Earth's Magnetic Field

The Earth’s magnetic field is generated by the motions of the molten outer core and acts as a dipole that protects the Earth from the solar wind. It is measured in microteslas (µT) and comprises a horizontal component, which orients the compass, and a vertical component.

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How to measure it in class

The FizziQ app displays the three components of the magnetic field measured by the smartphone’s magnetometer, as well as the magnitude of the total field, in microteslas (µT).

Steps:

Position yourself far from any metal structure, any magnet and any electrical device: a wooden table in the middle of the room, or better, the school courtyard.
Calibrate the sensor by tracing a large figure 8 in the air with the smartphone, then select in FizziQ the raw magnetic field rather than the field corrected by the system.
Record the magnitude of the total field while slowly rotating the smartphone in all orientations: it should remain roughly constant, between 45 and 50 µT in metropolitan France.
Lay the smartphone flat and record the horizontal components: their combination gives the direction of magnetic north.
Bring a magnet, a set of keys and then a radiator close, and observe by how much the magnitude deviates from the value of the Earth’s field.
Repeat the measurement at several places in the school and map the deviations to locate the metal structures of the building.

Scientific activities on this topic

The study of the Earth’s magnetic field makes it possible to address both the fields of geophysics and electromagnetism. For this study, the smartphone is an ideal scientific tool thanks to the extremely sensitive magnetometer present in most devices.

Here are three experiments that can easily be carried out with a smartphone or tablet, in class, in the field or at home:

Warning: recent smartphones have software that continuously adjusts the magnetic field to compensate for external disturbing elements (magnets, ferromagnetic materials, electric fields, …). To measure the magnetic field without interference, you must check that the displayed field is indeed the raw magnetic field (see our article on this subject).

Learn more

A shield for humanity

The Earth’s magnetic field is an invisible force that plays a key role in protecting our planet and in our ability to orient ourselves. It is generated by the motions of the outer core, composed of liquid iron in convection, which acts as a natural dynamo. This magnetic field forms a structure similar to that of a magnetic dipole, comparable to an immense magnet located at the center of the Earth.

A field in three dimensions

The Earth’s magnetic field is a vector field, which means that it has a direction and an intensity that vary depending on the position on Earth. It consists of two main elements:

The horizontal component: it is essential for navigation because it indicates the direction of magnetic north, used by compasses.
The vertical component: it represents the inclination of the magnetic field, which changes with latitude. At the magnetic equator, the field is perfectly horizontal, while at the magnetic poles, it is almost vertical.

Geographic North vs. Magnetic North

Magnetic north does not exactly coincide with geographic north (the Earth’s rotation axis). It moves slowly due to variations in the Earth’s core. This difference is called magnetic declination, which varies depending on where you are and must be taken into account in navigation.

An evolving magnetic field

The Earth’s magnetic field is not fixed: the magnetic north and south poles move over time, and magnetic reversals (where north and south switch) have already occurred several times in Earth’s history. These changes are studied by scientists to better understand the internal dynamics of our planet. You can follow the evolution of the magnetic pole via this link to the NOAA.

Magnetic field anomalies

The Earth’s magnetic field is not perfectly homogeneous. It presents magnetic anomalies, which are local variations of the field due to the presence of magnetic materials in the Earth’s crust. The average value of the Earth’s magnetic field is 50 µT, but some regions, such as the South Atlantic, show a very different magnetic intensity, influencing navigation and the operation of satellites. The study of these anomalies allows geophysicists to better understand the internal composition of the Earth and the history of the magnetic field, notably past magnetic reversals.

A field that protects the Earth

Beyond its role in orientation, the Earth’s magnetic field is a natural shield against charged particles from the solar wind. Without it, the Earth’s atmosphere would gradually erode, as happened on Mars, which no longer has a global magnetic field. On Mars you will not be able to use your compass, and in fact one of our activities deals with this challenge for Martian robots (see the activity).

How to measure it?

Today, thanks to the magnetometers built into smartphones, it is possible to measure and analyze the Earth’s magnetic field easily. These sensors make it possible to study the direction of the field, its intensity and even to locate the magnetic poles based on the measured inclination.

Orders of magnitude

Intensity of the Earth’s field: from about 25 µT (South America, South Atlantic anomaly) to about 65 µT near the magnetic poles. In metropolitan France: about 47 µT, with a horizontal component of about 21 µT and a vertical component of about 42 µT. Inclination: 0° at the magnetic equator, about 64° in France, close to 90° at the magnetic poles. For comparison, a refrigerator magnet produces 5,000 to 10,000 µT at its contact, more than one hundred times the Earth’s field.

Formula

The Earth’s magnetic field is a vector with three components. Its magnitude is calculated from the three magnetometer measurements:

B = √(Bx² + By² + Bz²)

where:

B: intensity of the total magnetic field (µT)
Bx, By, Bz: components of the field along the three axes of the smartphone (µT)

The horizontal component H and the vertical component Z are deduced from the magnetic inclination i:

H = B × cos(i) and Z = B × sin(i)

where:

H: horizontal component, the one that orients a compass needle (µT)
Z: vertical component (µT)
i: magnetic inclination, angle between the field and the horizontal (°)

In the dipole model, the inclination is related to the magnetic latitude L:

tan(i) = 2 × tan(L)

Application examples

A hiking compass orients itself using only the horizontal component of the field, about 21 µT in France: this is why it becomes unusable near the poles, where this component vanishes.
Migratory birds, sea turtles and certain bacteria detect the Earth’s field and use it as a navigation reference.
Mining prospectors locate iron deposits through local magnetic anomalies, which can exceed the average field by several hundred µT.
Auroras appear where the field lines guide particles from the solar wind toward the upper atmosphere, around the magnetic poles.
Archaeologists and deminers use magnetometers to locate buried ferrous objects without digging.
Paleomagnetism reads in the basalts of the ocean floor the orientation of the field frozen in during their cooling, which made it possible to demonstrate continental drift.

FAQ

Q: Is the magnetic north pole a north pole in the physics sense? A: No, and this is one of the most frequent pitfalls. Two poles of the same name repel each other. Since the north pole of a compass needle is attracted toward geographic north, what is located there is physically a magnetic SOUTH pole. It is called the “magnetic north pole” by geographic convention, because it is located in the northern hemisphere.

Q: What is the difference between magnetic declination and inclination? A: They are two different angles and they are often confused. The declination is an azimuth offset, measured in the horizontal plane, between magnetic north and geographic north. The inclination is the angle the field makes with the horizontal, measured in the vertical plane.

Q: Why does my measurement vary from one place to another in the room? A: The magnetometer measures the total field, not just the Earth’s. The metal frames of tables, radiators, power cables and loudspeakers add their own field. You must move away from any metal mass to recover the value of the Earth’s field.

Q: Where does the Earth’s magnetic field come from? A: From the convection motions of the liquid iron in the outer core, between 2,900 and 5,100 km deep. These currents of conducting matter generate electric currents that sustain the field: this is the dynamo effect. It is not a permanent magnet, because iron loses all magnetization above its Curie temperature, which is very largely exceeded in the core.

Q: Is the Earth’s field dangerous or perceptible by humans? A: No. At about 50 µT, it is ten thousand times weaker than the field of a medical MRI and humans do not perceive it. It is, however, indirectly essential, since it deflects the solar wind and protects the atmosphere from erosion.

Magnetometer - Magnetic Inclination - Magnetic Declination - Compass - Magnetic Field of a Magnet