Earth Magnetism — Page 56, Lesson 65

I want to walk you through a concept that ties together everything we've been discussing about Earth's magnetism and how it affects navigation. We're looking at something called the 6 µT zone — that's six microtesla, a unit of magnetic field strength. Let me be precise: a microtesla is one millionth of a tesla, and the tesla is the standard scientific unit for magnetic flux density. So 6 µT is a very small magnetic field strength — but it's a critical threshold for navigation. Figure 3.12 shows an illustration of the 6 µT zone around the North Pole. This particular diagram is dated around 1950, and it's been chosen because the type of projection used shows a reasonably representative shape near the Poles. The projection is a polar projection, which means it's looking straight down at the North Pole, so the distortion is minimal in that region. Now, here's the key point: there isn't just one 6 µT zone. Figures 3.13 and 3.14 show more recent surveys of the 6 µT zone, dated 01 January 2000. These have been included because they demonstrate that there is a 6 µT zone around the South Magnetic Pole as well as the North. So both magnetic poles have a region where the magnetic field strength drops to around 6 microtesla. But there's an important caveat about how these maps look. The projection used in Figures 3.13 and 3.14 is a Winkel projection. A Winkel projection introduces distortions near the True Pole — that is, near the geographic North and South Poles. This distortion has the effect of making the zone around the North Pole appear larger than the one around the South Pole when, in fact, they are approximately the same size and shape. So the takeaway here is: the 6 µT zone exists around both magnetic poles, they are roughly the same size and shape, but depending on the map projection you use, they can look very different. As a navigator, you need to be aware of these projection distortions so you don't misinterpret the actual magnetic field conditions near the poles.