Terrestrial Magnetism

The Earth behaves as though a huge bar magnet lay inside it, roughly along its axis of rotation. This field, terrestrial magnetism, is what makes a magnetic compass work: a freely suspended magnet swings until it lies along the field, giving a reference direction anywhere on the globe with no power and no ground equipment. The same field also limits the compass. Over much of the Earth it is steeply inclined to the horizontal, near the magnetic poles its horizontal part fades away, it drifts over the years, and it is disturbed by solar activity.

For pilots, terrestrial magnetism explains magnetic variation, the turning and acceleration errors of the direct-reading compass, why a compass swing is valid only over a range of magnetic latitude, and why polar routes are flown on true or grid references. It is examined in EASA subjects 061 General Navigation and 022 Instrumentation.

The Earth's magnetic field

Every magnet has two poles. The end that seeks north is called the north-seeking or red pole, the other the south-seeking or blue pole. Like poles repel and unlike poles attract, so the magnetic pole in the Earth's northern hemisphere, which attracts red ends, has blue polarity; the naming follows geography, not physics. The Earth's lines of force leave the surface in the southern hemisphere, arch over the equatorial regions and re-enter in the northern hemisphere.

At any place the field has a strength and a direction. Its direction in the horizontal plane defines the magnetic meridian, the local "magnetic north" of a compass, and the angle between that meridian and the true meridian is the magnetic variation (see true, magnetic and compass direction). Its angle below the horizontal is the dip. Variation, dip, the total field and its horizontal and vertical components are together called the magnetic elements of a place.

Magnetic poles and the magnetic equator

The magnetic poles are the places where the field is vertical and the dip is 90°. The north magnetic pole lies in the Arctic, some distance from the geographic North Pole; the south magnetic pole lies off the coast of Antarctica, south of Australia. The two are not diametrically opposite each other, and both move. That offset from the geographic poles, together with local irregularities in the field, is what produces magnetic variation. A compass does not point at the north magnetic pole as if it were a beacon: it follows the local magnetic meridian, which curves with the irregularities of the field, so the variation at a place cannot be worked out from the pole's position alone.

The magnetic equator is the line joining places where the dip is zero and the field is horizontal. It is also called the aclinic line. It wanders either side of the geographic equator without coinciding with it, and it is the boundary that matters for the compass: the northern-hemisphere rules for compass errors apply north of the magnetic equator, not the geographic one, and reverse south of it.

Horizontal and vertical components

The total field, T, can be resolved into two components:

• the horizontal component, H, lying along the magnetic meridian;
• the vertical component, Z, acting straight up or down.

With θ the angle of dip:

H = T cos θ, Z = T sin θ, tan θ = Z ÷ H

With a total field of 50 µT and a dip of 60°, H is 50 × cos 60° = 25 µT, only half the field, and Z is 50 × sin 60°, about 43 µT. At the dip of about 66° found over the United Kingdom, cos 66° is about 0.41, so less than half the field is horizontal.

The two components do different jobs. H turns the compass into the magnetic meridian. Z merely pulls one end of the magnet downwards, and it is the source of the direct-reading compass's errors and of the magnetism induced in an aircraft's vertical soft iron.

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Magnetic dip and isoclinals

Magnetic dip, or inclination, is the angle in the vertical plane between the field, the direction a freely suspended magnet would take, and the horizontal. It is zero at the magnetic equator and increases towards the magnetic poles, reaching 90°; over the United Kingdom it is about 66°. In the northern magnetic hemisphere the north-seeking end dips; in the southern, the south-seeking end.

Lines on a chart joining places of equal dip are isoclinals, or isoclinal lines. The aclinic line of zero dip is the magnetic equator. They should not be confused with isogonals, which join places of equal variation.

A compass designer cannot let the magnet dip, or it would not read. The magnet system is therefore hung pendulously, with its centre of gravity below the pivot, so that gravity holds it nearly level. A small residual tilt remains, and in turns and speed changes that offset lets inertia and the vertical component swing the card: the turning and acceleration errors of the magnetic compass. Because dip causes them, they vanish at the magnetic equator, grow with magnetic latitude and reverse in the southern magnetic hemisphere.

Directive force and magnetic latitude

The directive force of a compass is the horizontal component H, the part of the field that turns the magnet into the magnetic meridian and holds it there. It is strongest at the magnetic equator and falls to nothing at the magnetic poles.

Magnetic latitude describes a place's position between the magnetic equator and a magnetic pole. The higher the magnetic latitude, the steeper the dip and the weaker the directive force, with three consequences:

• the compass becomes sluggish and its turning and acceleration errors grow;
• deviation from the aircraft's permanent hard-iron magnetism increases, because the same aircraft field now acts against a weaker H;
• magnetism induced in vertical soft iron by Z increases, so a compass swing done at one magnetic latitude does not fully cancel deviation at a very different one, and a large change of operating area calls for a new swing.

Exam texts treat the direct-reading compass as unreliable beyond about 70° magnetic latitude; the actual limits for a particular aircraft are given in its documentation.

Secular change and the World Magnetic Model

The field is not constant. Secular change is its slow, long-term drift, seen as a gradual wandering of the magnetic poles; exam texts describe it as an apparent rotation of the poles about the geographic poles over roughly 960 years. The north magnetic pole, long located in the Canadian Arctic, has in recent decades moved out over the Arctic Ocean, north of Alaska. There are also regular daily, annual and 11-year changes, linked to the sun, but they are too small to matter for normal navigation.

The main effect of secular change is on variation. Charts give the year of their variation figures, runways are renumbered when their magnetic direction rounds to a new value, and aircraft variation tables must be kept up to date: the Boeing 737 prohibits operations based on magnetic heading or track where the installed IRS variation table is in error by more than 5°.

The World Magnetic Model (WMM) is the standard mathematical model of the Earth's main field, produced jointly by the US National Centers for Environmental Information and the British Geological Survey. It is revised every five years; the current version, WMM2025, covers 2025 to 2029. Navigation systems use it, or models like it, to compute variation from position, and inertial systems that compute true heading apply such stored variation to display magnetic heading. The WMM also marks blackout zones around the magnetic poles, where the horizontal field is too weak for compasses to be relied on.

Magnetic storms

A magnetic storm is an irregular, sometimes violent, disturbance of the Earth's field caused by solar activity, associated with unusually large sunspots and eruptions on the sun. Storms vary in intensity and last from hours to as much as three days.

For navigation, a storm temporarily changes the variation, by more than 5° in polar regions, and can reduce H there below what a compass needs to work reliably. Strong storms are often accompanied by aurorae at high latitudes. Crews on high-latitude routes during a storm rely on inertial and other non-magnetic heading references rather than the compass.

The magnetic polar regions

Near each magnetic pole lies a magnetic polar region in which magnetic direction cannot be trusted. The dip is close to 90°, H is too weak to align a compass or a flux valve, and variation changes rapidly both with position and with time. Direct-reading compasses become useless, and remote-indicating compasses have to be switched to free, unslaved gyro mode.

Airliner documentation defines these areas precisely. With all its inertial units using the same variation table, the Airbus A320 does not provide valid magnetic heading or magnetic track north of 73°N between 90°W and 120°W, the area its limitations call the magnetic polar region, north of 82°N or south of 60°S, and flight beyond these limits is prohibited. The Boeing 737 IRS stores variation only between 82°N and 82°S, and later versions of its standby display switch to true heading automatically where magnetic heading becomes unusable.

Ground aids follow the same logic: where variation changes too fast to be useful, as in Canada's Northern Control Area near the north magnetic pole, VOR beacons are aligned with true north instead of magnetic. Inside these regions aircraft navigate on true heading from the inertial reference system, or on grid heading referenced to a chosen meridian (see polar and grid navigation).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.