Earth Magnetism — Page 56, Lesson 60

I want to walk you through the changing nature of Earth's magnetic field, because this is fundamental to understanding how a magnetic compass behaves in the air. For a long time, people thought the Magnetic North Pole simply rotated in a near-perfect circle around the True North Pole, completing one full revolution every 960 years. But that was just an empirical observation — a guess based on watching it over a limited period. More recent and accurate observations over the last 50 years have shown something different. At least over the last few years, the Magnetic Pole has been moving northwards and westwards. So the old circular model is not correct. What we can say for certain is that the Earth's magnetic field is changing. By observing it over time and extrapolating that change — projecting the trend forward — we can make reasonably accurate forecasts of magnetic variation for up to about ten years ahead. Beyond that, the predictions become unreliable. Now, within that overall change, there appear to be at least three predictable cycles in the pattern. Let me name each one. First is the secular change. The word 'secular' here means long-term. This is the long-term drift I just described — the northward and westward movement of the Magnetic Pole over many years. Second is the annual change. Superimposed on that long-term secular drift is a sinusoidal — that is, a smooth wave-like — change with a period of one year. This annual cycle is associated with the Earth's orbit around the Sun. Third is the diurnal change. 'Diurnal' means daily. Superimposed on both the secular and annual patterns is another sinusoidal change with a period of one day. This appears to be associated with the daily changes in the height of the ionosphere as the Earth rotates. As different areas of the upper atmosphere are presented to the Sun, the ionosphere's height changes, and this affects the magnetic field. The variation can change by up to about 0.1 degrees over the course of a single day. So those are the three predictable cycles: secular, annual, and diurnal. But there are also unpredictable changes. Two main ones are mentioned here. One is associated with solar activity. The Sun experiences cycles of sunspot activity that peak every 11 years. During these peaks, huge solar flares are expelled far out into space. The 11-year cycle itself is predictable, but whether a particular flare will affect the Earth is not. If one of these flares is pointed towards Earth, a tongue of intense ionisation curls around the upper atmosphere, causing various effects on the magnetic field. The other unpredictable change is associated with local anomalies — local irregularities in the Earth's magnetic field caused by things like mineral deposits. We'll cover those separately. So to summarise what I want you to hold onto: the Earth's magnetic field has predictable long-term, yearly, and daily cycles, plus unpredictable disturbances from solar flares and local anomalies. And our forecasts of magnetic variation are only reliable for about ten years ahead.