We've already seen that variation can be east or west — Page 49, Lesson 55

I want to walk you through the final piece of the variation picture, and then we'll move into Earth magnetism itself. We've already seen that variation can be east or west. Now, the maximum possible value of variation is 180°, and this occurs at both the North and the South Poles. On the map, the zero variation line is shown in yellow, and the 180° variation line is shown in green. Now, let's talk about what's really going on inside the Earth. Earlier, we used the idea of a large bar magnet as a convenient analogy, but that's not actually what causes Earth's magnetism. The real cause is the swirling of molten magnetic magma below the surface of the Earth. The effect is more like a bent bar magnet — the North and South Magnetic Poles are not actually antipodal, meaning they are not directly opposite each other. There is no reason why they should be. Let me give you some real positions. The National Environmental Research Council, or NERC, produced a magnetic map for 1st January 2000. That map positioned the North Magnetic Pole at approximately 81° North, 110° West. The South Magnetic Pole was at 63° South, 135° East. By 2009, the North Magnetic Pole had moved to 84° North, 120° West. So these poles drift over time. The actual situation is shown on the charts at Figure 3.4, Figure 3.5, and Figure 3.6. The disposition of variation is not quite as geometrically neat as the idealized diagram in Figure 3.3, but there definitely are two lines of zero variation. One runs southwards from the True North Pole, and the other runs southwards from the Magnetic Pole. One of these agonic lines — that's the name for a line of zero variation — runs down through Europe. The variation near Stuttgart, in Germany, is zero. The other agonic line runs down through the USA. Let me trace the paths for you. The agonic line running down through the USA continues through South America, as you can see in Figure 3.4, and then continues all the way to the True South Pole in Figure 3.5. That behaves much as you would expect from the idealized model. However, the agonic line running down out of the North True Pole takes a much more complicated route. It passes through Stuttgart into Central Africa, then curves upward again back onto the North Polar chart in Figure 3.4, going into North Central Asia. From there it goes southwards again through Australia, and finally reaches the South Magnetic Pole. Now, here's a really important point that I want you to understand clearly. Isogonals — those lines of equal variation we talked about — are not the actual magnetic lines of flux. The magnetic lines of flux are a natural phenomenon. Isogonals are something different: they represent the difference between the alignment of those lines of flux and the local direction of True North at any given point. So an isogonal is a man-made line on a chart showing you how much the compass needle is偏离 from True North at that location. It is not the magnetic field line itself.