The Magnetic Field of the Earth

Fields:

A ‘field’ is a region in which a body is able to experience a force owing to the presence of other bodies. Fields fill the space between matters.

They determine how bodies are able to exert force on each other at a distance. There are different fields in nature: for example, the gravitational field, the electric field and the magnetic field.

Gravitational fields determine how bodies with mass are attracted to each other by the gravitational force. In electric fields, objects that have electric charge are attracted to each other by the electric force or repelled from each other depending on the kind of electric charge they possess (opposite or same, respectively). Magnetic fields determine how electric currents that contain moving electric charges exert force on other electric currents.

Magnetic Field:

In a magnetic field, electric currents flowing in the same direction will be pulled towards each other and currents flowing in directions opposite to each other will experience a repulsive force. A magnetic field is found around a magnetic body or a current carrying conductor.

The Earth’s Magnetic Field:

The Earth has four layers: the thin outermost layer of lighter rock, ‘crust’; the rocky ‘mantle’; a liquid-iron ‘outer core’ and the innermost layer, an iron ‘inner core’. The ‘inner core’ of the Earth rotates at a different rate as compared to the solid outer layers. This feature, together with currents in the molten ‘outer core’, generates the Earth’s magnetic field.

The magnetic field generally is similar to the field generated by a dipole magnet—a straight magnet that has a north pole and a south pole—placed at the Earth’s centre. But the magnetic field changes depending on the time and location on the Earth. The axis of the dipole is approximately 11 degrees from the axis of rotation of the Earth which means that the geographical poles and the magnetic poles in the north and the south are not in the same place.

The Earth has had a magnetic field for at least some 3.5 billion years.

Features of the Magnetic Field:

The magnetic field is different in different places but it possesses some regular features. A dip needle stands vertical at the magnetic poles (the north end of the needle down at the north magnetic pole; the south end down at the south magnetic pole); the horizontal intensity then is zero. A compass does not show direction here. At the magnetic equator, the dip is zero. The earth’s geographic Equator is fixed but its magnetic equator changes.

Measuring Earth’s Magnetic Field:

The direction and intensity of the Earth’s magnetic field is measured to arrive at the Earth’s magnetism at any given location. The parameters measured are the magnetic declination, the horizontal intensity and the vertical intensity; from these parameters, the other parameters— inclination, north and east components of the horizontal intensity and the full intensity—can then be calculated.

Magnetic Declination:

(Symbol D; measured in degrees) is the angle between the magnetic north and the true north; it is positive when the angle derived is east of the true north and it is considered negative when the angle measured is west of the true north.

Magnetic Inclination:

(Symbol I; measured in degrees) is the angle between the horizontal plane and the direction of the earth’s magnetic field. The inclination is 90° at the north magnetic pole and zero at the magnetic equator.

The intensity of the total magnetic field (F) is described by the horizontal intensity (H), the vertical intensity (Z) and the north and east components of horizontal intensity. The components of intensity have gauss (symbol G) as the unit (the c.g.s. unit) but are recorded in nano Tesla (nT) (SI unit). The earth’s magnetic field is about. 25,000- 65,000 nT or .25-.65 gauss.

The strength of the field varies—from less than 0.3 gauss (30 microteslas) in South America and South Africa to more than 0.6 gauss (60 microteslas) in the region around the magnetic poles, i.e., in the northern parts of Canada, south of Australia and parts of Siberia.

The Earth’s magnetic field strength was first measured in the year 1835 by Carl Friedrich Gauss; since then, it has been regularly measured. There has been a relative decay of about 5 per cent in the last 150 years.

Detecting the Magnetic Field of the Earth:

Animals and birds including turtles and migratory birds can detect the Earth’s magnetic field especially to navigate during their migratory spells.

Governments of some countries operate units —the ‘geomagnetic observatories’ which are part of a national geographic survey—to measure the magnetic field. For example, the British Geological Survey has its Eskdalemuir Observatory for the purpose.

The state military can use ‘metallic anomaly detectors’: the features of the local geomagnetic field are ascertained and then it is found out whether there are anomalies in the natural background which could be caused by metallic objects like submerged submarines. The detectors are usually placed in airplanes or used from surface ships. Companies engaged in geophysical prospecting may also use magnetic detectors to differentiate naturally occurring anomalies from ‘ore bodies’.

Laws:

Inverse square law:

The magnetic field strength decreases as the inverse of the square of the distance from a specified point, close to one pole of a magnet. This is because the pole that is near seems stronger than the farther pole, which therefore can be ignored. Gravitational field, unlike the magnetic field, obeys the inverse square law at all times.

Inverse cube law Away from earth, the geomagnetic field reduces as the inverse cube of distance. This is because, far from a magnet, the poles seem to be at the same point.

Generating the Magnetic Field:

Energy is the basic and essential need of most natural processes. So with Earth’s magnetism. Energy, in this case, comes from fluid motions in the ‘outer core’ of the Earth—from slow circulating flows that help in shedding the heat (such as the circulating air flows on the Earth that cool the ground). The heat may come from iron in the ‘outer core’ solidifying and becoming a part of the ‘inner core’ or by radioactivity. The energy involved here is a tiny part of the total heat energy present in the core.

Due to radioactive heating and chemical differentiation, the outer core experiences intense convection. The process that results is similar to what happens in an electrical generator where the convective kinetic energy is converted to electrical and magnetic energy. The electrically conducting iron is in motion. By moving through the magnetic field that exists, it gives rise to a system of electric currents.

These spread out in the core. The currents themselves create a magnetic field. In other words, the resulting magnetic field is the same input field that helps in the creation of electric currents. When the created magnetic field reinforces the already existing one, a dynamo is created and it sustains itself (the ‘Dynamo theory). As long as the convection is maintained in the outer core, the process continues.

Variations and Reversals in the Magnetic Field:

The Earth’s magnetic field is time-dependent. In other words, it undergoes normal secular variation as well as an occasional reversal of polarity. In the latter case, the magnetic field flips over.

Magnetic Field Variations:

Deviations of a small nature in the Earth’s magnetic field produced by iron artifacts, kilns, some stone structures, ditches and middens are detected by magnetometers. The variations across the floor of the ocean have been mapped with the aid of magnetic instruments that were developed from airborne magnetic anomaly detectors used during the Second World War to mark the presence of submarines.

Notable is the presence of magnetite, a strong magnetic material, in the iron-rich, volcanic rock, basalt that is present in the ocean floor. The magnetite yields magnetic properties to the ocean bed; the magnetic variations themselves are a means to study the floor of the ocean.

The short-term instability of the magnetic field is measured by using what is called the K-index.

According to one theory proposed by J. Marvin Herndon and his colleagues, change in the magnetic field of the Earth occurs due to the inner core of the planet not being made of iron but very dense atoms which undergo nuclear reactions as replicated in a fast breeder reactor.

Reversals in the Magnetic Field:

At times, the secular variation becomes very large with the result that the magnetic poles are located very distant from the geographic poles. This is termed an ‘excursion’ for the poles. After such a period of enhanced secular variation, when the magnetic field returns to its state of rough alignment with the axis of rotation of the Earth, it can have any polarity (the Earth’s dynamo does not prefer a particular polarity). This flipping over or reversal of the magnetic field is a random occurrence. It can happen every ten thousand years or every 50 million years or more. (The Sun’s magnetic field in comparison reverses every eleven years.) It is not necessary that magnetic fields have to reverse; they can also be steady, with no time-dependence feature.

Impact on the Compass:

The magnetic field reversals have a severe impact on the navigational compass. The compass now points roughly north at most geographical locations. But before the last reversal occurred (7, 80,000 years), the polarity was reversed and so the compass would have pointed roughly to what is south now.

During a reversal, the geometry of the magnetic field is complicated. In such an instance, a compass would point in any direction based on criteria like its location on the Earth and the form of the mid- transitional magnetic field. During a reversal, the Earth’s magnetic field is weaker than normal with multiple magnetic poles.

Some palaeomagnetists say that the dipole moment (a measure of how strong the magnetic field is) is likely to decay in 1,300 years from now. The current dipole moment is higher now than what it was in the last 50,000 years. So a decline (reversal) could start any time. But it is to be remembered that a reversal gets completed after several thousand years.

Lava flows of basalt have revealed that the Earth’s magnetic field reverses at an average interval of about 2, 50,000 years; the intervals, however, range from thousands of years to millions of years. The last reversal took place about 7, 80,000 years ago (the Brunhes-Matuyama reversal).

When the molten lava (basalt or even tholeiite) from the volcanoes cool, it adopts the magnetic field present at the time. Other lava flows that follow produce bands of other magnetic fields. So we are able to detect the variations in the Earth’s magnetic field over time. This kind of magnetisation is termed ‘palaeomagnetism’. Studying the sequence of lava flows, the historical direction of the planet’s magnetic field has been measured using a magnetic detector (working like a compass). It has been discovered that the Earth’s poles have shifted time and again.

Why do these reversals occur?

There is no fixed idea why these reversals occur. External events may not cause the reversals as there is no correlation between the age of impact craters and the times when reversals took place. Scientists opine that there is no cause per se. Moreover, self-contained dynamical systems (the Earth’s dynamo being a natural example of such a system) built in laboratories show reversing behaviour of a random nature even without any outside influence.

Some scientists believe that a quasi-stable magnetic field is the cause of poles migrating their orientation over hundreds or thousands of years. Some others are of the view that the geodynamo switches off, perhaps due to some external factor, and restarts itself with the magnetic ‘North’ pole showing the north or south. If the magnetic North re-appears in the same direction, it is a ‘geomagnetic excursion’ but if it re-appears in the opposite direction, we say a ‘reversal’ in the magnetic field has taken place.

Though the Earth’s magnetic field does not affect human health directly, it impacts on electrically-based technological systems that are crucial for the modern civilisation.

Main Field:

The geomagnetic field on the Earth’s surface is a combination of several magnetic fields. Over 90 per cent of the geomagnetic field measured is produced ‘internally, that is, in the planets outer core itself. This part of the geomagnetic field is the Main Field. On the other hand, magnetic fields are created by the external currents in the ionised upper atmosphere and magnetosphere formed by the differential flow of ions and electrons in these areas. These magnetic fields are even 10 per cent of the Main Field at times. Magnetic fields are also induced by currents that flow in the Earth’s crust.

The Main Field shows small variation in time. It can be compared to a bar magnet with north and south poles inside the Earth and magnetic field lines that go into space.

It is described by the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM).

Magnetic Poles:

Magnetic poles are defined as areas where the dip (I) is vertical. In practice, however, the geomagnetic field is not exactly vertical at the poles; it is vertical on a loci, oval in shape, which is traced with variation from one day to the next and is somewhat centered on the dip pole positions.

Magnetic poles (dip poles) can be computed from all the Gauss coefficients using an iterative method while referring to the IGRF and the WMM, the Main Field models. In contrast, the geomagnetic poles or geocentric dipole are measured from the first three Gauss coefficients only from a main model. Using the WMM model, the 2005 location of the north magnetic pole is 83.21 °N and 118.32 °W and of the south magnetic pole is 64.53 °S and 137.86 °E. The 2005 location of the geomagnetic North Pole is 79.74 °N and 71.78 °W and that of the geomagnetic South Pole is 79.74 °S and 108.22 °E.

Magnetic Compass:

The magnetic compass is used to show the direction of a magnetic field. It has a magnetic needle pivoted in the horizontal plane. The needle experiences a torque from the Earth’s ambient magnetic field. Reacting to the torque, the needle shows a preferred alignment with the horizontal component of the geomagnetic field. The north end of the needle points in the general direction of the geographic North Pole; the south end of the needle points in the general direction of the geographic South Pole.

Adjustments need to be made to give true north. Knowing the magnetic declination angle between true north and the horizontal trace of the magnetic field for a particular location would help one correct the compass for the magnetic field in that area.

Locating Principal Magnetic Pole:

The difficulty in locating the principal magnetic pole by instrument is due to the large area over which the dip (I) is about 90° degrees; the pole areas moving many kilometres owing to magnetic variations and magnetic storms; and the inaccessibility of the polar areas. However, the Geographical Survey of Canada has been active in examining the shifts in the North Magnetic Pole. Its surveys show that this pole is moving across the Canadian Arctic.

The survey finished in May 2001 stated that the pole was moving northwest at 40 km per year (81.3 °N latitude and 110.8 °W longitude). The estimate for 2005 was given as 82.7 °N latitude and 114.4 °W longitude. The observed position of the South Magnetic Pole in 2005 was 64.7 °S latitude and 138.0 °E longitude.

Magnetic Equator:

The area where the dip or inclination (I) is zero (there is no vertical component to the magnetic field) is called the magnetic equator. Again, the magnetic equator, like the magnetic field and poles, is not fixed. It changes but slowly. The north end of the dip needle dips below the horizontal north of the equator.

Here, inclination (I) and vertical intensity (Z) are measured positive. The south end of dip needle dips below the horizontal south of the magnetic equator. Here, inclination and vertical intensity are in the negative. Both inclination and vertical intensity increases as one moves farther from the magnetic equator.

Sun’s Magnetism:

The Sun has magnetism but it does not owe simply to the fact that the Sun rotates around its axis. Any electric circuit, rotating like a solid object, cannot yield dynamo currents. This is even if part of the circuit follows the axis of rotation and can be considered non- rotating.

The Sun, however, does not rotate like a solid body, like a ball. The rotation period of its equator is shorter than that of higher latitudes (about 25 days for the equator but two days more for latitude 40 degrees). The uneven motion deforms the surface and is capable of driving a dynamo. In the Sun’s case, it is responsible for sunspot magnetism.

At the magnetic poles, the magnetic compass attempts to align itself with the magnetic field lines, But the fields of force vertically converge at or near the magnetic poles (the inclination is about 90° and the horizontal intensity is weak). The strength and direction tilt the compass needle up or down into the Earth. The compass then ‘points’ to where it is tilted irrespective of the compass direction.

In zones around the north and south magnetic poles, where the horizontal intensity (H) is between 3000 nT and 6000 nT (the erratic zone), and H is less than 3000 nT (the unusable zone) the compass behaviour is not normal. When H is less than 2000 nT, the daily variation in declination (D) can be more than 10 degrees.

In the hemispheres, the compass would not work properly if its needle does not rotate freely. The needle, in turn, would not rotate freely if we do not change the location of the weight balance’) which is placed on the needle to ensure it remains in a horizontal plane to rotate freely. This is necessary to get a correct reading in the northern and southern hemispheres. In the northern hemisphere, as the magnetic field dips down iii to the Earth, the weight is on the south end of the needle.

Then only will the needle be on the horizontal plane. In the southern hemisphere, the weight should be on the north end of the needle. The preferred directionality of a compass is affected by local disturbances in the Earth’s magnetic field, such as those generated by an electrical system. It is also influenced by local magnetisation of the Earth’s outermost layer, the crust, especially near igneous or volcanic rock deposits of a large size.