection processes similar to those occurring in the solar wind with the field lines of the solar wind, but reconnection with the solar wind is an ongoing process and the magnetotail is convected outwards and eventually the dipolar configuration of the magnetic field is restored by dayside reconnection.
The solar wind flow around the dayside magnetosphere is a flow around an obstacle. The magnetosphere reduces or magnetically shields the solar wind magnetic field pressure from the side facing the Sun, i.e. about 10% of the surface area of the Earth. This pressure force causes a long terrestrial ‘talisman’ to form, stretched out in the antisunward direction. This is the magnetotail. Changes in the solar wind pressure cause changes in the configuration of the magnetosphere and energy flows directly from the solar wind into the magnetosphere.
Magnetic fields provide important information on the dynamics of the core, mantle, and crust. Measurement of the magnetic field at the Earth’s surface reveals the nature of the source of the field and its variation with time provides information on the dynamic processes in the Earth’s interior. The energy for geomagnetic activity is derived from the kinetic energy of the solar wind flow.
Magnetic storms are disturbances in the geomagnetic field caused by solar activity. They are felt as worldwide depressions of the horizontal field, strongest at the equator, with associated ionospheric disturbances and increases in high-energy particle radiation. Magnetic storms are a part of a worldwide geomagnetic substorm which is caused by the release of energy stored in the magnetotail when the reconnection with the solar wind results in a change to a more dipolar configuration. The ring current is an important part of magnetic storms. It is a dipole-like current system encircling the Earth, located at a distance of 3 to 7 Earth radii in the equatorial plane. This current system is associated with a decrease in field intensity at high latitudes and an increase at low latitudes. The magnetic effects of the ring current increase the ionospheric current systems which create large changes to the lower ionosphere and significantly modify the electrical conductivity and the density distribution in the ionosphere. An enhanced ionospheric conductivity is responsible for worldwide effects on long wavelength radio propagation and the increased energetic particle precipitation can cause damage to geostationary satellites and has been a cause of failure of some power transformers on long electrical transmission lines.
5. Applications of Geomagnetism
Even the increasingly popular tourist is not immune from the effects of space physics, for it is necessary to ascertain that flight paths run clear of regions where there is danger of high ionospheric radiation to aircrew and passengers. A still developing application of geomagnetic data is the use of detailed field models to gain information on the electrical properties of the Earth’s crust and upper mantle, for example in regions of volcanic and earthquake activity. Overall, such wide application of geomagnetic data keeps its practitioners in touch with a broad spectrum of earth scientists and technologists.
The strength of the field and its direction are crucial parameters in the operation of magnetic storage media such as tapes and disk drives, and an understanding of the effects of the solar wind and solar-generated magnetic storms on the magnetosphere and ionosphere is essential for plasma physics, meteorology, and satellite communication. This in turn affects a wide range of modern technology; for example, the operation of satellites involves corrections to their orbital dynamics, and the rate of corrosion of oil pipelines is influenced by their becoming long conductors in a changing magnetic field.
A classic case is the magnetic compass, in use for a millennium and a half, which still finds essential application in marine and aeronautical navigation. An understanding of the temporal and spatial behavior of the core-generated magnetic field is needed to correct compass readings to true north, and also to provide secular changes to be applied in gyromagnetic compasses. This knowledge is based upon a global network of magnetic observatories, each of which continuously records the Earth’s magnetic field by means of three components of flux density.
Applications of geomagnetism are as fundamental to life in the twenty-first century as the study of paleomagnetism itself. There is a wide variety of uses to which data on the nature and behavior of the Earth’s magnetic field is put, and these greatly influence both the methodology of geomagnetic measurement and the areas in which such measurements are made. Our dependence on a range of electrically-based technologies has led to a concomitant increasing reliance on geomagnetic data for calibration or correction of the associated apparatus.
