Earth Magnetism — Page 56, Lesson 65

Let’s pick this up with the 6 µT zone. That’s the region around a magnetic pole where the total magnetic field strength drops to 6 microteslas — µT is the symbol for microtesla, the unit of magnetic flux density. So 6 µT is a very specific, very weak field value, and the zone is the closed contour line on a chart that joins every point where the field is exactly that strength. I want you to look at Figure 3.12, which is an illustration of the 6 µT zone round the North Pole. This particular diagram is dated about 1950, but I chose it because the type of projection used shows a reasonably representative shape near the Poles. So the shape you see there — that’s the general form of the zone, even though the survey is old. Now, Figures 3.13 and 3.14 show more recent surveys of the 6 µT zone. They’ve been included because they demonstrate that there is a 6 µT zone around the South Magnetic Pole as well as the North. That’s the key point — the zone is not unique to the north; it exists at both magnetic poles. These two figures are dated 01 Jan 2000. The projection used in those two figures is a Winkel projection. Now, a Winkel projection introduces distortions near the True Pole, and that distortion has the effect of making the zone round the North Pole appear larger than the one round the South — when, in fact, they are approximately the same size and shape. So the apparent difference in size is purely an artefact of the map projection, not a real difference in the magnetic field. The true geometry of the two zones is essentially identical. So the takeaway: the 6 µT zone is a contour of constant field strength around each magnetic pole, both north and south, and any visual size difference between them on a Winkel chart is a projection distortion, not reality.