Earth's magnetic field

The Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that stretches from Earth's interior out into space. It interacts with the solar wind, a stream of charged particles coming from the Sun. This magnetic field is created by electric currents from the movement of molten iron and nickel in Earth's outer core.

The strength of Earth's magnetic field at the surface is between 25 to 65 microteslas. We can think of it like a giant bar magnet tilted at about 11 degrees compared to Earth's rotational axis. The North geomagnetic pole near Ellesmere Island, Nunavut, Canada, is actually the South pole of Earth's magnetic field, while the South geomagnetic pole is the north pole of the field.

These magnetic poles move slowly over time but are still close enough to the geographic poles that compasses work well for navigation. The area influenced by Earth's magnetic field is called the magnetosphere, and it protects our planet from harmful solar wind and cosmic rays.

Significance

Earth's magnetic field helps protect our planet. It pushes away most of the solar wind. The solar wind is made of charged particles from the Sun.

People have used compasses for over 900 years to find direction and help with travel. Even though the magnetic field changes slowly, compasses still work well. Some animals, like certain bacteria and birds, also use Earth's magnetic field to know which way to go.

Characteristics

The Earth's magnetic field is like an invisible force around our planet. We can think of it like a giant magnet. It has a direction and a strength. A compass can show us where magnetic north is. The strength of this field is measured in tiny units called microteslas (μT). It is much weaker than the magnet on your fridge.

The field's strength changes depending on where you are on Earth. It is strongest near the poles and weaker near the equator. Over time, the strength of the Earth's magnetic field can also change. The magnetic field looks like it comes from a big magnet at the center of the Earth, tilted a little bit from the Earth's axis. This helps protect us from energy from the Sun. The places where the magnetic field points straight down or up are called magnetic poles. They move around a bit each year.

Magnetosphere

See also: Magnetosphere

Earth's magnetic field is mostly like a magnet. It changes shape far away because of the solar wind. The solar wind is a stream of charged particles from the Sun's corona that moves very fast. These particles carry their own magnetic field, called the interplanetary magnetic field.

The solar wind pushes against Earth's magnetic field. Earth's magnetic field protects us. The place where the solar wind and Earth's magnetic field balance is called the magnetopause. The magnetosphere, the area protected by Earth's magnetic field, is not round. The side facing the Sun is about 10 times the width of Earth, while the other side stretches out much farther.

Inside the magnetosphere, there is a region called the plasmasphere that holds low-energy charged particles. There are also two doughnut-shaped areas called the Van Allen radiation belts that hold high-energy particles.

Earth's magnetic field also blocks cosmic rays, which are high-energy particles from outside our Solar System. Some particles do enter the magnetosphere and move along the magnetic field lines. These particles can create beautiful lights in the sky called the aurorae.

The conditions in the magnetosphere, known as space weather, change with the Sun's activity. Big events from the Sun can cause problems for satellites and other technology on Earth.

Time dependence

Short-term variations

The Earth's magnetic field changes quickly, from milliseconds to millions of years. These changes happen because of electric currents in the sky, called the ionosphere, and farther out in space, called the magnetosphere. Big bursts of energy from the Sun, called solar flares, can hit Earth's magnetic field. This can create beautiful lights in the sky called auroras. Scientists measure these changes using something called the K-index.

Secular variation

Main article: Geomagnetic secular variation

Changes in Earth's magnetic field over many years are called secular variation. Over hundreds of years, the angle of the magnetic field, called magnetic declination, can shift by tens of degrees. The strength and direction of the main part of the magnetic field, called the dipole, also changes. In the past two hundred years, this strength has been getting weaker.

One pattern in these changes is called westward drift, where parts of the magnetic field move westward each year. This drift changes in different places and times. Scientists find evidence of these changes in old rocks and lava.

Magnetic field reversals

Main article: Geomagnetic reversal

Sometimes, the magnetic North and South Poles switch places. Scientists call these switches geomagnetic reversals. We can find evidence of these reversals in rocks and deep-sea sediments. These reversals happen at random times. The most recent reversal happened about 780,000 years ago. During these events, the magnetic field can become weaker before settling into its new direction.

Earliest appearance

Studies of very old rocks show that Earth's magnetic field has been around for at least about 3,450 million years.

Future

Since the late 1800s, the overall strength of Earth's magnetic field has been getting weaker. The magnetic north pole has been moving from northern Canada towards Siberia.

Physical origin

Main article: Dynamo theory

The Earth's magnetic field is made by electric currents in the hot, liquid iron of its core. This happens because heat from the core makes the liquid iron move in patterns called convection currents.

The Earth, like many planets and stars, makes a magnetic field by moving electrically conducting liquids. The Earth's field starts in its core, which is made of iron alloys and splits into a solid inner core and a liquid outer core. The liquid outer core moves because heat flows from the hot inner core to the cooler area near the mantle. This movement is guided by the Earth's rotation and the solid inner core.

The process that makes the magnetic field is called a geodynamo. It works like a loop: electric currents create magnetic fields, changing magnetic fields create electric fields, and these fields affect the moving charges. This loop keeps the magnetic field going. The motion of the liquid iron is kept going by heat and by differences in composition as the core cools. The Earth's rotation also helps organize this movement.

The average magnetic field in the Earth's outer core is much stronger than the field we feel on the surface. Scientists study this using computer models to understand how the geodynamo works. These models have shown that the Earth's field can change direction over time.

Effect of ocean tides

The oceans also play a small part in Earth's magnetic field. Seawater, being a conductor, interacts with the magnetic field as tides move the water. The strongest effect comes from the regular lunar tide that occurs twice daily. Other movements in the ocean, like swells and eddies, also add smaller effects. The temperature of ocean water affects how strong this interaction is, and scientists can learn about the heat stored in the oceans by studying changes in the magnetic field.

Currents in the ionosphere and magnetosphere

Electric currents high up in the ionosphere create their own magnetic fields. These fields change daily as the ionosphere gets closer to the Sun, causing small shifts in the magnetic field we experience on the surface. Typical daily changes are very small, around one part in 2000, and even faster changes are even smaller.

Measurement and analysis

The strength of Earth's magnetic field was first measured by Carl Friedrich Gauss in 1832. Satellites like Magsat and Ørsted help scientists study the shape of this magnetic field from space.

Governments have special places called geomagnetic observatories to watch the magnetic field. These help us understand changes that can affect our phones and electricity. There are over 100 of these observatories around the world.

The military and companies that look for minerals use tools to measure magnetic changes. These tools can find hidden objects or places with lots of iron deep underground.

Magnetometers can find very small changes in the magnetic field caused by old objects like tools or ruins. Scientists have mapped the sea floor using these magnetic changes by flying over the ocean.

To understand the magnetic field everywhere, scientists use a special math trick called spherical harmonics. This helps them make models that show how the magnetic field looks in different places and times.

The International Geomagnetic Reference Field (IGRF) is a model updated every five years. Another model, the World Magnetic Model, is used for navigation by groups like the United States Department of Defense and NATO.

More detailed models exist for special uses. The Enhanced Magnetic Model (EMM) can find very small magnetic changes, helping scientists study the Earth’s surface in great detail.

Biomagnetism

Main article: Magnetoreception

Some animals, like birds and turtles, can sense the Earth's magnetic field. They use it to find their way when they travel. Scientists have noticed that cows and deer sometimes line up their bodies in a certain way when resting. This might be linked to magnetism. However, not all scientists agree on this.

Birds also use the Earth's magnetic field to navigate. But very weak electromagnetic fields can sometimes mess up their sense of direction. This can be caused by certain types of radio waves and everyday electronic devices.