4.1 Earth's magnetic field and its origin

Earth's Magnetic Field: Characteristics and Components

Earth's magnetic field originates deep within the planet's liquid outer core, where convective motion of electrically conducting iron generates a self-sustaining dynamo. This field shields the surface from harmful solar radiation, provides the basis for magnetic navigation, and preserves a record of tectonic and geologic history in magnetized rocks. The sections below cover the field's structure, its generation mechanism, how it changes over time, and how it interacts with the solar wind.

Dipole Field and Magnetic Poles

To a first approximation, Earth's magnetic field resembles that of a bar magnet (a dipole) tilted about 11° from the rotational axis. Field lines emerge from the south magnetic pole and converge at the north magnetic pole. Two angles describe the field's orientation at any point on the surface:

• Declination is the angle between geographic north and magnetic north. It varies with location and changes slowly over time.
• Inclination (or dip) is the angle the field vector makes with the horizontal. At the magnetic equator, inclination is 0° (field lines run parallel to the surface). At the magnetic poles, inclination is ±90° (field lines point straight into or out of the ground).

Field Strength and Composition

Surface field strength ranges from about 25,000 nT near the magnetic equator to roughly 65,000 nT near the poles. The total observed field is a superposition of three contributions:

• Main field (~95% of surface strength): Generated by dynamo action in the liquid outer core.
• Crustal (or lithospheric) field: Produced by permanently magnetized minerals in the crust. This component is relatively weak but carries important information about past field directions.
• External field: Arises from electric current systems in the ionosphere and magnetosphere, driven by solar wind interactions. It fluctuates on timescales of seconds to days.

Dynamo Theory: Generating Earth's Magnetic Field

Convection Currents and Electrical Conductivity

The geodynamo theory explains how Earth maintains a planetary-scale magnetic field. The liquid outer core is composed primarily of iron and nickel, making it an excellent electrical conductor. Vigorous convection in this layer is driven by two main energy sources:

1. Thermal convection from heat released at the inner-outer core boundary as the solid inner core slowly crystallizes.
2. Compositional convection from the release of light elements (such as sulfur, oxygen, and silicon) during inner-core solidification. These buoyant fluids rise through the denser surrounding liquid, sustaining flow.

Self-Sustaining Geodynamo

The dynamo becomes self-sustaining through a feedback loop:

1. Convective motions move electrically conducting fluid through an existing (even weak) magnetic field.
2. By electromagnetic induction, these motions generate electric currents in the fluid.
3. Those electric currents, in turn, produce magnetic fields that reinforce and reshape the original field.
4. Earth's rotation imposes a Coriolis force on the convecting fluid, organizing the flow into roughly columnar patterns aligned with the spin axis. This is why the resulting field is predominantly dipolar and aligned close to the rotation axis.

The geodynamo will persist as long as there is enough heat flux from the inner core and a continuing supply of buoyant light elements to drive outer-core convection.

Temporal Variations of Earth's Magnetic Field

Secular Variation

Secular variation refers to gradual changes in the field's strength, direction, and pole positions over timescales of years to centuries. These changes reflect evolving flow patterns in the liquid outer core. Observable examples include:

• The north magnetic pole has been migrating from the Canadian Arctic toward Siberia, accelerating in recent decades to roughly 50 km/yr.
• Global dipole moment has decreased by about 10% over the past 150 years, though this does not necessarily signal an imminent reversal.

Geomagnetic Reversals

At irregular intervals, the dipole field reverses polarity entirely: the north and south magnetic poles swap. Key facts about reversals:

• The time between successive reversals varies enormously, from tens of thousands to tens of millions of years. The most recent full reversal, the Brunhes-Matuyama reversal, occurred approximately 780,000 years ago.
• A reversal is not instantaneous. The transition typically takes several thousand years, during which field intensity drops significantly and the field geometry becomes complex and non-dipolar.
• The reversal record is preserved in rocks. As new oceanic crust forms at mid-ocean ridges, iron-bearing minerals lock in the ambient field direction as they cool through their Curie temperature. This produces symmetric magnetic stripes on either side of the ridge, which were a key piece of evidence for seafloor spreading and plate tectonics. On land, stacked lava flows record the same polarity history, forming the basis of magnetic stratigraphy used to date sedimentary and volcanic sequences.

Earth's Magnetic Field and the Solar Wind

Magnetosphere and Solar Wind Interaction

The solar wind is a continuous stream of charged particles (mostly electrons and protons) flowing outward from the Sun's corona at speeds of 300–800 km/s. Earth's magnetic field deflects most of these particles, carving out a cavity in the solar wind called the magnetosphere.

• On the dayside (Sun-facing), solar wind pressure compresses the magnetosphere; the boundary (the magnetopause) typically sits about 10 Earth radii from the surface.
• On the nightside, the field is stretched into a long magnetotail extending hundreds of Earth radii downstream.

Geomagnetic Storms and Auroras

During intense solar events such as coronal mass ejections (CMEs), a surge of plasma and enhanced magnetic fields strikes the magnetosphere. This can trigger geomagnetic storms, which pose real hazards:

• Induced currents can overload power grids (the 1989 Quebec blackout is a well-known example).
• Satellite electronics, GPS accuracy, and high-frequency radio communications can all be degraded.

Auroras (the northern and southern lights) are a visible consequence of solar wind-magnetosphere coupling. Solar wind particles that enter the magnetosphere are funneled along field lines toward the polar regions. When these energetic particles collide with atmospheric gases, the gases emit light: oxygen produces green and red hues, while nitrogen contributes blue and purple. Auroras are typically confined to oval-shaped zones around the magnetic poles, but during strong geomagnetic storms they can be visible at much lower latitudes.