Earth Magnetism — Page 56, Lesson 58

Right, let's pick this up with the real-world behaviour of variation at the poles. We've already established what isogonals are — lines joining points of equal magnetic variation — and we've seen the agonic line, which is the specific isogonal where variation is zero. Now I want to show you what actually happens to these lines at the poles, because it's a striking pattern. Look at Figure 3.6, which shows the actual variation at the North Pole. The agonic line running up from Europe — shown in yellow — changes to 180° variation, shown in black, at the True North Pole. So as that zero-variation line approaches the geographic pole, it flips to a 180° variation line. Now, the red isogonals around the True Pole go from 015°E variation up to 180°E, whilst the blue ones go from 015°W down to 180°W. The key point here is that the isogonals converge on the True North Pole — they all meet there. But here's the fascinating part: exactly the same thing occurs around the Magnetic North Pole. The agonic line running up from the USA — again yellow — changes to 180° variation, black, between the Magnetic and True Poles. And 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. So the isogonals converge on the Magnetic North Pole as well. And if you study Figure 3.5, you'll see the same phenomenon at the South Poles. So the general rule is: isogonals converge on both the True and the Magnetic North and South Poles. Now, let's move on to how variation changes over time, because it's not a fixed value. The value of variation at any point on the Earth changes over a period of time. For instance, the present value — and note this is as of 2012 — at Oxford is about 2°W. But this appears to be reducing at a rate of approximately one degree every 9 years. To put that in perspective, in 1960 Oxford's variation was about 8°W. If the present trend continues, the value should fall to zero in about 2040, and then become easterly subsequently. Other changes are taking place in other parts of the world too. So why does this happen? The reason 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. Now, 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. So the magnetic pole isn't fixed; it drifts, and that drift changes the variation you measure at any given location over the years.