Now, perhaps what may be surprising, to you because we live in this period of relative stasis where the Earth's magnetic field, has been undergoing very little changes on the scale of human life time, is that. The earth magnetic field can sometimes reverse. And what I mean by that, is that the south magnetic pole in the southern hemisphere, becomes the North magnetic pole in the Southern hemisphere and thus essentially compasses would, would point the other way. So where is now we've built compasses to point towards, the geographic north, after a reversal, it would be pointing towards the geographic south, the north magnetic pole. Now, we don't have to worry about this on the scale of a human lifetime. The last time this happened was 780,000 years ago when we had last reversed the polarity. But you can see over geologic time this being the present time and going back one, two million years that sometimes we're in normal polarity. Sometimes we're in reverse polarity and this happens. And it happens, a periodically you can't predict it and it seems to be consistent with as you'll see a chaotic convective fluid inside the Earth. And if we go back, back 5 millions years we can see 24 reversals in the last 5 million years. Again, now, with this larger perspective, you don't see any periodicity in here. There's no characteristic time scale of these reversals. And in fact, with in the last five million years, we've been actually in a normal polarity for quite a bit of time. In fact, the longest period. Since 5 Million years ago. To illustrate what that might look like, here is a graph of magnetic field lines, from a simulation by Gary Glassmeyer and Paul Roberts at in the University of California. And what they showed. First in 1995 was how the Earth's magnetic field could be maintained, through computer simulations. But then more importantly, how these reversals could happen. And it was almost serendipitous. They were trying to model the steadiness, of the Earth's magnetic field and instead what they modeled. and, and, and they did capture that but instead they also modeled one of these reversals by accident and I think that really illustrates that they were on to something here with their modeling. So here you can the earths magnetic field lines going in to the earth and then out of the earth and of course these would join up out here at some distance from the earth. And this is the simulation, that and Roberts produced. Here, you can see the Earth's magnetic field, on the surface of the Earth, with the inclination into the northern hemisphere, here. And, out of the southern hemisphere, here. Further down, in the outer core. The surface of the Outer Core, you can see much more much more detail, much more structure in the Earth's magnetic field. And that's due to the convection going on there. So if we put this into motion, we can see the convection occurring down here. We see occasional bubbles of reverse polarity and move up into the Northern hemisphere and then BOOM there we go. We've reversed the polarity here of the Earth from where it used to sit with the blue towards the North and the red towards the South. And you can see it was quite a chaotic process. Just like. The bubbling of liquid, at the base of the core. Now we can run that through, one more time. Here we are with blue to the north. As in normal polarity. We can see the convection start to get going in the northern hemisphere. Reds and yellows and oranges moving into the northern hemisphere. And then boom. We've reversed, the polarity. And so, these reversals as I said, are quite common in the geological record, and I'm going to talk in the next lecture about how we are able to ascertain that process. So, to summarize today lecture, the earths magnetic field is a result of a convecting, conductive fluid within a rotating earth. And we know that this layer, is the molten iron within the outer core. We also recognize that the Earth's magnetic field, has changed over geologic time. And from the geologic perspective, it can change relatively abruptly. These reversals may only take 1,000 or 2,000 years to happen. And the third point, is that the Earth has undergone unpredictable and aperiodic reversals. We don't know how to predict them, as you can see from the simulation. It just is a chaotic bubbling fluid inside the earth, and as a result there's no real way to predict these things. And the consequence of that is that they have no periodicity. There was nothing that we can say, after say, 500,000 years we can expect a reversal. So, with that, thanks for attending. And I'll talk to you in the next lecture.