Earth Magnetism — Page 56, Lesson 60

Right, let's pick this up with the changing nature of the Earth's magnetic field. We've established that the magnetic poles don't sit still, and now we're looking at exactly how and why they move. For a long time, the old theory was that the Magnetic North Pole rotated in a near-perfect circle around the True Pole, completing one revolution every 960 years. But that was just an empirical observation—meaning it was based purely on what people had seen over a limited period, not on a proven physical law. More accurate observations over the last 50 years have shown that, at least recently, the Magnetic Pole has actually been moving northwards as well as westwards. So the honest answer is that the Earth's magnetic field is certainly changing, and by watching it over time and extrapolating that change forward, we can make reasonably accurate forecasts of variation for up to about ten years ahead. Now, within that overall change, we can identify at least three predictable cycles. The first is the secular movement—that's the long-term change I just described. The second is the annual cycle. Superimposed on that long-term drift is a sinusoidal change with a period of one year, and this is associated with the Earth's orbit around the Sun. The third is the diurnal cycle. Superimposed on the other two is a sinusoidal change with a period of one day, and this one is tied to the daily changes in the height of the ionosphere as the Earth rotates, presenting different areas of the upper atmosphere to the Sun. The variation can change by up to about 0.1° over the course of a single day. But there are also unpredictable changes, and we have two of them here. One is linked to solar activity, and one to local anomalies. Let's take solar activity first. The Sun goes through cycles of sunspot activity that peak every 11 years. During those peaks, huge solar flares are expelled far out into space. Now here's the key point: the 11-year period itself is predictable, but whether a given flare actually affects the Earth is not. If one of those flares happens to be pointed towards Earth, a tongue of intense ionisation curls around the upper atmosphere, and that causes various effects on our magnetic field. So when you're planning a flight and you look up the variation for your area, remember you're reading a forecast, not a fixed value. It's a prediction built on these secular, annual, and diurnal cycles, with the understanding that solar activity and local anomalies can throw in unpredictable changes on top.