The Geomagnetic Field

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1 Magnetic fields as phenomena/concept

Definition (sort of): A common physical phenomenon of fundamental importance in all mechanical and electrical processes, a magnetic field is the product of charged particles in motion.

1.1 What is magnetism?
In the widest sense, all matter (even split into the tiniest pieces) has magnetic properties. This is due to the fact that electrons, by nature, are restless entities. Firstly, as electrons are set free from atomic nuclei, they can immediately be attracted by positive ions and create an electric current. Secondly, even electrons that are not released into a current but locked in (relatively) fixed positions, orbit the atomic nucleus they belong to and at the same time are in constant spin. (You could make an analogy with a soccer ball: when kicked, it is both put into rotation and set into flight along a curved path.)
In most materials, the myriads of electrons that a slice of the material consists of behave so chaotically that their respective charges cancel each other out, making the molecules and larger structures of the material non-magnetic (or dia­magnetic). But certain materials (such as iron) are ferromagnetic (or in some cases ferrimagnetic) and can under­go magnetization, which means that the electrons get symmetrically aligned in a manner that makes their charges “work together” in maintaining a relatively strong and long-lasting magnetic force. Even though the magnetization actually fades over time, on a human time scale an object with ferromagnetic properties can be seen as a permanent magnet.
All known magnets (including Earth) are dipoles with one North and one South Pole, attracting the opposite and repulsing the identical. Between the poles, magnetic forces act on particles in a way that is described with field lines. (Theoretically, from a relativistic standpoint, magnetic monopoles should exist, but detection tests have never supplied sufficient empirical proof.)

1.2 Measuring the magic: Magnetic flux density
Magnetic flux density is one factor in calculating the force that acts on each and every charged particle within a magnetic field (mathematical formula F = BQv, where B symbolizes magnetic flux density). The SI unit for measuring the strength of a magnetic field, or more accurately the magnetic flux density (a vector quantity), is tesla (T). (An alternative unit is gauss [G].)

2 The Magnet Earth

Already the legendary mathematician/physician C.F. Gauss and his colleagues – in the 1830''''s – initiated studies and measurements of the Earth''''s magnetic field.

2.1 How is the Earth''''s magnetic field generated?
How the existence of Earth''''s present-day magnetic field could be explained is not entirely clear (see below). According to a widely recognized theory, anyway, the magnetic field is generated by electric currents that occur rather randomly in the masses of molten iron in the Earth''''s (liquid) outer core. Thus, the planet would function as one huge electromagnet (constantly switched to “on”).
There is, however, a doubt. Under normal conditions, the physics of electromagnetism require that the electricity occurs around the body of a permanent magnet. But the inner core of the Earth is believed to be too hot for the molecules of iron and nickel to stay in an order that would allow the core to be magnetized permanently . Scientists therefore have to speculate on how the Earth''''s magnetic field is still able to persist. One possible answer is that the field is configured similarly to a self-reinforcing dynamo.
In any case, the interior of the planet is a thermodynamic environment and, in consequence, has much more varying physiochemical conditions than the standard-model electromagnet. Because of this it more convenient to perceive the Earth as one gigantic, permanent dipole, even though it is more or less a “super-simplification” of the real circumstances.

2.2 Field strength
The Earth’s magnetic field has a magnetic flux density of 30-60 µT, being weakest in the near-Equatorial south hemisphere and strongest (quite naturally) near the poles. The strength fluctuates, being weaker at times and sometimes it gets so weak it collapses (see 4.2).

3 Magnetic & geomagnetic poles

3.1 Unmatching poles
The axis of the Earth and its magnetic dipole are not in alignment; hence, the magnetic poles are not found in the same spot as the geographic equivalents. Presently, the location of the magnetic north pole is differing approximately 11° from the “real” North Pole, placing it on the floor of the Arctic Ocean to the north-west off Axel Heiberg Island (Canada). The magnetic poles

3.2 A navigational problem
A long-known problem with any compass is that its needle will point at the magnetic north pole, whereas north on a map is the direction to the geographic north pole. The angle between the different directions, or the declination, has to be compensated for in some way if navigation with the help of a compass is to be successful.
The angle at which the field lines of the magnetic field intersects the Earth surface in a given location is the inclination. At the magnetic poles the lines are totally vertical (90° inclination), otherwise the inclination vary with latitude and longitude.

3.3 Hypothesizing an axial magnet: Geomagnetic poles
Since the internal structure of the Earth is much more complicated and dynamic than that of a simple stick-shaped dipole, the ”Earth-magnet” is not exactly axial. While the term ''''magnetic poles'''' signifies the geo­graphical places where the magnetic field lines are actually pointed perpendicularly to the surface, the term ''''geomagnetic poles'''' refers to the endpoints of a hypothetical axial dipole (that would stick straight through the planet''''s interior).

4 Shifting polarities

4.1 Reversals (as seen in Remnant Magnetism)
Ferrimagnetic, magmatic materials (especially the iron-rich mineral magnetite) in the Earth''''s crust get aligned with the geomagnetic field as they cool to below their specific Curie Temperature and crystallize. So these materials, which are abundant in the basalt and gabbro layers of the oceanic crust, can be examined as registries of how the magnetic field has changed in the past. What this Thermal Remnant Magnetism (TRM) reveals is that the field polarity has regularly been reversed, as compared to the ''''normal'''' polarity of today. Additionally, there are Depositional/Chemical Remnant Magnetism (DRM/CRM) recordings to analyse (in the case of the latter, the mineral hematite plays an important role).
It has been observed that normals and reversals tend to dominate for vast periods of time, while there are short-time shifts within these periods. The mean number of years between shifts is roughly 200.000, but the time of each interval varies greatly. The last shift was 728.000 years ago.
Until recently, it was practically impossible to go further back than the age of the oldest oceanic crust (ca 250 Ma) when looking at Remnant Magnetism, because the analysis required rather large rock samples. This limitation is now history, as the newest technologies allows examination of minor pieces of silicates, which has been trapped as inclusions in crystals of much greater age (up to 4 Ga). In other words, variations in the geomagnetic field can now be mapped almost as far back as to the birth of the planet!

4.2 Geomagnetic excursions
Reversals occur after major collapses in field strength. Yet, most often when its strength has gone down to reach a critical state of near-collapse, the field retains the same polarity as before after it stabilized again. However, it has been discovered that during the critical period, it may happen that the magnetic poles are displaced or even disappear. This phenomenon is known as ‘geomagnetic excursions’.

5 The Magnetosphere

5.1 Far above the clouds: An invisible shield
The Earth''''s magnetosphere is the region of space around the Earth within which the magnetic field is actually doing some work, a region that stretches several 10.000 km out. In spite of its name, the magnetosphere lacks that symmetric, circular or elliptical shape you may expect a sphere or magnetic field to take. Instead, it has a distinctly twisted shape that it gets from interacting with the interplanetary magnetic field (IMF), which is generated by the sun and provides the primary distribution route for the solar wind.
The solar wind, in turn, can basically be described as a plasmatic flow of charged particles ejected by explosions on the surface of the Sun. Within the magnetosphere, the geomagnetic forces works as a deflector that sends the solar wind plasma past the planet and into the magnetotail. If it had not been for this protective shield, the solar wind had probably caused a devastating erosion of the Earth''''s atmosphere (as is suspected to have happened on Mercury and Mars) and ultimately made the planet uninhabitable (at least for such lifeforms we know of).

5.2 When storms get electric: Disturbances in the magnetosphere
Especially powerful showers of solar particles can distort the magnetosphere quite heavily. The effects of such disturbances range from a beautiful display of colours in the sky – the phenomenon of (polar) aurora – to geomagnetic storms that potentially causes damage to electricity-dependent apparatuses and systems even down on the Earth surface (by inducing currents that may short-circuit the electrical architecture).
Short-term fluctuations in the magnetic field due to cosmic “attacks” on...

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Inactive member [2007-03-27]   The Geomagnetic Field
Mimers Brunn [Online]. https://mimersbrunn.se/article?id=7833 [2024-06-14]

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