GS9: Basics of Earth's Magnetic Field
Updated: Jul 27, 2020
Magnets were discovered even before the scientific records were maintained, so there is no known precedent of how we came across the magnets? Mankind was introduced to magnetism with the very strange behaving ’lodestone- meaning leading stone ’. The loadstone was, actually, a stone containing naturally occurring iron ore magnetite that helped travelers to navigate. Nevertheless, Greek philosophers talked about lodestone around 800 BC and its unusual properties were known to Chinese by 300 BC. Being unaware of the origin, Greeks believed lodestone to possess a soul and ascribed to metaphysical power.
In the Han dynasty, between 300 and 200 BC, the Chinese created a rudimentary compass out of lodestone. In the Tang dynasty around 500 AD, travelers were well aware that magnetic compass did not point exactly to geographical north
Image Credit: NASA
Origin of the word Magnet
One of the greatest and wealthiest of the ancient Greek city-colonies in Asia Minor was the seaport of Ephesus, at the river Meander (term Meandering originates from the nature of this river) in the Persian province of Caria (presently Turkish province). A new colony was founded in the 5th century BC called Magnesia on the Meander. There was a large ready supply of lodestone available in the vicinity of this colony and subsequently known by the Latin word magneta from which the term magnetism derives. Because of the attractive property of the lodestone the shepherds of the region found it difficult to walk with their wooden shoes on which had irons nails. They had iron-tipped rods that were similarly affected.
In 1600, William Gilbert (1544–1603), an English scientist and physician to Queen Elizabeth distinguished clearly between electrical and magnetic phenomena. He was also the first person, perhaps, to demonstrate that the terrestrial globe behaves like a giant magnet.
The physical origins of terrestrial magnetism
By the end of the 18th century many characteristics of terrestrial magnetism were known. However, the origin and more fundamental understanding of magnetism was still a subject of research. Many of us live our lives completely unaware of the magnetic field overhead and around the Earth and take the field for granted. But not all planets have magnetic fields, even the ones which are more similar to Earth- Mars and Venus, do not have measurable magnetic fields. So, why does Earth have a magnetic field?
Search for the answer to this question began around 1926 when scientists learned that the planet’s outer core consists of liquid iron alloy based on geophysical evidence. Physicists already proved that the flow of metal, basically the negative and positive charges in it, can produce an electric current and a moving non-accelerated charge also generates a field of magnetism. So it seemed that the flow of the outer core must be responsible for the magnetic field. But how?
Outer core behavior as a dynamo was investigated by W. Elsasser in the USA and Edward Buller in England.
Clues to understanding the generation of the Earth’s magnetic field comes from the understanding of the working of a simple instrument called a dynamo. For a dynamo, when some mechanical power spins a wire coil (an electric conductor) wrapped around an iron bar (a permanent magnet), the motion of the wire in the bar’s magnetic field generates an electric current in the wire. In the case of Earth, thermal convection cells serve the role of the spinning wire coil making the outer core behave like an electromagnet. However, the inner core probably cannot serve the role of the permanent magnet, because it is too hot- magnets can only be permanent at a temperature below the Curie point; above which material loses its magnetic property. The only solution to this problem was a Self- exciting dynamo.
The dipole is displaced 300 km from Earth's center towards Indonesia and is inclined at an angle of 11.5 degrees to the Earth's axis.
Evidently, during the Earth’s early history, flow in the outer core started in the presence of a magnetic field. This flow generated electric current, and once the current was established it generated a magnetic field. Continued flow in the presence of the generated magnetic field produced more electric current that, in turn, maintained the magnetic field. Once started, the system perpetuates itself, as long as there is an input of energy to keep the liquid iron alloy of the outer core in motion (radioactive decay and heat from the inner core).
The Curie point of a material decreases with increasing pressure.
What causes the outer core to keep moving?
Recent studies suggest that as the Earth cools overall, the diameter of the solid inner core is increasing (by about 0.1 to 1 mm in diameter per year) at the expense of the outer core. The process of crystallization releases both heat (the latent heat of crystallization) and lower-density elements (such as silicon, carbon, sulfur, hydrogen, or oxygen) into the base of the outer core. These elements are expelled from the inner core as they are believed to be present in the lattice of solid in the inner core. The higher concentration and the higher temperature at the base of the outer core make it positively buoyant than the top, setting up a convention in the liquid outer core. And, the less dense materials rise from the base to top.
Earth’s Rotation and its Magnetic field
The understanding of the polarity of the Earth’s magnetic field comes from understanding the geometry of the convection cells in the outer core. The rotation of the Earth sets up Coriolis force in the outer core too, like on the surface of the Earth. The Coriolis force causes convective cells in the outer core to become spirals that align roughly with the Earth’s spin axis. The spirals wobble, therefore, the overall dipole axis is not exactly parallel to the spin axis. Also, the spirals may apply a force to the inner core, causing it to rotate slightly faster than the rest of the Earth. This phenomenon is known as superrotation. Also, the polarity reversals happen because the spirals are unstable and over time they slow and fade away causing the magnetic field to weaken or disappear temporarily. As new spirals become established, the field reappears, possibly with a different polarity. Earth has seen numerous polarity reversals since its inception which we know from the rock record where these reversals are recorded by the alignment of magnetic domains along the magnetic field present at the time of formation of the rock.
Watch the video at the end!
Reason for the initial magnetic field
Scientists are perplexed with the reason for the original magnetic field which generates convection currents in the first place. Few reasons are given by the scholars for the weak initial field which could have started in a variety of ways when the Earth was being formed like a planet. However, our understanding of the formation of the planet is still a little vague, the matter is no more than theoretical interest at present.
Three primary sources of energy for maintaining the field are considered:
Energy generated from radioactive decay. But our understanding of the content of radioactive substances in the core is not very clear. This source is still a subject of exciting research.
Another source of energy is downward migration of iron from the mantle to the core, a process that could release gravitational energy and generate thermal energy. This hypothesis cannot be theorized as if the process happened at all, it should have come to an end a long ago or have now become slow to supply any significant amount of heat.
Another hypothesis for the source of energy is latent heat which is liberated as a result of the phase change as a result of a decrease of pressure (based on the work of Dirac, Ramsey, and Egyed).
More possible reason is the release of gravitational potential energy in the downward migration and freezing out of iron from the liquid outer core to form the solid inner core which gradually leads to its expansion
Why Earth's Magnetic Shield Matters by Science Channel
1. William Lowrie: Fundamentals of Geophysics
2. Stephen Marshak: Essentials of Geology
3. Arthur Holmes: Principles of physical geology