When I tell people that I study the history of Earth’s magnetic field, I get a bit self-conscious – as if I just told someone I specialize in Santa Claus. Geologists call us “paleomagicians” for a reason. You can’t see magnetic fields. You can’t touch them. Unlike most geological stuff, nothing obvious happens if you hit a magnetic field with a hammer. Once you understand a few things about Earth’s magnetic field, though, it becomes a bit less mystical. In the next few articles, I’ll try to bring Earth’s magnetic field … um … down to Earth.
Number 1: Compasses line up with magnetic fields. Although you can’t see a magnetic field, you can see its effects. In the pre-GPS days, when we still used maps and compasses, we used those effects all the time. Compass needles (which are themselves magnets) line up with magnetic fields. One end of the compass needle is the “north seeking” end, which points toward Earth’s North Magnetic Pole [1]. But wait: Earth’s North Magnetic Pole is not its North Pole! And the North Magnetic Pole moves from year to year. Here is a movie showing the angle your compass would point (relative to True North… as in North Star North) at different places on Earth, over the past 400 years more or less. Scientists made this animation in part by looking through old navigation logs, matching ships’ compass readings with the same ships’ positions based on speed estimates (dead reckoning) and star sightings [2]. Keep an eye on the North Magnetic Pole – where the lines converge in the Northern Hemisphere – as it drifts aimlessly around the Arctic. How random is this drift?
We want to how Magnetic North changes through time because it helps us navigate. But that’s really not the main issue now that we have GPS. We want to know how Magnetic North wanders because it’s a puzzle, and because it brings up some even more fundamental puzzles about the Earth. Why does Magnetic North wander? Where has it wandered in the past? If we were to watch a compass for, say, a million years, would it point at the true North Pole on average? And what, if anything, does that wandering tell us about the Earth?
[1] Physicists (and geophysicists) represent magnetic field lines in a few different ways: as arrows that line up the way compasses would (field vectors), as lines that connect those arrows (field lines), or, confusingly, as lines that illustrate the strength of the magnetic field (contour lines). You can play around with some of these representations here.
[2] If you want to see the original work, it’s by Finlay and Jackson (2003) and Jackson et al. (2000). These are not meant to be entry-level papers.