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"The Exploration of the Earth's Magnetosphere"

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29. Space Radiation and our weakening Magnetic Field

Hi Dr. Stern:

I hope you can help me. I was just checking out your webpage and a question regarding the earth's Van Allen belts and solar flares/solar winds. I read that the earth's magnetic field has actually weakened by about 7% and field's actual total energy measured is less by 14% (since 1829). What is the impact of this weakening on the Van Allen Belts and the earth's Magnetosphere?

If solar flare activity increases (e.g. second-biggest geomagnetic storm ever measured hit the earth about a week ago) and the earth's magnetic field weakens, what impacts would we observe inside the atmosphere? Higher radiation exposure for folks on planes? Greater disruptions with electrical grids and radio transmissions? What's projected in the long term?

Can you recommend any websites that "a non-scientist lay person" might be able to read up on this. I guess the late August solar flare activity had nothing to do with the New York blackout (it occurred 2 weeks earlier in August).

Reply

When discussing risks and dangers from radiation in space, you should really distinguish two kinds of radiation:

(1) Trapped radiation is governed by the geomagnetic field. If you are below the belt (as in the international space station) or elsewhere outside its intense part, you should have nothing to worry about. It could well be that the belt is now 7% weaker than in the time of Gauss, 160 years ago, but that does not really change the preceding statement. These ions have about 50 MeV.

(2) Solar flares release unpredictable blasts of particles of higher energy, often 500 MeV and up to 10 GeV. In this case, people on the ground are still safe, because the atmosphere has enough thickness to stop the particles, equivalent to something like 4 meters of concrete. See the end of
http://www.phy6.org/Education/wsolpart.htm If you are in a spacecraft on your way to Mars, that can be dangerous. In Ben Bova's book "Mars" this does happen, and astronauts have to hide in a protected area--behind fuel tanks, probably.

On Earth, we have an additional shield, the Earth's magnetism, which will deflect all but the highest energies from regions at equatorial and middle latitudes. Jetliners crossing the polar region may perhaps find it useful to fly a little deeper in the atmosphere, maybe, and I heard the Concorde carried a radiation alarm.

The magnetic field would have completely protected the space station in its originally planned orbit, inclined 29 degrees to the equator (latitude of Cape Canaveral). As it happened, this was later increased to about twice as much, to enable Russian launch sites to resupply the station (which turned out quite important after the "Columbia" disaster). Twice each orbit, therefore, the station has relatively weak magnetic protection, near its closest approach to the magnetic poles. I heard a rumor that during the 3 big flare events at the end of October the astronauts did in fact hide, but that is strictly hearsay which I cannot confirm. The even bigger flare on November 4 did not produce such a radiation surge.

I am not sure about disruption of power grids, but I think it arises when the auroral electrojets shift to lower latitudes during storms. There are two large electric currents flowing along the auroral zone towards midnight, associated with the polar aurora (or more precisely, with the electric currents which produce big aurora; see in "Exploration of the Earth's Magnetosphere.") Like any electric currents, they produce a magnetic field which can be observed on the ground, and which changes fairly irregularly.

When they move equatorwards, into more inhabited regions (and out of them again), the changing magnetic field induces electric currents in the high voltage networks there. The induction is slow, so the transformers of the grid, configured to impede currents of 60 or 50 cycles/second, see essentially a DC current, to which they offer it no significant impedance, allowing it to grow big. Such a current can burn out transformers, unless appropriate circuit breakers are tripped in time. I don't know how serious that is: burn-outs happened in 1989, but as far as I know, not recently.

I am not an expert in disruption of radio. Flares emit X-rays, which modify the ionosphere, adding ionization deeper down. It then can absorb certain frequencies, but I am not sure whether, say, cell phones are affected, or ships and airplanes. I think the frequencies used by communication satellites are high enough to be immune, and of course a lot of land traffic these days uses optical cable. I am not sure about GPS.

30. Can the Sun trigger magnetic reversals?

Current thinking suggests that the dynamo effect runs the earth's magnetic field. Computer models have been run which suggest that simply applying he rules of interaction of fluid dynamics and magnetism can create a stable and reversing polarity field of magnetism. But we are troubled to explain why the field does not run down, and we really can't model accurately the fluid mechanics of the outer core.

This can be probably be better understood by the picture of many different dynamic movements creating magnetic fields, of a multitude of mini dynamos operating within the earth about areas of upwelling and vortices. The earth travels in the magnetic field of the sun and interacts with solar wind and radiation in the magnetosphere, and this will excite fields which support the current flowing in a particular direction. Hence the north-south orientation of the current field, slowly precessing but stable.

My theory is this, that a hitherto unknown solar eruption on a grand scale changes the exiting fields and as a result, the sum of dynamo fields is altered and could lead to a pole reversal. This would allow rapid pole reversals and does not require wholesale restructuring of the outer core.

I am alas a poor student of extraterrestrial physics and do not have the math, facilities or background to explore my theory further. I would like to discuss this with someone, and ask your advice as to who to approach for assistance.

Reply

Your concern seems to be with the way the magnetic field in space affects that of the core, and whether it can trigger reversals. Actually the effect is very small and thus probably not significant.

As I type this (4 November 2003), a big magnetic storm is in progress, and after dinner my wife and I will drive out of town to see if aurora is visible. According to the web, data from Kyoto on
http://www.antarctica.ac.uk/SatelliteRisks/lastweek.html
the average magnetic intensity at the equator has dropped by some 350 nT (nano-Tesla), in a region where the total field is about 30,000 nT. This seems to be the largest storm of the year so far, and the change is a little over 1% or 0.01 . That ratio also turns out to be of the order of the ratio between the energy of the disturbance and the magnetic energy of the core outside the surface of Earth (theorem of Dessler, Parker and Sckopke). Thus the internal energy is still much larger, even in a large disturbance.

You write "what would happen if, say, a pressure wave moved the so called 'bow shock' boundary say half the distance towards the earth". It does happen in magnetic storms--may even have happened today. When this happens, the result is in the opposite direction from that of trapped ions and electrons added by the magnetic storm. The 1% cited here is the net sum, and the sign (direction of the added field, today southward) suggests the added ions and electrons have a dominating effect.

And that is just at the surface of the Earth. Because of conductivity of the core, any disturbance is very severely attenuated inside it. A highly conducting material shields out external magnetic disturbances.

One thing about reversals. They seem to happen rapidly, but theory suggests (at least considering just the resistive nature of the fluid) that the magnetic energy of the core can only change very slowly. What it might be doing instead is redistribute itself--less in the main "dipole" part and more in the complex components ("higher harmonics") which diminish steeply with distance and therefore contribute less to the field observed on the surface. In recent years, I have read (work of Ned Benton) that the dipole field has weakened steadily, but the complex part has grown stronger, and the sum-total of their energy is roughly constant.

So during a reversal, complex magnetic fields are expected to persist and contain most of the energy. Since these fields are much more intense than anything created by electric currents in space (certainly in the core, but even on the surface), I do not expect external sources to significantly affect the production of the new dynamo field.

31. What is the smallest magnet?

I read with great interest you clear and concise description of magnets and magnetism and was hoping you could answer a question for me.

What is the smallest magnet that is possible ?

If I have read your page correctly there can be no magnetism without an electric current. If this is correct where does the electric current come from within the Earth ?

Reply

What is the smallest magnet possible? Maybe the electron. Imagine it (as people around 1900 imagined it) as a tiny sphere loaded with negative electric charge. If you make that charge rotate around some axis, its different parts will move in circles, each acting like a small current, and the result would be that the electron is magnetized along its rotation axis--"has a magnetic moment" in sciencespeak.

Something like that was discovered in 1925 by Uhlenbeck and Goudsmit, the so-called electorn spin. They deduced it while trying to explain the spectral lines of atoms--see

http://www.phy6.org/stargaze/Sun4spec.htm

These are phenomena of individual atoms, and at that scale, physics laws change to different laws, so-called quantum physics. What on our scale is smooth and continuous (e.g. the range of orbits allowed for a satellite) becomes choppy and discrete (e.g. range of orbits allowed to an electron in an atom). By those laws, even the picture of an electron as a rotating charged sphere, or of an electron orbiting inside an atom, are not really right. But they help our imagination get the main points right--such that electrons are magnetized.

Protons and even neutrons are also magnetized, and much more weakly. This is used in MRI magnetic imaging--see "optional excursion" for the teacher.
http://www.phy6.org/stargaze/Lprecess.htm

32. Isn't the Sun too hot to be magnetic?

Hello Sir,

A magnet loses its properties at a particular temperature, but we know that there is a very strong magnetic field around the sun where the temperature is way beyond the Curie`s Temperature. So what is the reason behind suns magnetism ? Thank you.

Reply

Your question shows an understanding of physics... but also, that you have not read my web pages, where this is discussed in detail.

The magnetism of the Sun indeed used to be a great puzzle: not only was the Sun extremely hot, much hotter than any Curie temperature, but it was also a gas. Yet sunspots were intensely magnetic.

Sir Joseph Larmor therefore proposed in 1919 that perhaps sunspot magnetism came from electric currents, driven by a "dynamo process" by flows on the Sun (which is hot enough to conduct electricity). Later the same idea was extended to the liquid core of the Earth, which is also hotter than any Curie temperature, but again, probably contains molten iron which conducts electricity.

For more, see the sections on the dynamo process in "The Great Magnet, the Earth", home page http://www.phy6.org/earthmag/dynamos.htm
http://www.phy6.org/earthmag/dynamos2.htm

They cover the dynamo process at an elementary level, and more details are in "A Millennium of Geomagnetism" linked from the home page of that web collection. As you will read there, the process for the Earth has now been successfully simulated on a computer, including polarity reversals. (The polarity of the Sun's magnetism also reverses with the 11-year cycle.)

33. "Artificial magnetic shields" for astronauts?

Could you please tell me if you have ever heard of a supposed phenomenon called "Magnetic Deficiency Syndrome" that affected early Russian and American Astronauts?

Did N.A.S.A. install "Artificial magnetic shields" to overcome the supposed condition ( Which caused bone density loss)?

The answers to these questions trouble me. I thought it was zero gravity that caused bone density loss.

Reply

Yes, I have heard about it, but only from letters like yours. Whichever way the story got started, it is false: I know of no magnetic problems in orbit, and of no artificial magnetic shields.

The magnetic field sensed in Earth orbit is very weak, and I know of no magnetic effects. Have you ever undergone an MRI scan--a medical scan using a magnetic resonance imaging (MRI) machine? You lie on a narrow pallet and are wheeled into the middle of a large magnet, with very intense field. It's a noisy procedure, but you can sense no effect due to magnetism, because to the best of my knowledge, there is nothing in the human body affected by magnetism.

Loss of bone density is still ascribed to weightlessness, and is counteracted by exercise machines. See also my web page http://www.phy6.org/stargaze/Sskylab.htm

Concerning various medical problems or cures ascribed to magnetism, and other unusual claims for magnetism, please look up
http://www.chem1.com/CQ/magscams.html

34. The movie "The Core"

Can the Earth's magnetic field just "dwindle away," as a recent movie "The Core" proposes? And if this were somehow to happen--what dire consequences (also described in the film) could we expect?

Reply

Sleep soundly: the premise is far fetched, and even if it did somehow happen, you would probably not notice any difference.

Could it happen? The magnetization of the ocean floor (among others) shows that the north-south "main magnetic field" of the Earth does occasionally reverse north-south polarity, at irregular intervals averaging about half a million years. Right now the intensity of this field is decreasing at about 7% per century. That is a typical rate of variation, suggesting that if a reversal happens, we will have ample notice.

However, at the reversal itself the field is not necessarily zero. Other, more complex parts of the field exist, and right now it looks as if these are soaking up magnetic energy lost from the main north-south field. At the time of reversal, the Earth may have for a while 4, 6 or more magnetic poles, and a somewhat weaker field--but it is not expected to lose its magnetism altogether.

And yet, if it ever did... would we be exposed to radiation bursts from the Sun--the kind which erupt of the order of once a year (or less) and can imperil future astronauts on their way to Mars? No, because we are shielded by the atmosphere, an absorber comparable to 10 feet of concrete. Solar bursts cannot penetrate that thickness. It is true that the magnetic field of Earth deflects the fast protons of those bursts even before they reach the top of the atmosphere--but that magnetic shield fails near the magnetic poles, yet no extra radiation is detected there at ground level.

Without magnetic protection, the solar wind emitted by the Sun would also reach the atmosphere. Could it perhaps strip our atmosphere away? Maybe, given a few billion years, but not quickly. Venus lacks a magnetic field and experiences a stronger solar wind, being closer to the Sun, yet retains a very dense atmosphere. Mars, without a global-size magnetic field, has only a thin one--but the gravity holding down its atmosphere is only 1/3 of ours.

So sleep soundly, even after you may have watched the movie.

35. Can we tell if a symmetric magnetic field rotates around its axis?

(shortened)
Question that puzzles me for a long time is basically very simple. Does earth's magnetic field rotate around it's magnetic poles axis Well, looking at the rotation of magnetic anomalies, and rotation of magnetic poles, this is true. But that cannot be, and should not be considered as rotation of magnetic field on it's magnetic axis. The experiments can show us that the rotation of any magnetic field on it's magnetic axis is impossible and contrary to the very laws of nature. That should be viable even for complex magnetic structures as earth's magnetic field is. Yet, it is obvious that majority of scientific community is even unaware of Faraday's experiments with rotating magnets..

Reply

Your question brings up something which has confused many people (even me, when I was younger).

Basically, the rotation of axially symmetric magnetic fields in a vacuum around their axis of symmetry is NOT observable. If you have a bar magnet and attach it to a shaft which rotates it around its symmetry axis (the line connecting its poles) you will not find any difference whether it rotates or not. The outside magnetic field--and the electric field, too--is not changed by the rotation, and the equations which determine these fields do not reflect rotation of the source.

However, that is in a vacuum (and to all practical purposes, also in air). When matter is present--especially, electrically conducting matter--rotation may make a difference. Take for instance Faraday's disk dynamo, where a conducting disk rotates in a magnetic field (see illustration in http://www.phy6.org/earthmag/dynamos.htm ). The magnetic field may be axially symmetric, and the disk and its motion have the same symmetry, but it makes a difference, because a moving CONDUCTOR in a magnetic field B, at a point moving with velocity V, experiences an electric field E = VB in a direction perpendicular to both B and V (mathematically you need use here a "vector product", but let me not go into that detail).

If instead the magnet rotates around its axis and the disk is at rest, nothing happens to the disk, which shows that it is the motion of the CONDUCTOR that matters, not that of the magnetic source. Except of course in non-symmetric sources, where the motion causes observable changes in B--these do matter, and such irregular components clearly rotate with the Earth.

If the disk is not connected, the only effect of this E is to move electric charges across the disk, until they create an opposite field which cancels E, and nothing more happens. However, if electrical contacts from the outside touch the disk as in the drawing, connecting it to a circuit which does NOT rotate, then the electrical charges which try to neutralize E are carried away to the outer circuit, and the device works as an electric generator.

Just for your interest: at one time it was suggested that any spinning matter, conducting or not, produced a magnetic field, but experiments showed that was untrue. See 2nd paragraph in section 14 on web page http://www.phy6.org/earthmag/ /mill_5.htm.
--------------------------

There exists another level of this discussion, but if you find it confusing, you may stop here. Suppose you have a fluid which conducts electricity very well--such as the metal mercury, or liquid sodium, or the molten iron in the core of the Earth, or even the ionized gas ("plasma") which surrounds the Earth in space (it may not conduct as well as mercury, but the dimensions of many thousands of kilometers help create the same result). Assume that it contains a magnetic field, as is certainly true for the Earth in space.

If this conducting fluid moves through the magnetic field, its motion generates electric currents, like the motion of the wheel in a Faraday dynamo. Those currents, in turn, create magnetic fields of their own, and modify the original magnetic field.

In the general case, finding the modified magnetic field is a complicated mathematical problem, requiring that one also calculates electric fields and electric currents. However, if the conductivity is very, very good, the equations show that the solution simplifies. Namely, it then turns out that FIELD LINES SEEM TO MOVE with the material. Let a field line be drawn at some specific time, and imagine you could mark all the portions of the fluid (all the atoms, if you wish) through which is passes at that time. If you then let some time pass, find where these atoms are now and draw a line through them, the mathematics of the problem tells you that they are ALSO connected by the same field line.

Physicists like to say that this is THE SAME field line, traveling with the fluid as if it were "frozen" to it. In this way we can say that magnetic field lines "move." This is a very convenient was of avoiding the calculation of electric fields and currents, although it is strictly true only for "infinite conductivity" limit. But one must always remember--this is not something happening in a vacuum, but in a very good electrical conductor

A study project involving this "freezing" is described in
http://www.phy6.org/Education/wimfproj.html

36. What causes permanent magnetism?

I realized a couple of days ago that I don't know the answer to this question and was hoping that you would answer it for me. Surprisingly, no one here at work knows the answer either! We are a bunch of engineers doing software, some people have EE backgrounds, some physics. I am embarrassed! I understand the business about electric currents being the real reason for magnetism (hence sunspots, etc.), and the loop of wire resulting in transformers. I just don't understand a permanent magnet or what is so special about iron that an electric field induces magnetism in a piece of iron. It doesn't happen in too many other materials. Why is stainless steel different (most of it is NOT magnetic)?

Reply

Yes, magnetism requires currents going around in loops (not electric fields but electric currents). However, a spinning electric charge also fills the bill. A few days ago someone sent an e-mail question "what is the smallest magnet?" and my answer was, a spinning electron.

The trouble is, by the time you get down to the scale of electrons, the rules of physics change to so-called "quantum physics." Electrons turn out to have a natural spin and associated magnetic properties, but the direction of the spin/magnetic axis is severely constrained, and the relation between the two is not what you expect from a rotating charge.

Iron and other "ferromagnetic" substances get their magnetization from spinning electrons. In other substances, neighboring spins arrange to cancel each other--the way two neighboring bar magnets do (N of one is next to S of the other). Iron atoms are not permitted to do so, because of properties of quantum physics. Sorry I cannot say more--it isn't my field--but look up
http://en.wikipedia.org/wiki/Ferromagnetism .

37. What types of metal are attracted to magnets?

We are students working on a science project involving magnetism. We are investigating what types of metals are attracted to magnets. Could you please send us your opinion on this subject along with any other information you may have.

Reply

(see also previous question and its answer)

Iron is attracted to magnets, but why? Because near a magnet, in a "magnetic field", ordinary iron turns into a temporary magnet. See web page
http://www.phy6.org/earthmag/inducemg.htm

on "Performing Gilbert's Experiment on Induced Magnetism."

Iron and some alloys (and I am not sure, but also maybe nickel) turn into fairly strong temporary magnets. These are "ferromagnetic" materials--look up ferromagnetism in an encyclopaedia. Some very strong magnets contain no iron but have compounds of rare earth elements.

Many other materials are "paramagnetic" and while they too turn into temporary magnets, these are very feeble, and are only weakly attracted. "Diamagnetic" materials also turn into feeble magnets, but of opposite polarity, so these are slightly repelled.

Now go do your work!

38. "If the earth is a giant magnet, why doesn't all iron stick to it?"

I am a second grade teacher in Massachusetts. One of my students, a little boy named Seamus, has really stumped me with a question while studying the make up of the earth. He asked;

"If the earth is a giant magnet, why doesn't everything made of iron stick to it, especially at the north and south poles??"

I have done some research, but am having a hard time putting it in terms

Reply

This is not an easy question to answer at the second-grade level, but I will try. The answer has two parts: (1) The force of the Earth's magnetism is not all that great. It takes a delicately balanced magnetic needle (known as a magnetic compass!) to show its effect. (2) The magnetic force on a magnetic needle consists of two effects: (a) it tries to TURN the needle (b) it tries to MOVE the needle, for example to attract it. The turning effect is MUCH stronger than any attraction. As noted under (1), we need a delicately balanced needle to detect the magnetically produced turning, but we would need an INCREDIBLY sensitive instrument to measure any attraction.

Here is the reason why. Suppose you have a pivoted magnetic needle or bar magnet, with "N" and "S" poles which, as in any magnet, have an equal strength. (At this point you might want to make a drawing). The N pole will be attracted by the north magnetic pole of the Earth and the S pole will be repelled from it, so the needle will rotate until the "N" pole is as close to the north magnetic pole as it can go, and the "S" pole as far from it. BOTH forces here help produce the same rotation.

But what would be the forces MOVING the magnet? The N pole is attracted, the S pole repelled, and both forces are ALMOST the same. If they were EXACTLY the same, they would cancel and the result would be zero--no force at all. As it is, since the needle has already rotated, the N end is a TINY bit closer to the north magnetic pole (say two inches, the length of the needle), and the force on it is a TEENY bit stronger. But that teeny difference is not nearly enough to move the needle, even if the pivot did not hold it back.

So far we have talked about magnetized needles, not plain iron. However, plain iron near a magnet itself becomes a temporary magnet--see

http://www.phy6.org/earthmag/inducemg.htm

So all what was said about magnetized iron holds in principle for iron (though in the weak field of the Earth, an iron nail is magnetized much more weakly than, say, a compass needle).

And because the attraction results from the cancellation of opposing forces, you now understand why an iron nail (say) is attracted to a magnet only when it is VERY close to the magnet's pole--when the magnetic pole at the attracted end of the nail is MUCH closer than the repelled end.

39. Risks from stormy "Space Weather"

Hi Dr. Stern:

Reply

When discussing risks and dangers from radiation in space, you should really distinguish two kinds of radiation:

(1) Trapped radiation is governed by the geomagnetic field. If you are below the belt (as in the international space station) or elsewhere outside its intense part, you should have nothing to worry about. It could well be that the belt is now weaker than in the time of Gauss, 160 years ago, but that does not really change the preceding statement. These ions have about 50 MeV.

(2) Solar flares release unpredictable blasts of particles of higher energy, often 500 MeV and up to 10 GeV. In this case, people on the ground are still safe, because the atmosphere has enough thickness to stop the particles, equivalent to something like 4 meters of concrete. See the end of
http://www.phy6.org/Education/wsolpart.html
If you are in a spacecraft on your way to Mars, that can be dangerous. In Ben Bova's book "Mars" this does happen, and astronauts have to hide in a protected area--behind fuel tanks, probably.

On Earth, we have an additional shield, the Earth's magnetism, which will deflect all but the highest energies from regions at equatorial and middle latitudes. Jetliners crossing the polar region may perhaps find it useful to fly a little deeper in the atmosphere when the sun emits high-energy particles, and I heard the Concorde carried a radiation alarm.

The magnetic field would have completely protected the space station in its originally planned orbit, inclined 29 degrees to the equator (latitude of Cape Canaveral). As it happened, this was later increased to about twice as much, to enable Russian launch sites to resupply the station (which turned out quite important after the "Columbia" disaster). Twice each orbit, therefore, the station has relatively weak magnetic protection, near its closest approach to the magnetic poles. I heard a rumor that during the 3 big flare events at the end of October 2003 the astronauts did in fact hide, but that is strictly hearsay which I cannot confirm. The even bigger flare on November 4 did not produce such a radiation surge.

I am not sure about disruption of power grids, but I think it arises when the auroral electrojets shift to lower latitudes during storms. There are two large electric currents flowing along the auroral zone towards midnight, associated with the polar aurora (or more precisely, with the electric currents which produce big aurora; see in "Exploration of the Earth's Magnetosphere.") Like any electric currents, they produce a magnetic field which can be observed on the ground, and which changes fairly irregularly.

That's about all I know on this subject. Let me know if you find anything more, or anything contrary to what I know.

40. Does our magnetic field stop the atmosphere from getting blown away?

Thank you for such an in depth and informative Web Site. I have a question about the force of the solar wind against the magnetosphere. If the Earth's magnetosphere turned off (magnetic fields no longer existed on Earth), how long could life on the Earth survive? Eventually the solar wind would blow the atmosphere away.

Reply

I don't have an exact answer, and it all depends on what you mean by "eventually." If the process happens it is probably too slow to make much difference.

Venus has no magnetic field, its gravity is less than ours and being closer to the Sun, it experiences a much stronger solar wind, yet it retains a dense atmosphere. True, because its atmosphere is so dense, it can suffer appreciable loss without much change.

I don't know what the loss rate is, but it could be calculated from observations downwind from Venus, and maybe someone has done so. I have found a long article (below) but it does not seem to give an answer
http://www-ssc.igpp.ucla.edu/personnel/russell/papers/interact_solwind/
Another web page suggests little change in the atmosphere:
http://pauldunn.dynip.com/solarsystem/Venus_B.html

41. Dynamos triggered by the sun?

(from new Zealand)

The dynamo effect is has a solid background, but we are troubled to explain why the field does not run down, and we really can't model accurately the fluid mechanics of the outer core.

Exciting an electrical field or a magnetic field is essential to provide a strong dynamo effect as with any typical rotating field generator, and in getting it to run in a particular direction. If the multitude of mini dynamos were operating within the earth about areas of upwelling, and rotational vortices, the fact that the earth travels in the magnetic field of the sun and interacts with solar wind and radiation in the magnetosphere, will excite those fields which support the current flowing in a particular direction, hence the slowly processing but stably north south orientation of the current field. My theory is this, that a hitherto unknown solar eruption on a grand scale changes the exciting fields and as a result the sum of dynamo fields is altered and could lead to a pole reversal. This would allow rapid pole reversals and does not require wholesale restructuring of the outer core.

Reply

Explaining the details of geomagnetic dynamos can't be done briefly, also it's not my field really, and very mathematical. Still, I'll try.

Two things to note.

First, a magnetic dynamo, like almost any process in nature, needs energy (in that respect, energy is like money). Geomagnetic dynamos are believed to be driven by flows in the liquid part of the Earth's core, and the flows are driven by heat generated there.

In one simple view, the interior of the Earth is heated by long-lived radioactive elements (uranium, thorium and potassium) in the crust, which keeps it at a temperature sufficient to keep iron fluid. If that alone happened, no flow would be needed (and hence, no dynamo would arise). However, some extra heat is generated--perhaps by a slight amount of radioactive elements left in the core, perhaps by very slow solidification of fluid iron onto the inner core (no one really knows). If the amount of heat is small, it may just be conducted away to the mantle and beyond. But if there is too much heat for this process to do the job, the fluid starts circulating and convecting it away, like boiling water in a pot. The atmosphere gets rid of heat by a similar convection process, see
http://www.phy6.org/stargaze/Sweather1.htm

As long as energy is supplied, the convection will continue, and if conditions are right for a dynamo, it will not run down. Instead it will settle down within some comfortable limits. It can't grow too large and become "runaway" unless more energy is made available somehow.

Secondly, you may indeed say there exists a "multitude of mini dynamos," not just a single "strong dynamo effect as with any typical rotating field generator." If you look at a magnetic map of Earth, you will note that although the field has a two-pole structure, it also contains many bumps. This "bumpiness" reflects a complex flow and field in the core. This is essential: Thomas Cowling in 1933 proved a dynamo cannot be symmetric around some axis.

The core has about half the radius of Earth, and it turns out that the distance filters the field. The 2-pole part (dipole) goes down like the 3rd power of distance, so at the surface of the core it is 8 times stronger. More complex parts, however (there is a mathematical technique for isolating them) go down like 4th, 5th, 6th... power of distance, which means that at the surface of the core they are 16, 32 and 64 times stronger than they seem at the surface. The dipole is still dominant there, but by a much smaller margin. The complicated parts are quite big there.

One may thus look at the flow as a collection of circulating eddies. A crude model was proposed by Rikitake (1971?) with two eddies--call them A and B. Each of them may be thought of as a Faraday disk dynamo--see http://www.phy6.org/earthmag/dynamos.htm

The motion of disk A creates an electric current which produces a magnetic field, and that field is sensed by B. Then the motion of B through this magnetic field generates a current, which creates a magnetic field... and that is the field sensed by A, the one which allows it to generate a current. Each disk creates the magnetic field used by the other. It turns out such a model can reverse, and oscillate.

All this sounds a bit like the chicken-and-egg paradox: you need some magnetic field to start either dynamo! Actually, if conditions are right, even a tiny field gets amplified more and more, and the process only saturates when the limited energy available won't support further growth. So you do not need events on the Sun, etc, whose magnetic influence is always very weak.

[By the way, this resembles somewhat the classical model of arms race, proposed by Lewis Fry Richardson(1881-1953). There, the arms production of country A is proportional to the arms country B is known to have, and vice versa. It can get out of hand. See http://shakti.trincoll.edu/~pbrown/armsrace.html where the following can be found:

"The Richardson Arms Race model was developed by English physicist Lewis Fry Richardson(1881-1953), who was troubled by WWI and WWII because of his Quaker beliefs. Based on the assumption that having a large available arsenal makes a given nation more likely to engage in conflicts, Richardson conjectured that an arms race was often a prelude to war. The ultimate goal of his model is to examine the stability (or lack thereof) of an arms race between two nations in order to predict whether a small incident could potentially start a large conflict."]

You may find more details and references on dynamos in my review article "A Millennium of Geomagnetism," starting at http://www.phy6.org/earthmag/mill_1.htm

I would not however recommend the "Nova" program "Magnetic Storms" shown here last night on public TV; it will probably reach New Zealand in due time. It supposedly discusses the Earth's dynamo and field reversals, but does so in a confused and confusing way. Even though some of its participants are top scientists, the overall narration and editing were of poor quality and often in error.

42. Could generated electricity affect Earth's magnetic field?

Earths magnetic field is weakening, leading to a pole shift (

question # 6d

I would like to know if the effect of all the electricity generated by power plants etc. on the Earth's magnetic field would also contribute to this problem.

Reply

A quick answer is no, for several reasons. The world uses alternating current with 50 or 60 reversal cycles per second. You can pick up this field almost anywhere--just tie a loose wire to an oscilloscope and you will see the waveform--but this does not affect the averaged magnetic field.

In addition, to create a widespread magnetic field, you need the current to flow around a large loop. Electric currents of the power grid do not do that.

Take for example the power cord of any electric applicance you have. It hardly produces any magnetic effect, because it has two parallel wires close to each other carrying current in opposite directions. (The third wire, connected to the ground, is just for safety.) Because the electric currents in the wires are equal and opposite, their distant magnetic fields cancel each other almost perfectly. Only in the small space between the wires does a sizable magnetic field exist. Power grid currents also come in pairs that cancel, even if the return flow is through the ground.

Furthermore, I am not sure power grid currents are strong enough. Given a choice to transmit energy using a large current at low voltage or a small current at high voltage, power companies choose the latter, because the power loss caused by resistance is proportional to the current. However, the magnetic field produced is proportional to that current, too.

43. "Magneto-therapy"

(from France)

Dear Sir,

I am in the search of documentation on magneto therapy. certain apparatuses, very expensive, are on sale by canvassers in residence and appear me to be swindle.

Can - you to inform me?

Reply

Your guess seems correct. There exist many magnetic devices on the market (bracelets, mattress pads etc.), and as far as I know, they are all useless.

Occasionally letters about this arrive, and one of them I put on the web. See

http://www.py6.org/Education/FAQs4.html#52

Concerning various medical problems or cures ascribed to magnetism, and other unusual claims for magnetism, please look up
http://www.chem1.com/CQ/magscams.html

44. Curie Point

Dear Dr. Stern:

I am sending this message on behalf of my 8-year-old son. Jacob is preparing a project on magnetism for his third-grade science fair. Jacob notes that iron magnets lose their ability to attract when heated sufficiently. Jacob's question, in his words:

Do you have information about why magnets lose their force when heated? Everyone can tell me that it happens, and I even did it myself, but no one can tell me WHY.

I know that Jacob will be very grateful for any explanation you can provide, or any child-accessible sources you could suggest.

Reply

The effect you are studying is called the Curie Point, after the French physicist Pierre Curie who discovered it in 1895. He went on to marry a young Polish chemist named Maria Sklodowska, who as Maria Curie went on with Pierre to discover radium and earn the Nobel Prize.

Magnets are a very special kind of material with strong magnetic properties--much stronger than those of other substances. Such materials are called "ferromagnetic" because iron is the best known one and "ferro" refers to iron, from its Latin name (e.g. the family name "Ferraro" means "Smith", someone who works with iron).

All matter consists of atoms, and atoms are made of nuclei and of electrons. Both are magnetic. The magnetism of protons (hydrogen nuclei) is what makes MRI scanners in medicine possible ("nuclear magnetic resonance") but electrons are much more strongly magnetized, and therefore they are the one that count in most magnetic properties of matter. Their magnetism is related to "electron spin," with the electrons behaving as if they were also spinning around the direction in which they are magnetized. An electron carries a negative electric charge, and a charge spinning around an axis indeed should indeed make it act like a small bar magnet (although the numbers do not fit exactly, for reasons related to quantum theory, the physics of atomic dimensions).

In iron and ferromagnetic materials, spins can line up inside the crystal structure and create small "domains," blocks in which all electrons are lined up. Obviously, each domain is a strong magnet. In a permanent magnet, many of these domains are also lined up (in an electromagnet they only do so temporarily), giving the strong magnets we know. By the way, there exists a limit to the strength of such magnets ("saturation")--if all domains line up, the magnetism is as strong as it can get, it cannot grow any more. Some of the strongest magnets in the world therefore rely just on electric currents.

Heat creates disorder. Heat an ice cube and at a certain temperature, the forces that make water molecules stick to each other give way. That would be the melting point. Heat a magnet and at a certain temperature the orderly arrangement of electron spins falls apart, too. That would be the Curie point.

I looked on the web for "Curie point". Some sites are:

http://www.exploratorium.edu/snacks/curie_point.html http://en.wikipedia.org/wiki/Curie_point http://physics.ucsd.edu/was-sdphul/labs/demos/modern/curie.html demo

You will also find some facts about magnets, and their history, on "The Great Magnet, the Earth"
http://www.phy6.org/earthmag/demagint.htm.

45. Blocking of magnetic fields

Silly question? Is there anything on the planet I could put between two magnets to completely block magnetism? I would call it a magnetic blocker. Is there such a thing? I'm sure some materials could weaken it, but I am not sure anything blocks it. I would be very grateful for a reply whether you know the answer or not. The internet is a vast ocean of @#$%&! to me, but when I find someone who knows what they are talking about it's much more tolerable.

Reply

Nothing can block magnetic fields, the way a slab of wood can block sunlight from going past it. The equations don't allow that. However, you can divert a magnetic field from the region where you don't want it to be, and that's often just as good. Example: in old days, TV picture tubes had a shield of soft iron around their necks. (Maybe they still do: haven't looked inside a TV for decades). The idea was to prevent stray magnetic fields from reaching the electron beam and jiggle or defocus it. Magnetic field lines directed towards the neck of the tube hit the shield and are channeled into it--they go around the glass tube and then come out of the other side of the shield and continue.

An interesting application of magnetic shielding is the experiment on cigarette smoking by Dr. David Cohen of MIT. See:
http://www.phy6.org/earthmag/magmeter.htm

46. Earth magnetism from rotating electric charges?

I have always wondered whether the magnetic field of the Earth is produced by electric charges rotating with it. It is a thought that has been puzzling me for many, many years.

Reply

A rotating electric charge could create a magnetic field, but Earth does not carry free charges--if it did, electrons or ions from space would be attracted and would quickly cancel them. Also, the magnetic axis of Earth is not its rotation axis, plus the field has complex parts with more than 2 poles--and of course, evidence of old lavas tells us the field reverses directions now and then.

There do exist electric currents in space, whose energy is supplied by the solar wind; their magnetic fields cover huge regions, but they are rather weak. The analysis method of Gauss allows one to tell how much of the Earth's field originates above us and how much below us. The result is that less than 0.5% comes from the outside.

47. Teacher seeks easy experiments

(From a letter by a high-school teacher)

... Also, does anyone have nice labs or demos they could share for magnetism/EM induction?

Reply

Quick demos in high school:

(1) Magnetize a sewing needle, then float it in a Petri dish filled with water, on top of a vu-graph projector, for the entire class to see. Kids will learn not juts about geomagnetism but also about surface tension.

(2) Magnetic induction can be demonstrated on top of a projector, in a magnetic analog of the electroscope. See
http://www.phy6.org/earthmag/inducemg.htm
(3) Oersted's experiment can also be demonstrated on the top of a projector. See end of http://www.phy6.org/earthmag/oersted.htm

All three are quickies with rather little preparation. Kids can also run them by themselves.

48. Local field does not always decrease!

Respected sir,

I am an undergraduate BSc (Hons) Physics student at the University of Mauritius doing my final year project entitled "Geomagnetism studies over the Indian ocean.". I have provided the computer program calculating the local magnetic field intensity F with my location--latitude 20.17 S, longitude 57.33 E and elevation (highest point) 828m. The program gave me values of the total field F (=magnitude of the magnetic vector B) for 1900-2004.

The value of the total field is found to be decreasing during the interval 1900-1997 (from 39834 nT to 36884 nT), but from 1998-2004, there is an increase! (From 36985 nT to 37420 nT). I have read that at present the magnetic field continues to weaken. Then how is it possible that here from 1998 to 2004 we are having an increase? Should there not be a decrease?

I am really stuck at this point and do not know how to explain this. I have applied the program to another location ( 39.25 N, 105.28 W elevation 735m) and there I get a continuous decrease in F for 1900-2004, from 59288 nT to 53390 nT. This suggests a decrease exists over the entire interval. But if so, how can the other result, at my location, be explained?

I would be very grateful to you if you could give an answer to my question.

Reply

I do not have the codes which you use and can only guess. What is decreasing all these years is the dipole component of the field. That is the biggest contribution to F and over a long time (such as 1900-1997, almost a century) usually dominates the local trend.

However there exist other terms, such as quadrupole (n=2) and higher. They seem to be increasing (if the total energy of the field is almost constant, as Ned Benton found) and more important, they are not axially symmetric, and slowly migrate. I suggest that they are the cause.

The trouble with F is that it is non-linear, involving the square of the vector field B. I do not know if you can modify your code. If you can, have it calculate separately B1, the dipole (vector) field and B2, the non-dipole field. Then (* is scalar multiplication)

F*F = (B1*B1 + 2 *B1*B2 + B2*B2)

Now track over the same years B1*B1 , which is the contribution to F*F from the dipole alone, and of 2*B1*B2 + B2*B2, roughly the contribution of the higher harmonics (B2*B2 is small, and you can perhaps neglect it). My guess is that the first term declines steadily over 1900-2004, but the second may grow. It may perhaps overtake the decline in 1997, but is probably present even before.

You may also discuss it with your professor.

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Author and Curator: Dr. David P. Stern
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Last updated 6 June 2002
Re-formatted 9-27-2004