Geomagnetism

Magnetism and gravity are two fundamental properties of the Earth and are innate to our planet’s existence. But, the discovery of magnetism was not as dramatic as that of gravity. By deciphering the reason responsible for the fall of an apple, Newton opened the gates to understanding many of the basic principles governing the Universe. The circumstances leading to understanding of magnetism, and in essence geomagnetism, however, were slow and gradual.

Earth is innately magnetic and owes this property to its dynamic internal physicochemical processes. The assertion that Earth is a giant magnet should be taken figuratively. The geomagnetic field is not produced by any bar magnet situated within the Earth, though the shape of the field is identical to a magnetic dipole with the south pole actually placed in the northern hemisphere. Visualising the Earth as a uniformly magnetized sphere, the lines of force near the poles are close together providing a relatively stronger field, while near the equator the field has about half the poles intensity. The farther one goes north or south from equator, the angle with the surface (magnetic dip) increases and it becomes vertical near the poles. The points on the surface of the Earth, where the continuation of the dipole axis cuts the Earth’s surface, are called magnetic poles. The south magnetic pole is located in the northern part of Greenland near Thule MO (74°N and 100°W), and the north magnetic pole is at the Victoria land in Antarctica (65°S and 145°E). The magnetic axis of the Earth’s geomagnetic field is situated ~436 km away from the Earth’s centre, and so is referred to as the eccentric dipole field. The position of the magnetic poles varies with the passage of time known as polar wandering.

The advent of balloons, rockets, satellites and space probes is of great help in exploring external magnetic field and aeronomic changes. This study requires sophistication achieved by USA, Russia, and European community. The contribution by India is limited to the analyses and studies of these procured satellite and space data. The geomagnetic surface data alone provide no clue, unless the process of solar wind and solar plasma stream interactions with the geomagnetic field are better understood and modelled. The cause in the form of solar wind interaction produces various effects such as ring current, charged particle diffusion, scattering and final precipitation in the auroral zone. Simple Ohm’s law and Ampere’s law are at work in the production of various currents and geomagnetic field changes on the global scale. The morphological changes in the geomagnetic field play an important role in generating micropulsations and accelerating charged particles by annihilating magnetic field at the X-type neutral point in the geomagnetic tail.

Any form of data whether acquired at an observatory or through experiments concerning studies of space and solid Earth described in this book, need instruments. These instruments, essentially magnetometers, are distinguished not only by the component of the field they measure, but also by the principle of their working. The principles employed in magnetometers range from the elementary laws of forces acting on a magnetic needle, to the technique of optical pumping. There are different types of magnetometers, which include theodolite magnetometers, torsion magnetometers, variometers, Lloyd’s and Schmidt’s balances and Earth inductors. The recent additions are the nuclear magnetic resonance magnetometers, saturable-core magnetometers (fluxgate magnetometers), induction magnetometers and more. The magnetometers introduced in the late twentieth century include proton precession magnetometers (PPM), optical pumped sensors, super quantum interference devices (Squids) and others. The most important change in instrumentation in the last half-century, however, is the automation of observation and the direct connection of sensors to data storage and computational facilities. This chapter outlines the importance and use of three categories of instruments in magnetic measurements: magnetic observatories, ground and marine magnetic surveys and laboratory magnetometers. Some “current” instruments are also included in these three categories of magnetic measurements.

The geophysical discipline of geomagnetism carries out scientific study of the Earth’s external and internal field comprising secular change, and magnetic reversals. This is a multidisciplinary science and magnetic observatories (MOs) provide data to those institutes, whose interests range from geology, subsurface structural mapping, geophysics (including seismology and earthquake prediction), meteorology, solar-terrestrial physics, and astronomy. Most of the surface magnetic phenomena exhibit strong dependence on both latitude and longitude. A worldwide network of magnetic stations is, therefore, absolutely essential for continuous monitoring of the ionospheric and magnetospheric phenomenon. Much depends on the accuracy and stability of the input magnetic data from magnetic observatories.

One of the fundamental issues in geosciences relates to formation and evolution of the crust, whose understanding has increased many-fold with advanced geophysical techniques. The geological history spans from Archaean, 3.8 Giga years (1 Giga=10⁹), to the present (Neogene). Geophysical methods and techniques investigate the structure, composition and physical state of the Earth by mapping crustal anomalies associated with mineral deposits, structural features, and hydrocarbons. Since these constituents lie hidden beneath the surface, geophysics has become a preferred tool of exploration compared to geological techniques. The present section deals with crustal anomalies, and deliberates on their form and detection using appropriate geophysical procedures.

Experimental geomagnetism deals with experimental observations and their potential applications of palaeomagnetic and environmental magnetic investigations. Palaeomagnetism is a specialized study, which has provided positive evidence of continental drift and development of the hypothesis of plate tectonics. The other major contributions relate to understanding of the generation of EMF, palaeointensity, relative movements of continental blocks, concept of tectonostratigraphic terrains and magnetostratigraphy as a dating tool. All these varied research activities require the development and use of extremely sensitive instruments.

The Earth’s atmosphere occupies some million times greater volume than the solid Earth. In this huge system, the charged plasma particles react strongly to electric and magnetic fields. Hence, electrical processes in one part of the system can influence the electrodynamical processes in another distant part. The redistribution of the charged particles in turn can modify the existing electric and magnetic fields in the atmosphere. Hence, an investigation of electrodynamical processes in various regions of the atmosphere and their coupling is very important for understanding the state of electrical environment of the Earth’s atmosphere.

Geomagnetism has applications in navigation, communication, space travel (manned or unmanned), power generation, in the search for minerals and hydrocarbons, in dating rock sequences and in unravelling past geologic movements such as plate tectonics. For studies related to different fields, suitably located MOs are operated throughout the country (Fig. 1.14) and worldwide (Fig. 5.1) to record strength, intensity, and direction of the geomagnetic field. It varies spatially and temporally on a very wide frequency spectrum of signals, whose characteristic times extend from geological to historical to millennial to centuries to subsec intervals (Figs 5.3b and 9.1). The longer timescales, typically those occurring over decades to millennia, are relevant to discern planetary magnetism. The long-period SVs of the main field are used for studying the dynamo action in Earth’s core and the overlying mantle as well as CMB. Short-term variability finds several important applications related to Sun-Earth connection as well as geophysical prospecting of the Earth’s deep interior. The transient type of short-period variations are used for studying the mantle and the crust (see EM induction methods). The small-scale anomalies which ride on the expected magnetic field give information about the crust of the Earth and the presence of petroleum and mineral deposits within it. The metal ores are concentrated in rocks rich in magnetites that are highly magnetic.

The genius of people like Norman, Gilbert, Faraday, Oersted, Ampere and Maxwell gave geomagnetism a strong footing to tackle fundamental problems related to Earth and interplanetary space. These discoveries whetted the curiosity of the inquisitive mind to unravel the causative agents and look for interrelationships between the Earth, the Sun and other planets or satellites and use this knowledge to predictive purposes. Intricacies of changes in the geomagnetic field are studied because they change over several timescales from few million years to fraction of a second in several spectral bands. Each band of frequencies is a goldmine of information indicative of various causative mechanisms with sources in the Earth’s interior, near space or far space environment as outlined in Chapters 1 to 8. Specifically, Chapters 5 to 8 list out applications of geomagnetic measurements to understand the chemical, physical and dynamical characteristics of the atmosphere and the interior of the planet Earth. In virtually every case presented in this book, there is considerable scope for further development in exploratory techniques, analysis, interpretation and application. Prospects for the future, lie in further extension and refinement of established approaches used in magnetic observatory, upper atmospheric and solid Earth studies.