Basics of Magnetic Fields
4. Earth’s magnetic field
Learning outcomes
- I can describe Earth's magnetic field.
- I can explain how compasses align with Earth's magnetic field.
- I can identify Earth's magnetic poles.
- I can describe the role of the magnetosphere.
- I can explain phenomena such as auroras using Earth's magnetic field.
Earth’s Magnetic Field
Earth Has a Magnetic Field
Earth is surrounded by a large-scale magnetic field that extends far into space.
Near Earth's surface, the field resembles the field around a large magnetic dipole, although the real field is more complex and changes over time.
Earth's magnetic field is important because it:
- allows magnetic compasses to work
- interacts with charged particles from the Sun
- creates the magnetosphere
- contributes to the formation of auroras
- provides important information about Earth's interior
Where Does Earth's Magnetic Field Come From?
Earth is not simply a giant permanent bar magnet.
Its magnetic field is generated mainly by processes deep inside the planet.
Earth contains:
- a solid inner core
- a liquid outer core
- a rocky mantle
- a solid crust
The outer core contains mainly liquid iron and nickel.
Motion of electrically conducting material in the liquid outer core produces electric currents. These currents generate Earth's magnetic field.
This process is called the:
geodynamo
The Geodynamo
The geodynamo depends on several processes working together.
Heat escaping from Earth's interior causes liquid material in the outer core to move through convection.
Earth's rotation helps organize these flows.
Because the liquid outer core is electrically conducting, its motion can generate electric currents.
These currents create magnetic fields.
The resulting system maintains Earth's global magnetic field.
Earth Is Not a Permanent Magnet
It is tempting to imagine a giant bar magnet inside Earth.
The bar-magnet model is useful for understanding the general field pattern, but it is not the actual cause of Earth's magnetic field.
The temperatures inside Earth's core are far too high for the core simply to behave as an ordinary permanent iron magnet.
Instead:
moving conducting fluid → electric currents → magnetic field
Earth's Magnetic Field Lines
Earth's field can be represented using magnetic field lines.
Like all magnetic field lines, they form:
continuous closed loops
Near Earth's surface, the pattern is roughly similar to the field around a bar magnet.
However, Earth's actual field is:
- tilted relative to its rotation axis
- irregular
- changing with time
- affected by the solar wind
Geographic Poles
Earth's geographic poles are determined by Earth's axis of rotation.
They are:
Geographic North Pole
and:
Geographic South Pole
The rotation axis passes through these two points.
These are different from Earth's magnetic poles.
Magnetic Poles
Earth also has magnetic poles, determined by its magnetic field.
The magnetic poles are not located exactly at the geographic poles.
They also move over time as the processes in Earth's outer core change.
Therefore:
geographic poles ≠ magnetic poles
A Subtle Point About Magnetic North
A compass's north-seeking end points roughly toward geographic north.
Since unlike magnetic poles attract, the region near geographic north behaves, in the simple bar-magnet analogy, like a magnetic south pole.
This naming can be confusing because the location is commonly called the north magnetic pole based on its geographic location and navigation convention.
For introductory work, it is often most useful to remember:
the north-seeking end of a compass points approximately toward Earth's magnetic north region.
How a Compass Works
A compass contains a small permanent magnet that is free to rotate.
When placed in Earth's magnetic field, the compass needle experiences a turning effect.
The needle rotates until it aligns approximately with the local magnetic field.
The north-seeking end points approximately toward magnetic north.
The opposite end points approximately toward magnetic south.
Why the Compass Needle Turns
A compass needle is itself a small magnet.
Earth's magnetic field exerts forces on its magnetic poles.
If the compass is not aligned with the field, these forces produce a turning effect or torque.
The needle rotates until it aligns with the local field.
Therefore, a compass does not point north because it is somehow "pulled directly toward the North Pole."
Instead, it aligns with:
the local direction of Earth's magnetic field.
A Compass Shows Local Field Direction
The compass needle points along the horizontal component of Earth's magnetic field.
If you move to a different location on Earth, the direction of the field changes slightly.
Therefore, compass direction depends on location.
Local magnetic materials and electrical equipment can also disturb compass readings.
Examples include:
- steel structures
- vehicles
- electrical cables
- magnets
- electronic equipment
True North and Magnetic North
True north points toward the geographic North Pole.
Magnetic north is the direction indicated by Earth's magnetic field.
These directions are usually not exactly the same.
The horizontal angle between true north and magnetic north is called:
magnetic declination
Declination varies with:
- geographic location
- time
Navigators may need to account for this difference.
Magnetic Inclination
Earth's magnetic field is not usually horizontal.
The angle that the field makes with the horizontal is called:
magnetic inclination
or:
magnetic dip
Near the magnetic equatorial region, the field is more nearly horizontal.
Closer to the magnetic poles, the field becomes more steeply inclined.
A freely rotating three-dimensional magnetic needle would therefore tilt as well as turn horizontally.
Earth's Magnetic Field Strength
Earth's magnetic field is relatively weak compared with many laboratory or industrial magnets.
Near Earth's surface, its magnetic flux density is typically on the order of:
tens of microteslas (μT)
Although weak, the field extends across an enormous region of space and has major effects on charged particles around Earth.
The Magnetosphere
The region of space dominated by Earth's magnetic field is called the:
magnetosphere
The magnetosphere interacts strongly with charged particles travelling from the Sun.
It acts as an important part of Earth's space environment.
The Solar Wind
The Sun continuously releases a stream of charged particles known as the:
solar wind
The solar wind contains mainly:
- electrons
- protons
- other ions
These particles travel through interplanetary space at high speeds.
When they reach Earth, they encounter Earth's magnetic field.
How the Magnetosphere Affects the Solar Wind
Charged particles moving through a magnetic field can experience magnetic forces.
As a result, many solar-wind particles are:
- deflected
- redirected
- trapped
- guided around Earth
The magnetosphere therefore reduces the direct interaction of much of the solar wind with Earth's atmosphere.
It does not form a perfectly solid or impenetrable shield, however. Some particles can enter or become trapped within the magnetosphere.
Shape of the Magnetosphere
The solar wind changes the shape of Earth's magnetosphere.
On the side facing the Sun, the solar wind compresses the magnetic field.
This creates a relatively compressed:
dayside
On the side away from the Sun, the field is stretched into a long:
magnetotail
Therefore, the magnetosphere is not a symmetrical sphere around Earth.
Bow Shock
The solar wind moves rapidly toward Earth's magnetosphere.
The interaction creates a region called the:
bow shock
It forms on the Sun-facing side of Earth, somewhat like the shock wave produced when an object moves rapidly through a fluid.
The solar wind changes speed and direction as it encounters Earth's magnetic environment.
Magnetopause
The outer boundary separating Earth's magnetosphere from the surrounding solar wind is called the:
magnetopause
Its location is not fixed.
Changes in solar-wind pressure can compress or expand the magnetosphere.
Therefore, Earth's magnetic environment responds continuously to changing conditions from the Sun.
Charged Particles in Magnetic Fields
A moving charged particle in a magnetic field can experience a force:
F = qvB sin θ
where:
- F = magnetic force
- q = charge
- v = particle speed
- B = magnetic flux density
- θ = angle between velocity and magnetic field
The force acts perpendicular to the particle's motion and the magnetic field.
As a result, charged particles can follow curved or spiral paths around magnetic field lines.
Charged Particles Spiral Along Field Lines
Charged particles entering Earth's magnetic environment do not necessarily travel directly along magnetic field lines.
Instead, many follow spiral or helical paths around the field lines.
Their overall motion can guide them toward particular regions of Earth.
This behavior is important for understanding:
- radiation belts
- auroras
- space weather
The Van Allen Radiation Belts
Some energetic charged particles become trapped within Earth's magnetosphere.
These particles form regions known as the:
Van Allen radiation belts
The belts contain energetic charged particles influenced by Earth's magnetic field.
Understanding these regions is important for:
- satellites
- spacecraft
- astronauts
- space missions
because energetic particles can damage electronics and biological tissue.
What Are Auroras?
An aurora is a natural display of light produced in Earth's upper atmosphere.
The best-known examples are:
aurora borealis — northern lights
and:
aurora australis — southern lights
Auroras are closely connected to interactions between:
- the Sun
- Earth's magnetosphere
- charged particles
- gases in Earth's upper atmosphere
How Auroras Form
The basic process is:
solar activity → energetic charged particles → Earth's magnetic environment → upper atmosphere → light
Energetic particles can be guided toward high-latitude regions by Earth's magnetic field.
They enter the upper atmosphere and collide with atoms and molecules.
These collisions transfer energy to atmospheric particles.
Excited Atoms and Molecules
When energetic charged particles collide with atmospheric atoms and molecules, they can transfer energy to them.
The atoms or molecules enter higher-energy states.
They are then said to be:
excited
When they return to lower-energy states, they release energy as photons.
Those photons produce the visible light of an aurora.
Why Auroras Have Different Colours
Different atmospheric gases can emit different wavelengths of light.
Important gases include:
- oxygen
- nitrogen
Oxygen can produce prominent green and red emissions under suitable conditions.
Nitrogen can contribute blue, violet, and reddish colours.
The colour depends on factors such as:
- gas involved
- altitude
- energy of collisions
- atmospheric conditions
Why Auroras Usually Occur Near the Poles
Earth's magnetic field guides many energetic charged particles toward high-latitude regions.
As a result, auroras commonly occur in oval-shaped regions around Earth's magnetic poles called:
auroral ovals
This is why northern auroras are commonly seen in places such as:
- northern Canada
- Alaska
- Iceland
- Norway
- Sweden
- Finland
while southern auroras occur around Antarctica and other high southern latitudes.
Auroras and Solar Activity
The Sun's activity is not constant.
Events such as:
- solar flares
- coronal mass ejections
- changes in the solar wind
can disturb Earth's magnetosphere.
During strong disturbances, increased numbers of energetic particles may interact with Earth's upper atmosphere.
Auroras can then become:
- brighter
- more active
- visible farther from the poles
Geomagnetic Storms
A strong disturbance of Earth's magnetosphere is called a:
geomagnetic storm
Geomagnetic storms can produce spectacular auroras, but they can also affect technology.
Possible effects include:
- satellite disturbances
- communication problems
- navigation errors
- increased radiation exposure for spacecraft
- induced currents in electrical power systems
This makes Earth's magnetic field important not only scientifically but technologically.
Space Weather
Conditions produced by activity from the Sun and its interaction with Earth's magnetic environment are collectively called:
space weather
Space weather includes phenomena involving:
- solar wind
- solar flares
- coronal mass ejections
- geomagnetic storms
- auroras
Monitoring space weather helps protect technological systems.
Earth's Magnetic Field and Life
Earth's magnetosphere helps reduce the direct exposure of the atmosphere and Earth's surface to many energetic charged particles from space.
However, it should not be described as Earth's only radiation shield.
Earth's:
atmosphere
also provides extremely important protection by absorbing and interacting with incoming radiation and particles.
The magnetic field and atmosphere together contribute to Earth's protective environment.
Earth's Magnetic Poles Move
Earth's magnetic field changes because the liquid outer core is constantly moving.
As a result, the magnetic poles:
move over time
This movement is known as:
polar wander or geomagnetic secular variation, depending on the context.
Maps and navigation systems that rely on magnetic directions therefore need periodic updating.
Magnetic Reversals
Earth's magnetic field has also reversed many times during geological history.
During a geomagnetic reversal, the overall magnetic polarity changes.
The magnetic north and south orientations effectively switch.
These reversals do not happen on a simple regular schedule.
Evidence for past reversals is preserved in magnetic minerals in rocks.
Evidence from Ocean Crust
When molten rock cools, magnetic minerals can align with Earth's magnetic field and preserve information about its direction.
At mid-ocean ridges, new oceanic crust forms continuously.
Scientists discovered alternating bands of magnetic polarity on either side of these ridges.
These magnetic stripes provide important evidence for:
- past magnetic reversals
- seafloor spreading
- plate tectonics
Compasses and Navigation
For centuries, Earth's magnetic field has been used for navigation.
A compass provides a simple way of determining direction without:
- GPS
- satellites
- electronic navigation systems
However, accurate navigation may require corrections for:
- magnetic declination
- local magnetic disturbances
- changes in Earth's magnetic field
Animals and Earth's Magnetic Field
Evidence indicates that some animals can detect aspects of Earth's magnetic field.
This ability is called:
magnetoreception
It may contribute to navigation in animals such as:
- migratory birds
- sea turtles
- some fish
- certain insects
Scientists continue to investigate the exact biological mechanisms involved.
Earth's Magnetic Field vs a Bar Magnet
| Bar Magnet | Earth |
|---|---|
| Has a north-south magnetic field pattern | Has an approximately dipolar field |
| Magnetism comes from aligned magnetic material | Field mainly comes from the geodynamo |
| Field is relatively localized | Field extends far into space |
| Field shape can be approximately symmetrical | Solar wind strongly distorts the outer field |
| Usually relatively stable | Field changes over time |
The bar-magnet model is useful, but Earth's magnetic field is much more complex.
Reading an Earth Magnetic-Field Diagram
When interpreting a diagram, look for:
Field direction
Follow the arrows on the magnetic field lines.
Field-line density
Closer lines indicate stronger magnetic fields.
Magnetic poles
Identify where the field becomes strongly concentrated near Earth.
Solar direction
Determine which side faces the Sun.
Magnetotail
Look for the long stretched region on the side away from the Sun.
Particle motion
Determine whether charged particles are being deflected, trapped, or guided toward the polar regions.
Magnetic Field vs Magnetosphere
These terms should not be confused.
Earth's magnetic field
The magnetic field generated mainly by processes within Earth's liquid outer core.
Magnetosphere
The large region of space around Earth in which Earth's magnetic field strongly influences charged-particle motion.
Therefore:
magnetic field = physical field
while:
magnetosphere = region dominated by that field
A Simplified Aurora Sequence
The formation of an aurora can be summarized as:
Sun
↓
solar wind and energetic particles
↓
interaction with magnetosphere
↓
particles guided toward high latitudes
↓
collisions with upper-atmosphere gases
↓
atoms and molecules become excited
↓
photons emitted
↓
aurora
This connects magnetism, atomic physics, astronomy, and atmospheric science.
Common Mistakes
Mistake: Saying Earth contains a giant permanent bar magnet.
The bar magnet is only a model. Earth's field is mainly generated by the geodynamo in the liquid outer core.
Mistake: Assuming geographic north and magnetic north are exactly the same.
They are different and their separation changes with time.
Mistake: Saying a compass points directly at the geographic North Pole.
It aligns with the local horizontal component of Earth's magnetic field.
Mistake: Saying Earth's magnetic poles never move.
They move as Earth's magnetic field changes.
Mistake: Saying the magnetosphere completely blocks the solar wind.
It deflects and redirects many charged particles, but the interaction is complex and some particles enter the magnetosphere.
Mistake: Saying auroras are produced directly by sunlight.
Auroral light is emitted by excited atmospheric atoms and molecules after interactions involving energetic charged particles.
Mistake: Saying Earth's magnetic field is Earth's only protection from space radiation.
Earth's atmosphere also provides major protection.
Mistake: Assuming Earth's magnetic field has always had its present polarity.
The field has reversed many times during Earth's geological history.
Key Terms
Earth's magnetic field — magnetic field surrounding Earth, generated mainly by motion within the liquid outer core.
Geodynamo — process by which moving electrically conducting fluid in Earth's outer core generates the planetary magnetic field.
Geographic pole — point where Earth's rotation axis intersects its surface.
Magnetic pole — region associated with Earth's magnetic field where the field is steeply directed.
Compass — small permanent magnet that aligns with the local magnetic field.
Magnetic declination — horizontal angle between magnetic north and true north.
Magnetic inclination — angle between Earth's magnetic field and the horizontal.
Magnetosphere — region of space dominated by Earth's magnetic field.
Solar wind — stream of charged particles continuously emitted by the Sun.
Magnetopause — boundary between Earth's magnetosphere and the surrounding solar wind.
Magnetotail — stretched region of Earth's magnetosphere extending away from the Sun.
Van Allen radiation belts — regions containing energetic charged particles trapped within Earth's magnetic environment.
Aurora — atmospheric light produced when energetic particles lead to excitation of atoms and molecules in the upper atmosphere.
Aurora borealis — northern aurora.
Aurora australis — southern aurora.
Geomagnetic storm — major disturbance of Earth's magnetosphere caused by solar activity.
Space weather — changing conditions in near-Earth space influenced mainly by solar activity.
Magnetoreception — ability of an organism to detect aspects of Earth's magnetic field.
Key Takeaways
- Earth is surrounded by a large-scale magnetic field.
- Earth's field is approximately dipolar near the planet but is more complex than a simple bar-magnet field.
- Earth's magnetic field is generated mainly by the geodynamo in its liquid outer core.
- Moving electrically conducting material creates currents that help generate the field.
- Geographic poles and magnetic poles are different.
- Earth's magnetic poles move over time.
- A compass is a small permanent magnet that aligns with the local magnetic field.
- Magnetic declination is the difference between magnetic north and true north.
- Earth's magnetic field extends far into space.
- The region dominated by Earth's field is called the magnetosphere.
- The solar wind contains charged particles from the Sun.
- Earth's magnetosphere deflects and redirects many solar-wind particles.
- The Sun-facing magnetosphere is compressed.
- The opposite side stretches into the magnetotail.
- Some charged particles become trapped in the Van Allen radiation belts.
- Charged particles can be guided toward Earth's polar regions.
- Collisions with atmospheric atoms and molecules can produce auroras.
- Different atmospheric gases and altitudes contribute to different auroral colours.
- Strong solar activity can produce geomagnetic storms and enhanced auroras.
- Earth's magnetic field has reversed many times during geological history.
Check Your Understanding
- Describe Earth's magnetic field.
- What process generates most of Earth's magnetic field?
- Which layer of Earth is primarily responsible for the geodynamo?
- Explain why Earth should not literally be described as a giant permanent bar magnet.
- What is the difference between a geographic pole and a magnetic pole?
- Explain how a compass aligns with Earth's magnetic field.
- Why does a compass not necessarily point exactly toward geographic north?
- Define magnetic declination.
- What is magnetic inclination?
- Define the magnetosphere.
- What is the solar wind?
- Explain how Earth's magnetosphere affects charged particles from the solar wind.
- Why is the magnetosphere compressed on the side facing the Sun?
- What is the magnetotail?
- What are the Van Allen radiation belts?
- Describe how an aurora forms.
- Why are auroras most commonly observed near Earth's polar regions?
- Explain why auroras can have different colours.
- Describe one technological effect of a strong geomagnetic storm.
- Explain how magnetic patterns in oceanic rocks provide evidence that Earth's magnetic field has reversed in the past.