The Earth's Magnetism

by David Fiedrich (avi)


1. Introduction

Imagine if you will the following situation: Two bodies (perhaps of a nondescript or even mundane appearance) irresistibly drawn to one another. It seems as though the closer they get, the more powerful this attraction gets. What's more, once they touch they are inseparable of their own accord. They appear to battle against all odds to stay together. Such is their attraction for one another; that one would actually need to be physically removed from one an- other. In this manner, these two companions stand the test of time, gravity, man, weather, Dick Clark, almost everything just so that they might be next to one another.

At this point, you may, with reason enough, ask yourself who these model tavarishchi are. What could they be made of so that they are able to stick together through all of their modern trials and tribulations? Is it love? Is it fate? Is it destiny?

No, its magnetism.


2. What Is It?

Webster's Ninth New Collegiate Dictionary defines this spectacle of science as, "a class of physical phenomenon that include the attraction for iron observed in lodestone and a magnet, are believed to be inseparably associated with moving electricity, are exhibited by both magnets and electric currents, and are characterized by fields of force."

This phenomena alone is a subject of such magnanimous proportions that to encompass it all would definitely be overkill. So, in an attempt to narrow the subject and come to the point, what happens when we, say, look at a compass?

Why, the needle points more or less north and south, aligning itself between these two poles (although, just for the record, keep in mind that this is not always precisely true). With this little compass experiment, we can safely say, however, that the Earth has a magnetic field about it. "Einstein spoke of it with awe, describing it as one of the biggest questions that physics still had to answer." (Robert Crum, 37). The convergence points for our little compass and its nose for north and south are not, unfortunately, the north and south poles, that is, those poles along which the axis of rotation runs (5, p. 45). We would instead find ourselves at the magnetic north pole or the magnetic south pole. There exists at this point in time a eleven degree difference in the axis of rotation and the magnetic poles. This "can be fairly well described by the model of a small but powerful permanent bar magnet located near the center of the Earth and inclined about eleven degrees from the geographic axis" (Michael Wysession, 477). With this in mind, we shall see below a deeper look into what this whole magnetic field is, what it looks like, what it does, and how it does what it does. This magnetic force surrounds the entire planet and does much more for us than make sure boy scouts can find their way home. If we had to draw how strong this giant magnetic field looked, we would use lines "pointing in the direction of that field" (3, p. 49) to represent the amount of force in that field. The greater the force found in a field , the more lines it would receive in a physical representation with the converse also being true. We can also assign the relative field strengths with a color and superimpose them on a map of the world.To find out just how strong or weak a magnetic field is, a machine called (strangely enough) a magnetometer is used (Michael Wysession, 139).

Also in the pursuit of glibly measuring magnetic fields, a special unit of measure is used, called the gauss; named so for the German mathematician of the same name (3, p. 49). The Earth's very own magnetic field, at its strongest points, would register .6 gauss on the old magnetometer while, by contrast, a child's play magnet would pull in a whopping 10 gauss (3, p. 49). If we wanted to describe what this field looks like or the direction in which the force lines will travel for our physical representation, we have to use quite a different procedure.

Mariners and seamen have been using compasses to plot their positions, map new worlds, find their way round the globe, etc. for hundreds of years. Jeremy Bloxham, a geomagnetist at Harvard, has taken full advantage of almost 400 years of navitagors' experience to map out the direction, from a cartographic point of view, in which these force lines should travel on paper by using ships' logs to see what they recorded as the magnetic poles at that time and how they differ from other points in history on up to the present relative to each time and place given (Michael Wysession, 138).

Along these same lines, we must direct ourselves to the study of palaeomagnetism, or "the study of the fossil magnetization of rocks of all ages "(3, p. 53). Upon the transformation from liquid to solid, many rocks will become magnetic or have traces of magnetic elements within them. When this occurs, the rock will keep indefinitely that magnetic imprint. "Thus, by comparing the remanent magnetism in any particular rock ..., scientists can determine how much the rock may have moved since its birth (Golden, 20)." This is a perfect process by which we can make archeological use of common stones. We can, for example, use palaeomagnetism to see how far the magnetic poles have moved from the geological axis (polar wondering) over the years (3, p. 53). We can see, based on the magnetic "fossils" on opposing shores and mountain ranges on continents, how much and where the continents have moved (3, p. 53). We can see that a most curious thing (actually nine times in the past 4,000,000 years) has happened to the Earth's magnetic poles (3, p. 53). They have for some reason and indiscriminate lengths of time completely flipped, so that north was south and south was north (3, p. 53). We have deducted that the

Earth has had a magnetic field, and thus a liquid core, for at least the last 3.5 billion years (4, p. 481). One more thing, although the list does not stop here;


3. What It Looks Like

From space, our magnetic field, or magnetosphere, looks much like a comet does traveling through space. Cattermole and Moore liken its shape to a teardrop (3, p. 51). The reason for this shape is the solar wind from our sun. As the solar wind "blows" on the Earth, it distorts through compression all of the waves between itself and the fore side of the Earth (5, p. 45). After, the magnetosphere is drawn out like the tail of a comet. Because of this distortion the magnetosphere is, of course, closer to the Earth on the side facing the sun, then the other side. Seeing as the edge of the magnetosphere, or magnetopause, star-side is only ten Earth-radii away, while on the other side, the magnetopause can be found no less than 60 Earth-radii away. Here, we are talking about a size difference of about 50 Earth-radii, or approximately 380,000 km from one end to the other (5, p. 45).

Now that we have explored what this magnetic field is and what it roughly looks like, allow me to tell you what it does. Just like the word "atmosphere", our magnetosphere totally engulfs the planet, encircling her in a protective "shield" (2, p. 49). This protective magnetic shield serves to shelter us from radio-active particles that stream from the sun. Without this shield we would all have long since roasted in our own juices. Not only is the sun a threat, but also Jupiter who insists on daily bombarding us with charged particles--to no avail. We receive a large majority of such cosmic rays from outside our native solar system (3, p. 49). As above, this steady stream of cosmic rays from the sun assaulting our magnetic field is what causes its unique shape.

So, now that you have seen this peculiar magnetic magic show, I am sure you are all asking yourselves, 'How does he (the Earth) do it?' Well, I'll tell you.


4. Why It does What It Does

The Earth is largely liquid. I don't just mean the two-thirds of the entire surface mass comprised of water. Below the crust, which is mostly solid, we know that the make-up is a sort of very viscous fluid in which the lighter stuff floats (continents) and the heavier stuff sinks down (Crum, 35). To figure out just what the interior of the Earth looks like, geologists used seismic tomography. This is the process of tracking earthquakes around the globe to see just what the vibrations were passing through.

Richard O'Connel, a Harvard professor of Geophysics, says of it, "Seismic tomography gives us a snapshot of the Earth's structure ...and it confirms that the Earth is a heat engine "(Crum, 37). The concept of this "heat engine" is the key to what produces the Earth's magnetic field. We know that the Earth's innards are stratified based on temperature and composition. Two strata which interest us especially are the mantle and the outer core. Both layers being comprised of rock and liquid nickel and iron to varying degrees, it is here - actually the boundary (or CMB) - that we see the effects of convection, or heat transfer by conduction. Says Adam Dziewonski; "All of a sudden you go from solid rock of the lower mantle to the liquid outer core, which is mostly iron, highly conductive, probably the area where the magnetic field is generated" (Crum, 37). We know that the CMB acts as a convection oven fueled by gravity and the contractions of cooling rock (Wysession, 140).

Through research, it has been established that the liquid outer core creates a "self-generating dynamo which in turn produces a kind of electromagnetic field--or the Earth's magnetic field" (Crum, 38). On the other hand, we know that because the Earth passes a magnetic field, the outer core must convect (Wysession, 144).


5. Conclusion

So, as you can see, the Earth's magnetic field is nothing to scoff at. It affects us much more than the average Joe Smoe would even care to guess. From keeping two imaginary, magnetic bodies stuck fast to one another through thick and thin to providing us with palaeomagnetic fossils and protecting us from intergalactic charged particles, this wonder of nature is exactly that. Magnetism is a benefactor among phenomenon. The Earth's magnetic field, a prime example of magnetism at work, is formed by the complex swirling and convection in the bowels of the Earth. It radiates from the planet to protect us from all sorts of galactic bad stuff. We humans have been learning to harness and discribe its motions and power. Who knows what good deeds the future holds for our magnetic field.


Glossary


References

1. Broeker, Wallace "How to Build a Habitable Planet" Eldigo Press, Palisades. 1985

2. Calder, Nigel "The Restless Earth" The Viking Press, New York. 1972

3. Cattermole, Peter and Moore, Patrick "The Story of the Earth" Cambridge University Press, Cambridge. 1985

4. Press, Frank and Sevier, Raymond "Earth" W. H. Freeman and Company, New York. 1992

5. Smith, Peter J. "The Earth" Macmillan Publishing Co., New York. 1986


On The Web...

http://unix.nerc-murchison.ac.uk/modeling.html

http://unixb.nerc-murchison.ac.uk/intermagnet.html


Thanks

Thanks to Nerc-Merchison for the pictures.

I would like to thanks Jasna Gojkovic, pudgy bunny. I hope the library lets you back in.

Thanks to whoever the hell it is that grows all those coffee beans. Keep up the good work.

Thanks again to Ben van der Pluijm for all the nifty gadgets and eternal patience.