Terrestrial Magnetism — Page 115, Lesson 132

Let’s start with the directive force, because it’s the single most important idea on this page for understanding how a compass actually works. The earth’s total magnetic field, the full strength of it, is called T. But that total field doesn’t act on a compass needle all at once — it’s split into components. The horizontal component, the part of the field that lies flat along the earth’s surface, is called H. And H has a special name: the directive force. It’s called that because it is the component which aligns the magnetic compass needle with the magnetic meridian, so providing a directional reference. In plain terms, H is the part of the earth’s magnetism that physically turns the compass needle so it points along the magnetic north-south line. Without H, the needle has nothing to line up with. Now, what happens to H as you move around the globe? When you approach either of the earth’s magnetic poles, H approaches zero strength. Meanwhile, the vertical component, which is called Z, approaches the value of T — meaning nearly all of the total field is now pulling straight down. Over the pole itself, the dip angle is 90 degrees, the directive force H is zero, and the magnetic sensor — the compass — becomes useless. That’s a critical operational point: at the magnetic poles, the compass simply cannot give you a heading. Now go the other way, to the magnetic equator. There the situation reverses. The directive force H approaches the value of T, so almost the entire field is horizontal. And Z approaches zero, as does the angle of dip. So at the magnetic equator the compass works at its strongest, because the full field is lying flat to pull the needle. So you can see the pattern: the directive force H decreases as the angle of dip increases, and vice versa. Figure 9.9 illustrates this relationship. But I want to be careful here — the book warns us that the relationship between H and dip angle is not quite as simple as it appears. That’s because of irregularities in the pattern of the earth’s field, and variations with position and time of the total magnetic force T. So the simple inverse relationship is the general rule, but real-world irregularities complicate it. Here’s a useful rule of thumb to remember: the strength of the horizontal component H at a latitude about 60 degrees north of the magnetic equator is very roughly half the value of H at the magnetic equator. So at 60 degrees north, your directive force is only about half what it is at the equator — that’s a big practical drop in compass authority. Now let’s move on to the changes in earth magnetism, because the field isn’t steady — it changes over time. The book splits these into regular changes and unpredictable changes. First, the regular changes. The most significant of these are called 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. So the magnetic poles don’t sit still — they drift slowly around the geographic poles on a cycle of roughly 960 years. The north magnetic pole is moving slowly westward, and this wander mainly affects magnetic variation. Variation, remember, is the angular difference between true north and magnetic north. So as the pole wanders, the variation changes. Here’s a concrete example from the book: 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. So the variation is slowly shrinking, and eventually, in London, it’s predicted to be zero — true north and magnetic north will coincide there. Now, how do we keep charts up to date with this drift? The annual rate of change of variation is shown on navigation charts, so that the variation printed against the isogonals can be readily up-dated. Isogonals are the lines on a chart joining points of equal variation. So the chart gives you the current variation and the annual rate of change, and you can project it forward. There are other regular changes too. They 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 the daily, yearly, and eleven-year cycles are real, but they’re too small to worry about in normal flying. Now the unpredictable changes. These are the magnetic ‘storms’. They occur at irregular intervals, have varying intensity, and can last for as long as three days. These phenomena appear to be produced by unusually large sunspots. What’s the main effect of a magnetic storm? It’s a temporary but significant change in magnetic variation. The alteration is unlikely to exceed 2 degrees in the UK, but in the Arctic and Antarctic the change may exceed 5 degrees and last for as long as an hour. So in polar regions, a storm can swing your variation by more than 5 degrees — that’s a serious error for navigation. And there’s one more effect I want you to note. The value of the directive force H can also change during a storm, and in high latitudes it may fall below the minimum required for efficient compass operation. So not only does your variation shift, but the very force that aligns your compass needle can drop below the level needed for the compass to work properly. That ties right back to what we discussed at the start — H is the directive force, and if it falls too low, your compass is compromised. So to pull it all together: H is the horizontal component that drives the compass, it weakens as dip increases and strengthens toward the magnetic equator. The field changes regularly — secular changes over a 960-year cycle, plus smaller daily, annual, and eleven-year cycles — and it changes unpredictably during magnetic storms, which can shift variation by over 5 degrees in polar regions and drop H below the minimum for reliable compass operation.