Earth’s Magnetic Field

The earth’s magnetic field is also called the geomagnetic field and extends tens of thousands of kilometers outside the earth into space, and this is the earth’s magnetosphere. According to a paleomagnetic study of the Australian red dacite and pillow basalt, the magnetic field has been estimated to be at least 3.5 billion years old.

The actual magnetic poles of the earth are always moving because of the movement of the fluid in its outer core. The direction averages to the rotation axis of the earth over thousands of years. This means that the poles reverse after a certain period of time.

Dynamo theory can be used to explain the cause of the magnetic field. While a magnetic field does extend infinitely, it will weaken with the distance from the actual source. The placement of the poles is responsible for how we can use a compass for navigation.

What Causes Earth's Magnetic Field

The earth’s magnetism is generated by the convection currents of nickel and molten iron found in the earth’s core. These currents have streams of charged particles, and this results in the magnetic fields being generated.  The magnetic field deflects all of the ionizing charged particles that come from the sun, otherwise known as the solar wind, and prevents these particles from entering the earth’s atmosphere. The solar wind could slowly destroy the atmosphere and prevent life on the earth if there was no magnetic shield. Mars, for example, does not have an atmosphere that can sustain life as it does not have a magnetic field that can stop these particles.

Let’s talk about the earth’s magnetic field direction and how it works with the compasses we use. The earth’s magnetic poles are not aligned to the geographic north and south poles, instead, the magnetic north pole is in Antarctica and the magnetic south pole in Canada. These poles are inclined by about 10 degrees to the earth’s rotational axis, and because of this, the compasses point not to the true north but to Canada. So the magnetic north pole is at the south in Northern Canada, and the center of the Antarctica region is where you will find the geographic south pole. The magnetic pole, however, is near the coast and hundreds of miles away.

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Components of Earth’s Magnetic Field

The elements of the earth’s magnetic field that are responsible for the direction and the magnitude of the magnetic field around the earth are

• Magnetic Declination- This is defined as the earth’s magnetic field-the magnetic north and true north. The true north is never in a constant position on the horizontal plane and will keep shifting based on the time and the position on the earth’s surface.

• Magnetic Inclination- This is also called the angle of dip, and this refers to the angle made by the horizontal plane on the earth’s surface. The angle of dip is 0° at the magnetic equator and 90° at the magnetic poles. The angle of dip or the magnetic inclination provides the relationship between the horizontal and vertical components of the magnetic fields. 

The angle of dip formula is given by the following formula:

\[\Rightarrow tan \delta = \frac{{B_{V} }}{{B_{H} }}\]

Where, 

\[B_{V}\] - Vertical component of earth's magnetic field 

\[B_{H}\]- Horizontal component of the earth's magnetic field 

• Horizontal Component of the Earth's Magnetic Field- The total intensity of the earth’s magnetic field can not be found in any horizontal plane. But, the intensity lies along the direction at an angle of dip (\[\delta \]) to the horizontal. The component of the earth’s magnetic field along the horizontal at an angle \[\delta \] is known as the Horizontal Component of Earth’s Magnetic Field and it is given by the following equation.

\[\Rightarrow B_{H} = \text{B cos } \delta\]

Similarly, the vertical component of the magnetic field is given by  \[\ B_{V} = \text{B sin } \delta\] such that, \[B = \sqrt{B^{2}_{H}+B^{2}_{V}}\]

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Intensity

When we talk about the earth’s magnetic field value,  we refer to the intensity of the earth’s magnetic field. Gauss (G) is the unit of measurement for intensity, but it is reported in terms of nT or nanoteslas. Here 1 G = 100,000 nT. Gamma is another way to refer to a tesla, and the field range of the earth ranges between 25,000 and 65,000 nT approximately. 

The magnetic field intensity refers to the magnetomotive force per unit length of a magnetic path. This means that:

\[H=\frac{f_{m}}{l}\]  or  \[H=\frac{N_{I}}{l}\]

H= Magnetic field intensity \[\frac{A-t}{m}\]

\[F_{m}\]= magnetomotive force \[{A-t}\]

l = average length of the path (m)

N - number of turns

I = current (A)

Magnetic field intensity shows the effort that must be given by a given current into establishing a specific flux density in the material.

The intensity usually decreases from the poles to the equator with a maximum over Siberia, northern Canada, and Antarctica south of Australia and the minimum in the South Atlantic Anomaly. The magnetic field’s intensity changes over time and studies show that it cycles with intensity over every 200 million years.

Importance of the Magnetic Field

As mentioned before, there are energetic charged particles emanating from the sun, also called the solar wind, that the magnetic field protects the earth from by deflecting these particles. Some of these particles from the solar wind get trapped in what is known as the Van Allen radiation belt. Some charged particles from the solar wind manage to travel to the ionosphere and the upper atmosphere of the earth in the auroral zones though on an electromagnetic energy transmission line.

The only time that this is visible is when the particles are strong enough to result in phenomena like geomagnetic storms or the aurora. Bright auroras heat the ionosphere strongly, and this causes the plasma to extend into the magnetosphere, and this increases the size of the plasma geosphere, and the atmospheric matter escapes into the solar wind. When the pressure of the plasmas inside the magnetosphere is large enough to inflate and magnetosphere geomagnetic field, geomagnetic storms occur.

The solar wind is what is responsible for the shape that the magnetosphere of the earth takes. Aspects that have an effect on the local space environment of the earth include any fluctuations in the entrained magnetic field, speed, direction, and density. For example, when we consider the levels of radio interference and ionizing radiation, there can be variations by factors of up to thousands. Additionally, there can be a change by a few earth radii because of the position and shape of the bow shock wave and the magnetopause that is found upstream. What this does is that the direct impact of solar wind is faced by the geosynchronous satellites. 

This is what is collectively known as space weather. Atmospheric stripping is something that is caused by gas getting caught in the bubbles of the magnetic field that then are ripped off by the incoming solar winds. Rainfall variation within the tropics also has to be correlated with variations in the strength of the magnetic field.