Earth's Magnetic Field
Declination, dip angle I, horizontal component H. tan I = 2 tan λ.
Key Notes
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.
Formulas
Total field from elements
Decomposition into horizontal and vertical at any latitude.
Vertical from horizontal
I = magnetic dip / inclination angle.
Magnitude (vector)
Pythagorean addition.
Earth's dipole moment
Typical estimate.
Important Points
Compass needle aligns with the HORIZONTAL component of Earth's field — declination corrects for true vs magnetic N.
At magnetic poles: I = 90° (field purely vertical). At magnetic equator: I = 0° (field purely horizontal).
Geomagnetic north pole is actually a SOUTH magnetic pole (attracts the N pole of compass needles).
Field strength varies with location and time — gradually weakens, reverses ~every 100,000-1,000,000 years.
Solar storms (CMEs) compress the magnetosphere — cause auroras, GPS errors, and grid disturbances.
Field protects life on Earth from harmful solar radiation — Mars lost its field, lost its atmosphere.
Earth's Magnetic Field notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.