5.1 Earth's magnetic field and its origin

Earth's magnetic field is like a giant invisible shield protecting us from space radiation. It's created by swirling molten iron in the planet's core, acting like a massive dynamo. This field is crucial for life on Earth and helps us navigate.

The field isn't static – it changes over time. Sometimes it weakens, and rarely, it even flips completely. By studying these changes, scientists can learn about Earth's past and predict future shifts in our magnetic shield.

Earth's Magnetic Field

Characteristics and Structure

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• Earth's magnetic field is a dipole field, with north and south magnetic poles near the geographic poles
• The magnetic field lines emerge from the southern hemisphere and converge in the northern hemisphere, forming closed loops
• The strength of Earth's magnetic field varies with location, ranging from about 25,000 to 65,000 nanoteslas (nT) at the surface
• The magnetic field is not perfectly aligned with Earth's rotational axis, causing a slight inclination between the geographic and magnetic poles (approximately 11.5 degrees)

Magnetic Inclination and Declination

• The angle between the horizontal plane and the magnetic field lines is called the magnetic inclination, which varies with latitude
  • At the magnetic equator, the inclination is 0 degrees, and the field lines are parallel to the surface
  • At the magnetic poles, the inclination is 90 degrees, and the field lines are perpendicular to the surface
• The magnetic declination is the angle between true north and magnetic north, which varies with location and changes over time
  • Declination is essential for accurate navigation using a magnetic compass
  • Declination values can range from 0 to 180 degrees, depending on the location (eastward or westward)

Dynamo Theory and Geomagnetism

Dynamo Theory Basics

• The dynamo theory proposes that Earth's magnetic field is generated and maintained by convection currents in the liquid outer core
• Convection in the outer core is driven by heat from the inner core and the release of light elements during inner core solidification
• The convecting, electrically conductive fluid in the presence of a weak magnetic field can amplify and sustain the magnetic field through a self-exciting dynamo process

Earth's Rotation and the Dynamo Process

• The rotation of Earth plays a crucial role in the dynamo process by causing the convection currents to spiral, which helps to align and reinforce the magnetic field
  • The Coriolis effect, caused by Earth's rotation, deflects the convection currents and creates helical flow patterns
  • These helical flows are essential for the generation and maintenance of the magnetic field, as they help to organize and amplify the field

Liquid Outer Core's Role

Composition and Physical Properties

• The liquid outer core is composed primarily of iron and nickel, with a small percentage of lighter elements such as sulfur, oxygen, and silicon
• The high temperature (4,000-5,000 K) and pressure in the outer core maintain its liquid state, allowing for convection currents
• The electrical conductivity of the liquid outer core, due to the presence of free electrons in the metallic fluid, is essential for the dynamo process

Interaction between Convection Currents and Magnetic Field

• The interaction between the convection currents and the existing magnetic field in the outer core generates electric currents, which in turn create and maintain the geomagnetic field
  • The moving, electrically conductive fluid in the presence of a magnetic field induces electric currents (Faraday's law of induction)
  • These induced electric currents generate their own magnetic fields, which can reinforce or modify the existing magnetic field (Ampere's law)
• The continuous feedback loop between the convection currents, induced electric currents, and the magnetic field is the essence of the self-sustaining geodynamo

Temporal Variations in Earth's Magnetic Field

Short-term and Long-term Variations

• Earth's magnetic field exhibits both short-term and long-term variations in intensity and direction
• Secular variation refers to gradual changes in the magnetic field over decades to centuries, caused by changes in the fluid motion within the outer core
  • These changes can affect the position of the magnetic poles and the strength of the field
  • Secular variation is monitored by satellites and ground-based observatories to update magnetic field models

Geomagnetic Excursions and Reversals

• Geomagnetic excursions are short-lived, rapid deviations of the magnetic poles from their average locations, lasting a few thousand years
  • During an excursion, the field intensity may weaken, and the poles can move significantly from their usual positions
  • Examples of geomagnetic excursions include the Laschamp event (~41,000 years ago) and the Mono Lake excursion (~34,000 years ago)
• Geomagnetic reversals are complete reversals of the magnetic field polarity, with the north and south magnetic poles switching positions
  • Reversals occur irregularly, with intervals ranging from tens of thousands to millions of years, and the process can take several thousand years to complete
  • The most recent reversal, the Brunhes-Matuyama reversal, occurred approximately 780,000 years ago
• Paleomagnetic records in rocks provide evidence for past geomagnetic reversals and are used to create geomagnetic polarity timescales
  • Igneous rocks, sediments, and archaeological materials can preserve the direction and intensity of the magnetic field at the time of their formation or deposition
  • These records help scientists understand the long-term behavior of Earth's magnetic field and its implications for plate tectonics, climate, and the evolution of life