The magnetic phenomena exhibited by the earth and its atmosphere is known as geomagnetism. The strong magnetic field generated from deep within the earth is continually monitored and studied, allowing scientists valuable information for precise indexing of time (geochronology, paleomagnetism), determining crustal position (paleogeography), and abetting perhaps the most important revolution to unify modern geology: plate tectonics.
Magnetism is a force of attraction and repulsion between various substances brought on by the motion of electric charges of atoms. All planets in our solar system and several corollary moons have some variety of magnetic field that is distinctive to each body.
The Earth’s magnetic field is similar to a typical bar magnet acting as a dipole, where the magnetic force is concentrated at the north and south poles. The north magnetic pole is defined by the direction that a north-seeking magnet would tend when used near the planet. These poles are not generally located at the poles of rotation for a planet. At present, the Earth’s magnetic poles are approximately 11.5 degrees off the axis of rotation and are continually in motion.
A magnetic pole sets up a field in the space around an object that exerts a force on magnetic materials. The field can be visualized in terms of lines of induction or isolines drawn on an isomagnetic chart. The strength of the main magnetic field is mapped on isomagnetic charts. The contour lines representing constant values for horizontality are called isolines. However, the main polar fields are not the only sources of magnetism in a planet. Charts are individually drawn for the other components as well, such as nondipolar elements and for the total intensity. On Earth, there are many nondipole elements composing the total magnetic field that are local and more complicated than the main field. These additional elements make the Earth’s total magnetic field more intricate and can be furthermore utilized for a more comprehensive understanding of the Earth’s interior. The Earth can be seen to have an amalgamated field consisting of a strong dipolar main field and a superimposed nondipolar field, also known as the lithospheric field. Localized lithospheric magnetization, subsurface counterflow fluids processes, and magmatism create the independent multiple nondipolar poles.
From the clues and measurements of the ferromagnetic rocks and direct reading of the earth’s magnetic field, scientists are able to glean some information as to how this phenomenon works and how it perpetuates. The field is self-perpetuating, and the present source of the field is called the “Magnetohydrodynamic Dynamo.” Paleomagnetized rocks show that the Earth’s magnetic field is continuously regenerated and has been doing so for over 3.5 billion years. This is thought to be due to a well-working convection mechanism inside the Earth in which hot fluid rock circulation facilitates electric currents and magnetic fields as Earth rotates. The differential rotational movement of Earth’s layers furthermore complicates the motion of the fluid through a resistivity at the boundaries and a torque exerted by Earth’s rotational movement known as the “Coreolis effect.” The currents and the induced magnetic flux interact, with the existing magnetic field dissipating or reinforcing it continuously. These processes in concert with the Earth’s rotation are predicted by the Magnetohydrodynamic Dynamo theory, which attempts to explain the perpetuation of the earth’s magnetic field.
The configuration of the earth’s magnetic field as a dipole allows for a phenomenon of colorful curtains that appear to dance across the sky. Known as auroras, they can extend for 100 km arcing near the northern (boreal auroral zone) and southern (austral auroral zone) latitudes. Auroras are the signatures of charged particles that are constantly bombarding the upper atmosphere. As the electrons traveling along a magnetic field line enter the high-altitude atmosphere, the collision with a higher-energy particle strips an electron from the air molecule. This ionizes the molecule and increases its electrical conductivity, releasing a photon of light and giving the auroras their distinctive appearance.
Through the science of geomagnetism, the study of the earth’s interior has been greatly advanced. The complexities of the magnetic field beyond the dipolar orientation have allowed for a more complete picture of the interior of the earth. However, it is the dipolar element to the field that humanity relies on most for beauty and protection. The same field that generates the striking auroras in the northern and southern latitudes also protects the earth from the sun’s harmful radiation. Without such a field, the earth could have never developed and flourished, teaming with life and wonderful splendor. Humans, as indeed all life, owe our very existence to the dynamic processes deep within the earth that we are just starting to recognize and understand.
- Backus, G., Parker, R., & Constable, C. (1996). Foundations of geomagnetism. Cambridge: Cambridge University Press.
- Biskamp, D. (2003). Magnetohydrodynamic turbulence. Cambridge: Cambridge University Press.
- McElhinny, M. W., & Merrill, R. T. (1983). The earth’s magnetic field: Its history, origin and planetary perspective. Orlando, FL: Academic Press.