On the Earth, a pivoted magnet will swing round to point roughly north-south and, if it is free to tilt, will take up an inclined position with the northern end pointing downwards in the northern hemisphere and upwards in the southern. Such a magnet can be used to plot the direction of the Earth’s magnetic field which is causing it to move. Similarly it is not difficult to measure the strength of the field. The strength is measured in units called TESLAS; the field varies over the surface of the Earth from 7 X 10-5 to 2.5 X 10-5tesla. GEOMAGNETISM, the study of the Earth’s magnetic field, is a complicated subject because the field varies not only from place to place but also with time. Part of the place-to-place variation is due to permanently magnetized rocks. When molten rock forces its way to the surface and appears as lava from a volcano, it cools and is magnetized by the Earth’s general magnetic field. The direction of the magnetization is the same as that of the Earth’s field at the time when the rock cools. By studying such rocks, scientists are able to compare the Earth’s field in the past with what it is today. Laboratory measurements during the last 400 years have shown that the Earth’s MAGNETIC FIELD varies substantially over periods of a hundred years or so. The study of magnetized rocks has revealed the effects of these changes but has also shown that the field completely reverses in direction every few hundred thousand years. Each FIELD REVERSAL seems to take only one or two thousand years.

The field outside the Earth is roughly the same as that outside a uniformly magnetized sphere or, which is the same thing, that of a dipole at its centre. (A magnetic dipole is a simple magnet whose two poles are very close together.) Such a field has two magnetic poles where a magnetized needle stands vertically and a magnetic equator where it lies horizontally The dipole that best fits the Earth’s field has an axis inclined at 11 1/2 from the rotation axis so that the magnetic poles are at latitude 78 ½ N and S: the northern pole is in the northwest of Greenland. The differences between the best-fit dipole field and the actual field are quite substantial and reach 20 per cent in places. There are regions a few thousand kilometers across, over each of which there is a systematic difference. These regions are not correlated with any surface features such as oceans, continents or great mountain chains, so the cause of the non-dipole field is certainly not to be found in the surface layers of the Earth.

The variations of the field with time point to the same conclusion. 300 years ago the non-dipole field was quite different and, if present trends continue, it will be quite different again in another 300 years. The dipole part of the field, however, has hardly changed in this time. Typical of the changes are a 35° swing in the direction of the compass between 1580 and 1820 at London and a 30 per cent decrease of the horizontal component of the field in 100 years at Cape Town. The timescale for these changes is so short that it is inconceivable that they are due to changes in the solid part of the Earth, which would have other cataclysmic consequences From measurements of the field over the whole surface of the Earth it is possible to deduce that apart from some minor effects, the cause of the charges in the field must lie within the Earth. We have already excluded the solid part of the Earth; this leaves the fluid outer core where we can hope for motions that are sufficiently rapid to explain the variations in the field.

One explanation of the; magnetic field can be immediately excluded : that the Earth is a permanent magnet. There is no known fluid that can he permanently magnetized, and even if there were, the motions in the core would soon mix up the various parts so that there would be no general magnetism. Another common way in which a magnetic field can he produced is by electric currents, and it is now generally accepted that these are indeed responsible for the Earth’s field. The outer; core is thought to be composed largely of molten iron and is therefore a good conductor. If a cur¬rent were started in the core and just left, it would die away on a timescale of ten or twenty thousand years so there must be some mechanism to maintain the currents. The only satisfactory explanation is that there is some kind of dynamo generating the cur¬rents and hence the magnetic field. A simple type of dynamo, the disc dynamo, and shows how the external circuit can be arranged to generate the magnetic field and produce a SELF-EXCITING DYNAMO. If a constant torque is applied to the axle, the dynamo will settle down to a steady state with a constant field. If both the current and magnetic field are reversed, the dynamo will work as before. This is what we want: a dynamo which will give a field in either direction. However, reverse of its own accord, although more complicated systems will.

Obviously the dynamos discussed here are not what we expect to find in the Earth’s core. They have a very simple motion but a rather complex structure. In the Earth we will have a simple structure and so to produce a self-exciting dynamo, the motion must be complicated. It is a very difficult problem to decide whether there are any possible motions that are suitable, but physicists have now demonstrated that there are. This is, of course, not the same as determining what the motions going on inside the Earth are and this is one of the major research problems of geomagnetism. We know already that for any dynamo to work the flow speed in the Earth’s core must be at least 0.3mmS”1, which appears to be quite reasonable.

We do not know what the forces are that drive the dynamo. The outward flow of heat through the Earth, is likely to cause convective motions in the Earth’s core and the combined effect of these and the Earth’s motion is generally believed to be responsible. An alternative view ascribes the motions to the precession of the Earth. The fluid core may not be able to follow the precession exactly and an eddying motion may be produced. Quite possibly both these mechanisms play a part. It is generally true of fluid motion that the motion is more complicated than its cause. We can see this in meteorology: the Sun shines on and warms up one hemisphere and out of this-simplicity grows all the complexity of weather and climate. Similarly we can expect that the motions in the Earth’s core are sufficiently turbulent to cause the observed irregularities and changes in the field. Finally we can note that at the speed of 0.3 mms-1 the time to move 1000km, a distance comparable to the core size, is 100 years, the timescale for variations of the field.

Measurements from spacecraft have shown that none of the other inner planets has a magnetic field as large as the Earth’s. Typically, the surface fields of Venus, the Moon and Mars are no more than a few times 10-8 tesla, i.e. less than 0.001 of the Earth’s field. In each case it seems that a dynamo mechanism cannot work; Mars and the Moon are probably too small to have a sufficiently large conducting fluid core and Venus is rotating too slowly. If the driving force for the Earth’s dynamo is precession, which is caused by the Moon, rather than convection currents, then similar dynamos on Venus and Mars would still be quite impossible. The magnetic field of the Moon appears to be due to permanent magnetism; the same is presumably true for Venus and Mars. On the Moon this magnetism is widespread and it is definitely established that there are magnetized lava flows (due to domains of iron ha the rocks) there. If a material is heated above its CURIE TEMPERATURE, it loses its magnetism and can only be remagnetized if it cools in a magnetic field. The Curie temperature of iron is 780°C so the rocks must have lost their magnetism when they were melted to form lava. The Moon must therefore have had a field of internal origin to magnetize the lava flows when they cooled. The most likely explanation is that the Moon once had a field generated by a dynamo. When the Moon was young it is likely that the radioactive energy sources driving the convection were stronger than they are now so that a dynamo was possible. As the convection became weaker, a time would come when the dynamo could no longer function. Another possibility is that the fluid core of the Moon is probably smaller than it used to be because of cooling and subsequent solidification of its outer regions. A combination of both these effects is also a possibility.

Because of its small size and slow rotation, astronomers predicted that Mercury would have no intrinsic magnetic field, except possibly for a small field due to permanent magnetism induced by the solar wind and the Sun’s field. However the Mariner 10 spacecraft in 1974 and 1975 measured a field with a strength about one per cent of the Earth’s and showed that it was due to an intrinsic dipole . This result was a great surprise even though Mercury probably has a large iron core. It may not be sufficient to say that dynamos can work in less favourable circumstances than astronomers previously thought, for if Mercury has one then why does not Venus ? Now that information about the fields of all five inner planets, with their ranges of radius and rotation rate, is available it should be easier to sort out the valid theories from the invalid ones. Perhaps Mercury’s field is not due to a dynamo at all but to permanent magnetism (which is unlikely), an interaction with the solar wind, or to some cause not yet imagined.

The effect of the solar wind on the Earth’s magnetic field and the way in which it is confined within the magnetosphere has been discussed . Mercury- has a similar magnetosphere although it is much smaller because the field is weaker and. being nearer to the Sun, the solar wind is stronger. For both Mercury and the Earth, the particles of the solar wind cannot enter the magnetosphere and so they do not reach the planetary surface. This effect cannot operate on Venus or Mars because of their minute fields, but both are surrounded by an ionosphere. A sufficiently high electron density here can permit electric currents to flow and to induce magnetic fields which are strong enough to divert the flow of the solar wind and prevent it impinging directly on the surface. There is then a dividing surface, the Ionosphere, analogous to the magneto-pause. Spacecraft measurements show that the electron densities for both Venus and Mars are greater than the theoretically-calculated minimum values required to form an ionosphere. The Moon has no atmosphere and hence no ionosphere and it seems that the solar wind impinges directly onto
Its surface

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