Earth's Magnetic Field

Earth's magnetic field is approximately that of a bar magnet inside the Earth, tilted ~11° from the rotation axis.

Magnetic North pole is in the geographic SOUTH (Antarctic) — that's why a compass needle's N pole points to geographic N.

Three elements describe the field at any point: (i) declination D (angle between true N and magnetic N), (ii) dip angle / inclination I (angle between B and horizontal), (iii) horizontal component B_H.

Total field: B = B_H/cos I. Vertical component: B_V = B_H tan I.

Magnitude of Earth's B at the surface: ~25 to 65 μT. Strongest at poles (~65 μT), weakest near equator (~25 μT).

Earth's field has REVERSED polarity many times over geological history (last ~780,000 years ago) — recorded in basaltic rocks.

Origin: convection currents in the molten iron-nickel OUTER CORE — geodynamo theory.

Magnetosphere shields Earth from solar wind; auroras occur near magnetic poles where charged particles spiral down field lines.