If one of these flares is pointed towards Earth, a tongue of intense… — Page 56, Lesson 61

We've been talking about the Earth's magnetic field and how it's not constant. Now I want to walk you through what happens when the Sun throws a tantrum, and why that matters for your compass. You see, the Sun occasionally emits these bursts of energy called solar flares. Now, if one of these flares is pointed towards Earth, a tongue of intense ionisation curls around the upper atmosphere. That's a stream of electrically charged particles. This causes various effects, the most notable of which is the Aurora Borealis — that's the Northern Lights. There's also a similar effect around the South Magnetic Pole, called the Aurora Australis. So the Northern Lights and the Southern Lights are both caused by this same solar activity. These events are known as 'magnetic storms', and the effects can be very intense. Here's a number you need to remember: during the most recent of these 11-year cycles, variation changes of up to 7° were observed. That's a huge shift in your compass reading. Now, sunspot activity and solar flares occur at other times as well, so minor magnetic storms can happen even outside the 11-year peak of the period. So you can't just assume it's calm because we're not at solar maximum. Next, let's talk about Local Anomalies. These are caused by magnetic deposits or rock formations that cause the field to be different within a particular area. So if you're flying over a region with certain iron-rich rocks, the local magnetic field will be distorted compared to the surrounding area. In addition, scientific surveys have shown that variation can change slightly with altitude. However, few aircraft systems would be sensitive enough for this effect to be noticed. So it's a real effect, but practically negligible for you. Now, here's the big takeaway. Because of all these factors — magnetic storms, local anomalies, altitude effects — it is very difficult to know the precise instantaneous value of variation affecting an aircraft to better than about 2 degrees. And even if you take great care over finding and correcting for it, you can only get to better than about half a degree over a period of time. So your magnetic heading is never perfectly accurate. This is why the emergence of systems based on highly accurate gyros, particularly INS — that's Inertial Navigation Systems — in the 1960s and 70s was such a major advance in navigation technology. It wasn't merely that they calculated present position, important though that was. It was because, for the first time, there was a source of accurate, reliable heading. That's the key point: INS gave you a heading that didn't depend on the fluctuating magnetic field. Now, let's move on to updating isogonals. Pilots should always fly with the most up-to-date chart for flight safety reasons. Aeronautical information is constantly changing. Airways are re-aligned, the positions and frequencies of VORs and DMEs are changed — VOR is VHF Omnidirectional Range, DME is Distance Measuring Equipment — and danger areas alter shape or are moved as civil and defence requirements change within a country. If the chart is republished at frequent intervals, the isogonals should be comparatively recent as well, and for most radio navigation charts it is not normally necessary to update the isogonals. However, the interval between reissues of topographical maps may be considerably longer — perhaps every 5 or 10 years. In this case, if the variation has changed significantly, it may be necessary for the pilot to bring the isogonals up to date during the flight planning process. So you, as the pilot, have to do the updating yourself. On most maps and charts, the year of origin is shown, and some indication of the annual change is given. This annual change is due to the movement of the magnetic poles. This may be done in one of two ways. First, by a small arrow showing the direction and distance of the annual change of the position of the isogonal, as in Figure 3.7. Or second, by a statement giving the annual change in the variation quoted on the isogonal, as in Figure 3.8. So to summarise: magnetic variation is not a fixed number. It changes with solar activity, local geology, altitude, and time. And because of that, you need to know how to update your isogonals from the chart's annual change information. That's the practical skill you'll use in flight planning.