A magnetic pole is a location on or near a planet's surface where the local magnetic field is directed straight down or straight up — i.e., the magnetic inclination (dip) is vertical. On Earth these points are commonly called the magnetic north and magnetic south poles. They are features of the planet's magnetic field rather than fixed geographic landmarks, and their positions change with time as the magnetic field evolves.
Characteristics and terminology
Technically, the magnetic poles are the places where the magnetic vector has no horizontal component and is purely vertical. Observers using a freely suspended compass needle find it points either directly toward or away from these locations. The phrase geomagnetic poles is sometimes used with a different meaning: those are the poles of the best-fit dipolar approximation to the whole planetary field, and they do not coincide exactly with the actual magnetic (dip) poles.
Origin and movement
Earth's magnetic field is generated by electric currents in the fluid outer core. Those currents change on many time scales, producing continuous variation in the field known as secular variation. As a result, magnetic poles wander across the surface over years to decades. Over much longer intervals, the field can undergo polarity reversals in which the roles of magnetic north and south swap; such reversals are recorded in volcanic and sedimentary rocks.
Uses and practical importance
- Navigation: Compasses align with the magnetic field, so knowledge of the magnetic pole positions and local magnetic declination is essential for accurate compass-based navigation and mapping.
- Biology: Many species use the magnetic field for orientation and migration; shifts in the field can affect those cues.
- Geoscience: Changes in pole position and past reversals provide information about the geodynamo and plate tectonics, and are used in paleomagnetism.
Notable distinctions and facts
Do not confuse magnetic poles with geographic poles: the Earth's geographic north pole is defined by the planet's rotation axis, while the magnetic north pole is defined by the magnetic field. Naming can also be counterintuitive: the magnetic north pole is the pole toward which a compass needle's north-seeking end points, even though it is, by magnetic polarity, a magnetic south pole in physics terms. Measurements of pole location are maintained by international observatories and models; for more on the large-scale field see the magnetic field entries and related datasets.
Magnetic poles exist on other planets and moons that possess intrinsic magnetic fields or strong crustal magnetization. In all cases, the poles are dynamic when the generating currents or magnetized materials change, and they remain a focal point of study because of their influence on navigation, radiation shielding and the interpretation of the planet's interior processes.