Origin
>the dynamo theory
No one knows for sure about the origin of the magnetic field, however, it is widely accepted that a dynamo within the Earth generates it. Walter M. Elsasser is considered to be the pioneering scientist behind this theory
Earth's interior predominantly consists of highly electrical conductive materials such as Iron and Nickel. These fluids are good conductors but needs to be molten in order to create the dynamo that is required for the magnetic field. Heat trapped with the Earth from its formation and from continuous radioactive decay keeps part of the core molten. Impurities (such as Sulphur) in the Earth can also have an effect on the state of the interior since it has the ability to alter the melting point of materials such as Iron; impurities would lower the melting point and fluids within the core must stay above the eutectic point in order to stay liquid.
The heat trapped in the Earth would have escaped into the coldness of space overtime and this cooling would lead to the formation of the solid inner core. Cooling of the interior appears right above the solid inner core where it borders the liquid outer core, thus there is a chemical discontinuity. The fluid deep in the Earth's interior is less dense and would rise to the surface of the outer core. Fluid close to the mantle on the other hand hand, cools by losing heat to the cooler mantle and is therefore less dense then the fluid below it, this would cause it to sink back into the interior. The result of this are the convection currents within the Earth which is crucial to driving the planetary dynamo.
In order for this theory to work, the core cannot be completely solid or liquid. If it's completely solid, it would be impossible for the material to move. If it's completely liquid, there would be no temperature gradient across the core and thus no material movement or convection currents. Mapping of the Earth's interior with the help of seismology confirms that the Earth's core is partially liquid and partially solid.
One additional requirement which is vital for the dynamo is the rotation of the planet. The rotation would “stir up” the materials within the core by the Coriolis effect. The latter is in fact the effect engendered by the rotation of the planet, deflecting movements in the clockwise direction in the Northern hemisphere, and anticlockwise in the Southern hemisphere.
Thus it deflects the convection currents and organizes them into “rolls” that are thought to give rise to the magnetic forces. Faster rotation would lead to more “rolls” therefore giving rise to a stronger magnetic field.
According to the geodynamo theory, the fundamental requirements of a magnetic field are convection currents, rotation of the planet and an electrically conductive core.
Earth's interior predominantly consists of highly electrical conductive materials such as Iron and Nickel. These fluids are good conductors but needs to be molten in order to create the dynamo that is required for the magnetic field. Heat trapped with the Earth from its formation and from continuous radioactive decay keeps part of the core molten. Impurities (such as Sulphur) in the Earth can also have an effect on the state of the interior since it has the ability to alter the melting point of materials such as Iron; impurities would lower the melting point and fluids within the core must stay above the eutectic point in order to stay liquid.
The heat trapped in the Earth would have escaped into the coldness of space overtime and this cooling would lead to the formation of the solid inner core. Cooling of the interior appears right above the solid inner core where it borders the liquid outer core, thus there is a chemical discontinuity. The fluid deep in the Earth's interior is less dense and would rise to the surface of the outer core. Fluid close to the mantle on the other hand hand, cools by losing heat to the cooler mantle and is therefore less dense then the fluid below it, this would cause it to sink back into the interior. The result of this are the convection currents within the Earth which is crucial to driving the planetary dynamo.
In order for this theory to work, the core cannot be completely solid or liquid. If it's completely solid, it would be impossible for the material to move. If it's completely liquid, there would be no temperature gradient across the core and thus no material movement or convection currents. Mapping of the Earth's interior with the help of seismology confirms that the Earth's core is partially liquid and partially solid.
One additional requirement which is vital for the dynamo is the rotation of the planet. The rotation would “stir up” the materials within the core by the Coriolis effect. The latter is in fact the effect engendered by the rotation of the planet, deflecting movements in the clockwise direction in the Northern hemisphere, and anticlockwise in the Southern hemisphere.
Thus it deflects the convection currents and organizes them into “rolls” that are thought to give rise to the magnetic forces. Faster rotation would lead to more “rolls” therefore giving rise to a stronger magnetic field.
According to the geodynamo theory, the fundamental requirements of a magnetic field are convection currents, rotation of the planet and an electrically conductive core.
An image showing the Coriolis effect in the core creating "rolls" of material that create the magnetic field. Image from: http://www.abc.net.au
>Non-dynamo origins
Although the dynamo theory is most widely accepted, there are other theories. There have been suggestions that the magnetic field is induced by the Sun. There are also theories that simply the rotation of a planet is enough to create a magnetic field. Some have also suggested that gravity from neighbouring satellites is the cause of convection currents in the Earth but none of these pose a real challenge to the dynamo theory.
Earth's Interior
Earth's density is 5,515 kg/m3, which is of a much higher density than the minerals found on the surface; signifying that the interior must consist of denser material (found to be predominantly nickel and Iron). The interior can be divided into 5 main pa: lithosphere, asthenosphere, mesosphere, outer core, and the inner core. It is the outer and inner cores that are responsible for the magnetic field.
Using seismic waves produced by earthquake, we've been able to prove that Earth's core is partly solid and liquid. S-waves do not pass through liquids and causes a “shadow” on the other side of the origin of the wave. P- waves on the other hand can travel through gases, liquids and solids although it travels slower in liquids. S-waves do not travel through the outer core and P-waves are deflected by the outer core. By studying seismic tomography data from seismometers placed in different parts of the Earth, seismologists have mapped the interior of Earth with great accuracy.
Using seismic waves produced by earthquake, we've been able to prove that Earth's core is partly solid and liquid. S-waves do not pass through liquids and causes a “shadow” on the other side of the origin of the wave. P- waves on the other hand can travel through gases, liquids and solids although it travels slower in liquids. S-waves do not travel through the outer core and P-waves are deflected by the outer core. By studying seismic tomography data from seismometers placed in different parts of the Earth, seismologists have mapped the interior of Earth with great accuracy.
Earth's Interior mapped by using P & S waves Image from: ck12.org
Other planets
A comparison to other terrestrial planets in our solar system have supported the dynamo theory.
Despite its small size, Mercury has a magnetic field which came as a surprise when first discovered by Mariner 10. Of the 3 fundamental properties needed for the dynamo theory to work. It is almost the same density as the Earth meaning it is made of dense material similar to that of Earth's and could be partially solid; this could give rise to convection currents. It also has a relatively fast rotational speed orbiting its axis 3 times for every 2 orbits around the Sun.
Venus is the similar size to Earth and is geologically active, however, it lacks a magnetic field. Venus has a dense core consisting of electrically conductive material but most scientists believe that it lacks the dynamo because of its slow rotation of 243 days. Scientists also suspect that the core is either completely solid or liquid meaning it would lack
Despite its small size, Mercury has a magnetic field which came as a surprise when first discovered by Mariner 10. Of the 3 fundamental properties needed for the dynamo theory to work. It is almost the same density as the Earth meaning it is made of dense material similar to that of Earth's and could be partially solid; this could give rise to convection currents. It also has a relatively fast rotational speed orbiting its axis 3 times for every 2 orbits around the Sun.
Venus is the similar size to Earth and is geologically active, however, it lacks a magnetic field. Venus has a dense core consisting of electrically conductive material but most scientists believe that it lacks the dynamo because of its slow rotation of 243 days. Scientists also suspect that the core is either completely solid or liquid meaning it would lack