Welcome back to Our Earth Its Climate History and Processes. I'm David Schultz. This lecture we're going to talk about the Earth's Magnetic Field. As early as 720 A.D. the Chinese had mapped the orientation of the Earth's magnetic field. And we're using that, to, for compasses, and navigation. And the western world first began to think about the Earth's magnetic field around 1600, when William Gilbert, who was a physician to Queen Elizabeth the first in England, he provided the first scientific explanation for the Earth's magnetic field, and that is that the Earth is a magnet. And, you can see in this diagram here, the similarities between the magnetic field of a bar magnet on this side and the Earth here. Now, I want to point out that, compasses, the north pole of the compass actually points to the south magnetic pole. Magnetic pole of the earth, even though that south magnetic pole is in the geographic northern hemisphere. As you know, opposites attract so the magnet, the north pole of the magnet will be attracted to the south magnetic pole. As we go on, later it wasn't until 1838 that Carl Gauss, the famous mathematician, determined that the source of the magnetic field must lie within the Earth, that it was not some kind of externally imposed magnetic field. So fast forward to the present where we've determined how generically how the magnetic field occurs and it requires four different things. First, you need a conducting fluid. And that comes from molten iron in the outer core of the Earth's interior. We need convection of a sufficient magnitude in order to produce the currents that are needed in order to induce the magnetic field. And so you know, the question about how fast the convection occurs inside the outer core is certainly an issue. We don't really have a lot of direct observations of this. We also know that you need the rotation of the Earth on it's axis. Venus which the next planet closest to us, closer to the Sun. Does not have a magnetic field but it's got a very slow rotation rate compared to Earth, and so that is the reason that's suspected for why Venus doesn't have a magnetic field. And you also need a pre-existing magnetic field of some type, and here we can simply use the magnetic field that's induced by the sun. So, we were talking about the convection inside the outer core, and, you know, it's not known for certain where this comes from. But one big suspect is that when the molten outer core solidifies, on the solid inner core, that you get a release of energy from this process, from this solidification. And it's called latent heat of condensation. It's exactly the same kind of thing that happens when ice freezes. when, when liquid water freezes into ice, you get a release of that energy that kept the liquid water in a liquid form. When it freezes, it turns into a solid and that releases latent heat energy. At this point, I want to talk a little bit about the structure of the Earth's magnetic field and how it's changed over time. And you can see here from this graph from the World Data Center for Geomagnetism in Kyoto, Japan, that the Earth's magnetic field has been moving. And here you can see that in 1900, it lay in the northern, islands of Canada, in the Canadian Artic, and slowly, over time, it's been moving northward. Since about 1980, 1990, it's been accelerating pretty rapidly, and it's projected to be, much closer to the geographic North Pole than it's been in the recent past. And as you might expect similarly, the Southern magnetic pole in the Southern Hemisphere, which as you remember is also a North magnet, magnetic pole, has also changed and is also moving in this case, further away from the southern pole. So although it's not exactly a bar magnet, the, it's not a symmetric feature. The magnetic poles are in flux, and we know that they don't change over time. If we look at the distribution of the horizontal component of the magnetic field. We can see if we map this on the earth the orientation of the magnet field. And here you can see the south polar magnetic field and if we were to continue this graph up towards the North Pole, we would see that it lies up there. And then these numbers represent the orientation of that relative to local north. So in this case, here we've got 20 degrees. That means a orientation this way, over here, we've got an orientation of 30 degrees, that means that the magnetic field at this place on the earth, is oriented 30 degrees. And then it's pointing towards the left of north, to the west of north. So on this graph, what we see is the magnetic inclination. This represents the vertical component of the magnetic field. In the southern hemisphere, at the south magnetic pole, you can see, minus 90 degrees. That means that, the magnetic field lines are coming out of the South Magnetic Pole out of the earth. And here in the Northern Hemisphere, the North Magnetic Pole, we can see a positive. 80 degrees, that means these magnetic field lines are going, into the earth at, a rather steep angle. And as you might expect, near the equator the lines, are running roughly parallel to, the earth neither going up or down. And the green line here represents the zero contour.
