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Images courtesy the COMET program and Windows to the Universe original artwork.
Earth's Magnetic Poles
Earth has a magnetic field. If you imagine a gigantic bar magnet inside of Earth, you'll have a pretty good idea what Earth's magnetic field is shaped like. Of course, Earth DOESN'T have a giant bar magnet inside it; instead, our planet's magnetic field is generated by swirling motions of molten iron in Earth's outer core.
Earth's magnetic poles are located near, but not quite exactly at, our planet's geographic poles (the "spin axis" on a globe). Compasses work as direction-finding devices because Earth has a magnetic field. The needle of a compass, itself a small bar magnet, points in the direction of the magnetic pole. When you are far from either pole, a compass is very useful for navigation, for the direction it points is pretty close to true North (or South if you are in the Southern Hemisphere). However, if you were near either pole, the magnetic pole and the geographic pole could lie in very different directions (they could even be on opposite sides of you!), so a compass would not be very useful at all. Early explorers in the Arctic and Antarctic found navigation difficult at high latitudes for this reason.
A magnetic field that has two poles, like the field around a bar magnet, is called a "dipole field". Earth's magnetic field is pretty much (though not exactly) a dipole field. Earth's dipole is tilted slightly in comparison to its spin axis. The difference between the dipole's orientation and the spin axis is about 11 degrees. If you imagine a giant bar magnet inside Earth, tilted at an 11° angle away from the spin axis, you'll have a sense of what this is like. That's why the magnetic poles and the geographic poles are not in the same place. If you were standing at one of the magnetic poles, the magnetic field lines would be straight up and down. If you were holding a compass and turned it sideways, its needle would be oriented vertically!
Earth's magnetic poles and its geographic poles are actually pretty far apart. As of 2005, the North Magnetic Pole was located in the Arctic Ocean north of Canada about 810 km (503 miles) from the Geographic North Pole. The South Magnetic Pole was just off the coast of Antarctica, in the direction of Australia, about 2,826 km (1,756 miles) from the Geographic South Pole. Why, you may wonder, are the magnetic poles different distances from their respective geographic poles? Besides being tilted by 11°, our imaginary bar magnet representing Earth's (almost) dipole field is offset to one side; it doesn't pass directly through the center of Earth. In fact, it misses Earth's center by about 530 km (329 miles).
Did you notice how we said that these were the locations of the magnetic poles in 2005? The poles actually move over time! During the first part of the 20th century, the North Magnetic Pole moved an average of about 9 km (5.6 miles) per year. Around 1970 it speeded up dramatically, and has been moving about 41 km (25 miles) per year in recent years. Remember, Earth's magnetic field is generated by swirling motions in Earth's molten metal outer core (NOT by a gigantic bar magnet inside of our planet); so it should come as no surprise that changes in this swirling motion would cause shifts in our planet's magnetic field. You may have heard that Earth's magnetic field actually flips over from time to time, so that the North and South Magnetic Poles actually switch positions with each other. Does the increased rate of movement of the magnetic poles herald the coming of such a flip? Nobody knows for certain!
Speaking of flipping, did you know that the North Magnetic Pole is actually a south pole? Huh, what? When compasses were first invented and people noticed that one end of the compass pointed towards the North, they called the end of the compass needle that pointed North the "north end" of the needle (logically enough!). Remember, though, that the needle of a compass is actually a small magnet. If you recall how magnets work, like ends (a north and a north OR a south and a south) repel one another, while opposites (such as a north and a south) attract. So, since the north pole of a compass needle (a tiny bar magnet itself) points toward (that is, is attracted by) Earth's North Magnetic Pole, Earth's North Magnetic Pole must be the south pole of a magnet! Doesn't that just make your brain ache! Likewise, Earth's South Magnetic Pole is actually at the north pole of the planetary dipole field.
Earth's magnetic poles also wander around on a daily basis. Huge quantities of charged particles flowing outward from the Sun, called the solar wind, carry a magnetic field along with them. When this "wind" of charged particles collides with Earth's magnetosphere, it causes Earth's magnetic field to shift slightly. Since Earth's dipole field is tilted, this shift wobbles around over the course of each day. Recall, for example, how the North Magnetic Pole is off the coast of Canada. When Canada is on the daytime side of Earth, the magnetic field flowing from the Sun with the solar wind pushes Earth's magnetic pole one direction. When it is nighttime in Canada, the push is in the other direction. The amount of daily motion of the magnetic poles depends on the strength of the solar wind "blowing" in from the Sun, which varies substantially over the course of the 11-year sunspot cycle. During times of "stormy" space weather, Earth's magnetic poles can shift position by 80 km (50 miles) or more in a single day!
What significance do the magnetic poles have for life on Earth? As we've mentioned above, navigators through the centuries have used compasses to get their bearings. The aurora, the Northern and Southern Lights, are generated by collisions between charged particles from space and gases in Earth's atmosphere. The charged particles spiral downward into the atmosphere along magnetic field lines; hence, the aurora are most commonly seen near the magnetic poles where the field lines come to Earth. For similar reasons, airline flights near the poles must take precautions regarding radiation exposure to passengers and crews, for the same particles that light up the auroras also pose a radiation danger at the altitudes at which modern jetliners cruise.
