Terrestrial Magnetism — Page 115, Lesson 132

Let’s pick up right where the compass needle’s behaviour comes from — the earth’s field itself. I want to walk you through the concept of the directive force, because it’s the single most important idea for understanding why a magnetic compass works at all, and why it fails in certain places. The earth’s magnetic field, at any point, can be split into two components: a horizontal component, which we call H, and a vertical component, which we call Z. Now, the horizontal component H is given a special name — the directive force — because it is the component which actually aligns the magnetic compass needle with the magnetic meridian. In other words, it’s the part of the earth’s field that physically pushes the needle around until it points along the magnetic north-south line, and that’s what provides your directional reference. Without H, the needle has nothing to align with. Here’s the critical relationship. When you approach either of the earth’s magnetic poles, the horizontal component H approaches zero strength, while the vertical component Z approaches the value of the total field T. Think about that — at the pole, the field is pointing almost straight down, so there’s essentially nothing left horizontally to swing the needle. Over the pole itself, with dip at 90° and zero directive force H, the magnetic sensor — your compass — becomes useless. That’s a hard operational fact: at the pole, the compass simply cannot do its job. Now flip to the other extreme. In the region of the magnetic equator, the strength of the directive force H approaches the value of T — because the field is almost entirely horizontal there — while Z approaches zero, and so does the angle of dip. So you can see the pattern forming: the directive force H decreases as the angle of dip increases, and vice versa. Figure 9.9 illustrates exactly this effect of latitude on the components of dip. But I want to be honest with you about a subtlety. The relationship between H and the dip angle is not quite as simple as it appears, because the earth’s field has irregularities in its pattern, and the total magnetic force T itself varies with position and with time. So don’t treat H versus dip as a clean mathematical curve — it’s a general trend, not a precise law. Here’s a useful rule of thumb to anchor your mental picture: the strength of the horizontal component H at a latitude of about 60°N of the magnetic equator is very roughly half the value of H at the magnetic equator. So as you fly north, your directive force is being cut in half by the time you reach 60° north — that’s a big deal for compass reliability. Now let’s move on to how the earth’s field changes over time, because it’s not static. The field not only lacks symmetry, it’s subject to several known periodic changes. The most significant of these are the secular changes. These are produced by the slow movement of the magnetic poles about the geographic poles, and the period of this cycle is apparently about 960 years. The north magnetic pole is moving slowly westward, and this wander mainly affects magnetic variation — that’s the angular difference between true north and magnetic north. Here’s a concrete, practical example. In the UK, the value of westerly variation is currently decreasing at a rate of 7 minutes per annum. And the predicted variation in London in the year 2240 is zero — meaning true and magnetic north will coincide there at that time. Now, because variation changes year by year, the annual rate of change of variation is printed on navigation charts, so that the variation shown against the isogonals — those are the lines of equal variation on a chart — can be readily up-dated. That’s how you keep your charts current. There are other regular changes too: changes that occur diurnally — that’s daily — annually, and over an eleven-year period. That eleven-year cycle is apparently related to the eleven-year cycle of sunspot activity. But here’s the key point: these changes, unlike the secular type, are not of sufficient magnitude to affect normal navigation. So you can ignore them for practical flying. Finally, let’s talk about the unpredictable changes — the ones that can actually bite you. These are magnetic ‘storms’ of varying intensity, lasting for as long as three days, occurring at irregular intervals. They appear to be produced by unusually large sunspots. The main effect of these storms is a temporary but significant change in magnetic variation. In the UK, that alteration is unlikely to exceed 2°, but in the Arctic and Antarctic the change may exceed 5° and last for as long as an hour. And critically, the value of the directive force H can also change — and in high latitudes it may fall below the minimum required for efficient compass operation. So in a magnetic storm at high latitude, your compass can become unreliable not just because of variation shifts, but because the very force that aligns the needle has dropped out from under you. So to tie it all together: the directive force H is what makes your compass work, it weakens as you approach the poles and strengthens at the equator, it changes slowly over centuries due to pole wander, and it can be temporarily disrupted by magnetic storms. That’s the full picture of terrestrial magnetism as it affects your instruments.