How does the strength of the magnetic fields at each end of the poles compare to that at the center of the earth?
LATHROP: The magnetic field is the strongest inside the outer core, right where it’s being generated. And as you move out away from the core, it just gets weaker as you move further away. So that the main magnetic field of the earth all comes from the core, which is thousands of miles down below us.

…relatively far away from the core, all the rest of that fine structure has been washed out. And what you’re left with is mostly north/south.

And is it stronger at either end?
LATHROP: Oh, I’d have to look at a map. The place where it is strongest is inside the core. But it’s not necessarily stronger at one part of the core than the other. The field is actually inside the core, more wrapped around in circles than it is coming out like you’re thinking of, north/south.

And actually, you know, a simple picture of what the earth has is this north/south field. In fact the magnetic field is much more complicated than that, with lots of small structure going in and out of different places. It’s just that most of that is confined to the core. And once you get back where we are, relatively far away from the core, all the rest of that fine structure has been washed out. And what you’re left with is mostly north/south.

However, describing the magnetic field in terms of north or south poles is not the best way. The magnetic field pattern and shape is not best understood in terms of poles, but instead in terms of maps of strength and direction. Imagine trying to understand where are the continents in terms of where are two poles of land?

And how big of an area is the core from where the magnetic field is spread around?
LATHROP: Well, if you start at the surface and descend down into the earth, the outer core is 2,890 kilometers down. So starting at around 2,900 kilometers is the start of the core. Then from there to 15,100 kilometers is the outer core, which is this liquid, iron layer, which is conducting and causing the magnetic field, causing the dynamo. And then that goes down to the center of the earth, which is about 16,400 kilometers down. So it’s roughly half of the earth in radius is inner and outer core together.

And this is thought to be liquid iron that is conducting electricity?
LATHROP: Yes. The outer core is liquid iron, mostly– nothing’s ever pure on the earth, of course, there are lots of other things mixed in. But we know that from examining earthquakes, that the inner core is likely mostly solid iron. By looking at the way that the waves travel through the earth, you can tell which parts are liquid and which parts are solid. So the outer core is convecting, that is just undergoing some churning motion. And that churning motion then couples into magnetic fields and electric currents to cause the magnetic field itself.

And this magnetic field can flip over time?
LATHROP: Right. We know from the rock record that the earth’s magnetic field has reversed many hundreds of times.

…since people started measuring the magnetic field in detail, which is about 160 years ago, the field has dropped more than two percent, just steadily downward.

Is there a measurable effect from the reversals?
LATHROP: The answer is, unfortunately, we don’t know, in the sense that the last time it reversed, it was discovered, in 780,000 years ago. So it’s never reversed while people were around to record what happens. So one can speculate as to what it might be like during a reversal, or you could just wait until the next one, which we may well be heading for now. That’s not certain because we have no way to predict at this point.

But you have some indications that it could be about to flip?
LATHROP: Well, since people started measuring the magnetic field in detail, which is about 160 years ago, the field has dropped more than two percent, just steadily downward.

So the magnetic field –you know, of course it affects compasses, but the biggest effect is the magnetic field strength, as it goes down, the bubble around the earth called the magnetosphere, which shields us from solar charged particle radiation, that shrinks.

The sun is just continuously spitting large amounts of radiation out into space. It’s not just light that we’re used to, but all sorts of electrons and protons, and many of them of high energy. And these are, actually, relatively dangerous for people or other types of life because of the sorts of damage they cause in cells. But the earth has this big bubble around it inflated by the magnetic field, the magnetosphere, most of those charged particle radiation go around outside of the bubble, never making it down to the earth. There are little bits that leak in toward the poles and cause the aurora, but that’s just the tiniest bit of the overall particle radiation coming away from the sun.

You have constructed a stainless steel sphere. How big is that sphere?

LATHROP: Our biggest one is three meters in diameter, or about 10 feet in diameter and it is made of stainless steel. And the one immediately before that that we’re still running active experiments in is about two feet in diameter, or 80 centimeters, and is made of aircraft alloy titanium.

And in the experiments that you are performing, there is water inside of the sphere, and that’s to simulate molten earth?
LATHROP: Well in each of these experiments, ultimately, we’ll run liquid sodium as the main working liquid, as it has very strong interaction with magnetic fields. So the big system, the three-meter system, is filled with water right now, in part as a debugging process. So we’ve spun for the first time, and we’re checking out all the mechanical and other systems just with water, which is lower hazard, before we drain it and dry it and later fill it with sodium.

When do you expect to conduct the sodium experiments?
LATHROP: I think later this year is likely, but that depends on how the debugging goes. It’s not always possible to know the schedule in advance.

And sodium is highly flammable?
LATHROP: I would say just flammable. It burns in the air relatively mildly, but produces an irritating smoke. And the reaction is worsened with water, so we have to keep water out of the lab and well away from it because it would produce hydrogen gas, which is explosive. But as long as there is no water around, the biggest hazard is to have a leak that, if it touches something hot, could catch on fire.