earth magnetism chapter 3 - aircraft spruce & specialty co · present (2002) value of variation...

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Chapter 3Earth Magnetism

THE REAL VARIATION MAP

Figure 3.3 showed a somewhat idealised situation. As we said earlier, the North and South Magnetic Poles are not actually antipodal (i.e. not directly opposite each other). There is no reason why they should be. Earth magnetism is not really caused by a large bar magnet - this is simply a convenient analogy. It is actually caused by the swirling of molten magnetic magma below the surface of the earth. The effect is more like a bent bar magnet. The National Environmental Research Council (NERC) magnetic map for 1st Jan 2000 positions the North Magnetic Pole at approximately 81N 110W whilst the South Magnetic Pole is at 63S 135E. The position of the North Magnetic Pole in 2009 was 84N 120W.

The actual situation is shown by the charts at Figures 3.4, and 3.6 The disposition of variation is not quite as geometrically neat as the diagram in Figure 3.3, but there definitely are 2 lines of zero variation, one running southwards from the True North Pole and the other running southwards from the Magnetic Pole. One runs down through Europe (the variation near Stuttgart, in Germany, is zero) and the other runs down through the USA.

Fig 3.4 Variation at the North PoleFigure 3.4 Variation at the North Pole

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Chapter 3 Earth Magnetism

Figure 3.6. Variation at the South PoleFigure 3.6 Variation at the South Pole

(Charts supplied by the British Geological Survey (NERC) in March 2002)

The isogonal running down through the USA continues through South America (Figure 3.5) and then continues to the True South Pole (Figure 3.6), much as one would expect from the idealised model. However, the isogonal running down out of the North True Pole passes through Stuttgart into Central Africa, then curves upward again back onto the North Polar chart (Figure 3.4) into North Central Asia, then southwards again through Australia to the South Magnetic Pole.

It is important to realise that isogonals are not the actual magnetic lines of flux (which are a natural phenomenon). They are the difference between the alignment of the lines of flux and the local direction of True North at any point. This difference is a man-made concept. This is not the same thing. This point becomes more obvious in the next paragraph.


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REAL VARIATION AT THE POLES

Figure 3.7 shows the actual variation at the North Poles. The agonic line running up from Europe (yellow) changes to 180° (black) at the True North Pole. The red isogonals round the True Pole go from 015°E to 180°E variation whilst the blue ones go from 015°W to 180°W.

Figure 3.7 Variation at the North PoleFigure 3.7 Variation at the North Pole(Chart supplied by the British Geological Survey (NERC) in March 2002).

Therefore the isogonals converge on the True North Pole.

However, exactly the same thing occurs round the Magnetic North Pole. The agonic line running up from the USA (yellow) changes to 180° variation (black) between Magnetic and True Poles. Again, the red isogonals round the Magnetic Pole go from zero to 180°E variation whilst the blue ones go from zero to 180°W. Therefore the isogonals converge on the Magnetic North Pole as well. A study of Figure 3.6 will show the same phenomenon at the South Poles also.

Isogonals converge on both the True and the Magnetic North and South Poles.

CHANGES IN VARIATION OVER TIME

The value of variation at any point on the Earth changes over a period of time. For instance, the present (2002) value of variation at Oxford is about 4°W. However, this appears to be reducing at a rate of approximately one degree every 9 years. In 1960, Oxford’s variation was about 8°W and, if the present trend continues, the value should fall to zero in about 2040, becoming easterly subsequently. Other changes are taking place in other parts of the world.

The reason for the change is that the position of the Magnetic North Pole (which is to the West of Oxford, but by more than 90°W of longitude) is moving westwards round the North True Pole, thereby reducing the variation. The reasons for this movement are not fully understood, but it is evidently associated with movement of the molten magma in the Earth’s core.


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At one time it used to be thought that the Magnetic North Pole rotated in a near-perfect circle round the True Pole at a rate of one revolution every 960 years. However, this hypothesis was based simply on empirical observation over a period of time and more recent and accurate observation over the last 50 years has shown that, at least over the last few years, the Magnetic Pole has moved northwards as well as westwards. All that can be said is that the Earth’s magnetic field is certainly changing, and by observing it over a period of time and extrapolating the change, we can make reasonably accurate forecasts of variation for up to about ten years ahead.

There appear to be at least 3 predictable cycles in the pattern:

Secular. The secular movement is this long-term change described above.

Annual. Superimposed on this long-term change is a sinusoidal change with a period of one year. This is associated with the Earth’s orbit round the Sun.

Diurnal. Superimposed on these 2 patterns is a sinusoidal change with a period of one day. This appears to be associated with the daily changes in the height of the ionosphere as the Earth rotates, presenting different areas of the upper atmosphere to the Sun. The variation can change up to about 0.1o over the course of a day.

In addition, there are unpredictable changes. One is associated with solar activity and one with local anomalies.

Solar Activity. The Sun experiences cycles of sunspot activity which peak every 11 years. Huge solar flares are expelled far out into space. The period is predictable, but whether it affects the Earth is not. If one of these flares is pointed towards Earth, a tongue of intense ionisation curls around the upper atmosphere, causing various effects, the most notable of which is the Aurora Borealis (the Northern Lights). There is also a similar effect round the South Magnetic Pole, called the Aurora Australis.

These are known as ‘magnetic storms’ and the effects can be very intense. During the most recent of these 11-year cycles, variation changes of up to 7° were observed. Sunspot activity and solar flares occur at other times as well and minor magnetic storms can occur outside the 11-year peak of the period.

Local Anomalies. Local magnetic anomalies are caused by magnetic deposits or rock formations that cause the field to be different within a particular area.

In addition, scientific surveys have shown that variation can change slightly with altitude, though few aircraft systems would be sensitive enough for this effect to be noticed.

Accordingly, it is very difficult to know the precise instantaneous value of variation affecting an aircraft to better than about 2 degrees and, even if great care is taken over finding and correcting for it, to better than about half a degree over a period of time. This is why the emergence of systems based on highly accurate gyros, particularly INS, in the 1960s/70s was such a major advance in navigation technology. It was not merely that they calculated present position, important though that was. It was because, for the first time, there was a source of accurate reliable heading.


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UPDATING ISOGONALS

Pilots should always fly with the most up-to-date chart for flight safety reasons. Aeronautical information is constantly changing, airways are re-aligned, the positions and frequencies of VORs and DMEs are changed and danger areas alter shape or are moved as civil and defence requirements change within a country. If the chart is republished at frequent intervals, the isogonals should be comparatively recent as well and for most radio navigation charts it is not normally necessary to update the isogonals.

However the interval between reissues of topographical maps may be considerably longer - perhaps every 5 or 10 years. In this case, if the variation has changed significantly, it may be necessary for the pilot to bring the isogonals up to date during the flight planning process. On most maps and charts the year of origin is shown and some indication of the annual change (due to the movement of the magnetic poles) is given. This may be done by a small arrow showing the direction and distance on the annual change of the position of the isogonal as in Figure 3.8,

Figure 3.8Figure 3.8

or by a statement giving the annual change in the variation quoted on the isogonal as in Figure 3.9.

Figure 3.9Figure 3.9


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MAGNETIC DIP ANGLE

Consider the diagram at Figure 3.10, showing the lines of magnetic force round the Earth’s surface. Imagine aircraft at positions A, B, and C. The lines of force will make different angles to the horizontal in each location. This angle is known as the Angle of Dip and is shown diagrammatically in Figure 3.11.

Figure 3.10. Lines of ForceFigure 3.10 Lines of Force

The Earth Magnetic field is along the total line of force, shown as T. This can be resolved into a horizontal component H and a vertical component Z.

Vertical Component. The vertical component Z is of no value in determining horizontal direction. In fact, it is undesirable for two reasons.

Firstly, it causes the needle of a direct reading magnetic compass to dip from the horizontal. This is partially corrected by the use of pendulous suspension, but the end result is that the needle still hangs down to some extent, so that the centre of gravity is no longer directly below the centre of suspension, thereby resulting in the well-known problems of turning and acceleration errors.

Figure 3.11. The Angle of DipFigure 3.11 The Angle of Dip


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Secondly, the vertical component of the Earth’s field induces vertical soft-iron magnetism in the aircraft, thereby increasing the deviation.

Horizontal Component. The horizontal component is the part which is detected by the compass needle in order to determine magnetic north and is known as the directive force. In the region of the magnetic equator the strength of the directive force H approaches the value of T, while Z approaches zero as does the angle of dip.

Figure 3.12 considers positions A, B, and C on the Earth’s surface as shown in Figure 3.10. It becomes apparent that the directive force H decreases as the angle of dip increases, and vice versa.

Figure 3.12. The Effect of Latitude on the Componentsof Dip

Figure 3.12 The Effect of Latitude on the Components of Dip

When either of the earth’s magnetic poles is approached, this component approaches zero strength, while the value of Z approaches that of T.

Magnetic field strength is measured in units of micro-teslas. The generally accepted figure at which the horizontal component of the Earth’s field becomes too small to be detected by a compass is 6 micro-teslas. Clearly, in practice, the actual detection threshold will depend on the design of the particular compass being used, but 6µ teslas is the notional figure normally quoted.

The maximum possible dip angle is 90° and this occurs overhead the North and South Magnetic Poles.


earth magnetism chapter 3 - aircraft spruce & specialty co · present (2002) value of variation...

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