Earth science

Earth’s magnetic field

The dynamo, the dipole, inclination and declination, paleomagnetism and seafloor stripes.

Basics

The geodynamo

Liquid iron in the outer core moves with convection and rotation and sustains current and field — a dynamo. Internal heat and buoyancy of light elements drive the flow. Intensity wanders over millennia; poles sometimes reverse. A bar-magnet Earth is only a first sketch; real fields have higher multipoles and a westward drift.

Dipole and inclination

At the surface the main part is a tilted dipole. Inclination is the angle of field lines to the horizontal: steep near the magnetic poles, near 0 at the magnetic equator. Declination is the offset between true north and magnetic north. Navigation and field geology use local declination charts. Regions such as the South Atlantic Anomaly are weaker than a pure dipole.

Paleomagnetism

Magnetic minerals can lock the field as lava cools or sediment settles. Ancient inclination hints at the latitude of that continent. Seafloor stripes record normal and reversed field as crust is born at a ridge. That is independent evidence that plates move. Remagnetisation and tilt corrections keep interpretations honest.

Magnetosphere

The solar wind compresses Earth’s field on the dayside and stretches a tail on the night side. Charged particles are trapped in the Van Allen belts; spilling into the polar atmosphere makes aurora. Strong solar storms shake field and ionosphere and can disturb radio, satellites, and grids. A world such as Mars, with a weak global field, leaves more atmosphere exposed to the solar wind. For space-weather advice, follow official alerts.

Formulas

Dipole field (on axis, sketch)

B ∝ μ / r³

Moment μ, distance r. Earth’s dipole is tilted and slightly offset.

Symbols

μ magnetic moment
r distance from the centre

Dipole inclination

tan I = 2 tan λ_m

Magnetic latitude λ_m. I → 90° at the magnetic pole.

Symbols

I inclination
λ_m magnetic latitude

Cyclotron frequency

ω_c = q B / m

Gyrofrequency of a charge in B. Van Allen sketch.

Symbols

q charge
B magnetic field
m mass

Key table

Surface \|B\|	roughly 25–65 μT, larger toward the poles
Magnetic north	not true north; wanders year to year
Brunhes	present normal chron; last major reversal ~0.78 Ma