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Earth’s Magnetic Field


I know just a little about the Earth’s magnetic field – also called the geomagnetic field. The following are from some notes I wrote a few years ago and came across lately (thought maybe worthwhile sharing in my own words). My notes were based on: Ency. Britannica; Wiki article on geomagnetic field; and ‘chapter 3′ of a physicist’s notes (whose name I’m missing).

1. Magnetic north and Earth’s true north aren’t the same!

2. In fact, it is the magnetic south pole that’s closer to the Earth’s north, by something like 11 degrees. (That is called ‘magnetic declination’, the angle difference from true north.) For precise navigation this 11 degrees could be taken into account.

3. The magnetic field lines (usually written as B in physics) start from magnetic north and end at magnetic south. (At least, that is the convention.) Magnetic fields affect only charged particles (like electrons and protons). These particles move along the field lines by spiraling around them (like a coiled wire).

(They go back and forth. The reason they spiral in doing so is explained by the magnetic force being F = q v x B, where q is the charge on the particle, v is its velocity, and B is the magnetic field. The force is always perpendicular to B and v, which is why they spiral.)

4. The Chinese appear to have been the first to discover the geomagnetic field in their effort to perfect their navigation technology. (About 1100′s AD or so.) Later Sir Edmond Halley (of Halley’s comet) mapped the magnetic field.

5. It was believed 100s of years ago that the geomagnetic field had extra-terrestrial origin. It was the brilliant mathematician Carl Friedrich Gauss (mid 1800s) who showed that the field actually had its origin from the Earth itself, and he gave a mathematical expression for it (using spherical harmonics).

6. The magnetic field of the earth can experience reversal — where magnetic north and south poles are interchanged. But this happens irregularly, from some 700,000 or so years to a few million years. (I think it’s still a mystery as to why this happened in the past.) These reversals are recorded in rocks that register the field’s direction in the past. And in turn, this has been valuable in determining the history and motion of plate tectonics – and discovery of the mid ocean ridges which affect continental motion.

7. The motion of molten iron in the core of the Earth is generally credited for the creation of the geomagnetic field — this is called the geodynamo theory of Sir Bullard (about 1940s-50s). (The Earth’s crust has its contributions too, but they are fairly smaller.) Scientists study the inside of the Earth in part from how the geomagnetic field behaves and changes.

8. There are still some mysteries about geomagnetism. For example, why do the magnetic poles move about 10 km each year?

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