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FAQ: "Exploration of the Earth's Magnetosphere"

Listed below are questions submitted by users of "The Exploration of the Earth's Magnetosphere" and the answers given to them. This is just a selection--of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again--the danger of solar eruptions, the reversal of the Earth's magnetic field, etc.--or else the answers make a special point, going into extra details which might interest other users. Because this is a long list, it is divided into segments

## Items covered:

If you have a relevant question of your own, you can send it to
education("at" symbol)phy6.org

1. ### Space Tether

The space tether seems to extract kinetic energy from the orbital motion of the shuttle.

But as the shuttle loses energy, it descends to a lower orbit and must speed up. How can the shuttle lose energy AND speed up?

You are right, the kinetic energy of the shuttle increases. But it loses potential energy, like any object which descends from a high location to a lower one. In the final balance, the sum of potential+kinetic energy gets smaller.
2. ### Does the Earth's magnetic field rotate?

Hello David,

My 8 year old son and I are conducting simple experiments involving electricity and, particularly, magnetism.

We noticed that iron filings sprinkled upon a horizontal plane suspended above a pole of a dipole magnet do not appear to move when the magnet is rotated about the line extending between its two poles.

Yet, we understand that the accepted scientific view is that the Earth's magnetic field rotates (nearly) synchronously with the Earth.

So, our question is: Does the Earth's magnetic field rotate? If so, how do we know this?

Your question is not a simple one, and has in the past confused quite a few people. A rough outline of the answer follows below; it is not exactly simple, and I can only hope that you and your son will have the patience to follow it to the end.

I take it you refer to the axially symmetric parts of the field. The observed field also has many irregularities, and these certainly rotate with the Earth. For instance, the north magnetic pole (and the south one, too) is separated by something like 1000 kilometers from the geographic pole, the pole around which the Earth rotates. And indeed, the magnetic pole rotates every day a full 360° around the geographic one. What you have in mind--what your experiment involves--is something else: a symmetric field rotating around it s axis of symmetry.

If you have a source of magnetic field in empty space--iron magnet, coil--and you rotate it around an axis of symmetry, there is no extra effect, and therefore, nearby objects will not feel any force to make them share the rotation.

For instance, if you have a bar magnet and twirl it around its length, around the line connecting the poles, you get no observable change. At any point in the surrounding space, the magnetic force sensed (say, by a compass needle) is not changed by the rotation.

All that is true in empty space. And to a very good approximation, it also holds if the space contains substances which do not conduct electric currents--air, wood, paper, glass etc. In all these cases, just having the source of magnetism rotate has no measurable effect.

But if space is filled with a substance which can conduct electricity, and the rotating magnet also conducts electricity, the situation is different. Under certain conditions, electric currents may then be produced, and in that case, two effects are added:

• First electric currents are SOURCES of magnetic fields, and therefore the magnetic field may be modified.
• Secondly--more important here--magnetic fields exert a FORCE on the carrier of electric current, and in this case, in general, that force tends to make it share the rotation.

Space around Earth--except for the lower atmosphere--does conduct electricity. In the ionosphere--say 120 kilometers up (70 miles) and higher, sunlight rips off electrons from atoms, creating a "plasma," a mixture of free floating electrons and positive ions (left-over atoms, which miss an electron or more), and a plasma conducts electricity. If a free electron collides with an ion, the two may recombine again--but densities are so low that in most regions recombination does not catch up with "ionization" (the break up of atoms), even during the night, when the ripping-off of electrons stops. Also, higher up in space additional sources of plasma exist, e.g the "solar wind" blowing from the Sun.

Such magnetic forces try to make the plasma "co-rotate" with the Earth. Their effect turns out to be the same as would occur, if magnetic field lines which thread the plasma are viewed as attached to it. Then everything--Earth, plasma and field lines attached to it--rotate together. Once more--be aware that this only happens when space is filled with a good conductor of electricity. Without electric conductivity, the magnetic field does not transmit rotation. Iron does conduct electricity, but iron filings scattered on paper are not enough, they do not provide a good enough path for electric currents.

I guess this is about as far as one can go with an 8-year old. As for the "simple experiments on magnetism" you conduct with him, I would recommend three more, described in
http://www.phy6.org/earthmag/MagTeach.htm

For your own interest--if you want to know more about the way rotation CAN produce electric currents--look up the sections on dynamos in "The Great Magnet, the Earth", home page
http://www.phy6.org/earthmag/demagint.htm

3. ### Dynamo currents at Jupiter's moons

Are phenomena similar to the Io Dynamo also happening on Europa and Ganymede - the other big moons of Jupiter? Do they have footprint aurorae on Jupiter?

In principle, the same electric current generation which exists between Io and Jupiter can also exist with other moons, but if it does, it must be very weak.

Observationally, the finger was first pointed at Io because the radio emissions from Jupiter (even before space probes were sent to Jupiter) were known to be strongly dependent on the position of Io, not of any other moons.

Theoretically, we know Io has volcanoes and therefore an atmosphere and an ionosphere, and is the source of gas and ions around its orbit. I think these help conduct the electric current. Other moons do not have those features.

4. ### A Russian space tether experiment?

Perhaps you would be so kind as to help me. I can find no trace of a Soviet space probe which, maybe ten years ago or more, attempted to collect and measure spatial electricity by means of a long trailing conductor. As I recall it failed because of a burn-out.

There was in 1989 a Russian AKTIV satellite with a wire loop. See: cfa-www.harvard.edu/spgroup/TetherHBK_file4.pdf pages 190-191
5. ### How come a magnetic field can block particle radiation but not light?

I know the earth's magnetic field blocks high energy radiation/protons from the sun, but why does it not block all radiation i.e.: visible light, UV light, infrared? also, does the ability to block different wavelengths/energy levels depend on the strength of the field?

The best way to reply to your question is through an analogy. Imagine you sit in an armored vehicle, and an enemy machine gun is firing at you. The bullets never reach you, they are stopped by the armor, but you can clearly hear the sound of their impact. How come the sound gets through and the bullets don't?

The reason, of course, is that the two are quite different. The bullets are chunks of matter, and to let them through, the matter of the armor has to give way, something it strenuously resists. The sound is a wave: the bullets make the armor oscillate and propagate the wave through it, without breaking its integrity.

Similarly with particles and electromagnetic radiation from the Sun (or from any source): they are completely different. The issue is confused by the fact that streams of high-energy particles are also popularly called "radiation," as in "radiation belts" and "alpha-radiation" (or "beta radiation") from radioactive substances. There are historical reasons for this usage, but unfortunately, it is now too deeply rooted to be easily changed.

Particles found in space carry an electric charge, and a moving electric charge can be viewed as equivalent to an electric current. Electric currents react to magnetic forces--in fact, magnetism may be viewed as interactions between electric currents--so it's no wonder charged particles get deflected, sometimes even trapped.

6. ### What is a "magnetic moment"?

Another user asked you "what is the smallest possible magnet" and you wrote

I am a journalist, explaining science to the public (for a university research team). Could you define the term "magnetic moment" for me? I'm a bit baffled...

"Magnetic moment" is a measure of the strength of a magnetic source. A good comparison is provided by the electric force, whose source is electric charge." Given two electric charges, one 100 times larger than the other, the electric force produced by the bigger one, at any distance, is 100 times larger than what the other charge produces at the same distance.

Magnetic forces are more complicated. Imagine you have a bar magnet. A good approximation to its behavior is obtained by regarding it as a pair of "magnetic poles", an "N-pole" and an "S-pole." I won't use the words "north" and "south," because they confuse people--see

Each pole is the source of magnetic force, which like the electric force from an electric charge (and like the force of gravity produced by some mass), weakens with distance R like 1/R2.

However, poles ALWAYS come in pairs, equal in strength but opposite in type. Their forces therefore interfere with each other--while one pole pulls, the other repels. For that reason, at a great distance R, the magnetic field decreases with R at a faster rate, like 1/R3.

Intuition will tell you (and math confirms) that this interference--the mutual near-cancellation of push and pull--is greatest when the poles are close to each other, i.e. when the bar magnet is SHORT. The magnetic force produced by a bar magnet, at a given point in space, therefore depends on two factors--on both the strength Q of its poles, and on the distance D separating them. The force is in fact proportional to the product M = QD ; of course it also depends on the distance R, and its direction depends on the angle between R and the axis of the bar magnet.

M is known as the "magnetic moment" or "dipole moment" of the magnet. Sorry it took so long to get so far, but I know of no simpler way.

--- --- --- --- ---
...and in case you wondered

Loops or coils carrying an electric current are also sources of magnetism, and such a loop or coil behaves (at a distance) like a small bar magnet (see wmfield.html). It turns out that the magnetic moment of a loop--i.e. of the bar magnet whose action it mimics--is proportional to the strength of the current flowing in it, times the area of the loop. A coil of (say) 20 turns multiplies that area 20-fold. (All that, without adding any iron, which magnifies the effect).

7. ### Is fire a plasma?

The fire given off when burning for example, paper, wood, gasoline--is that another of the manifestations of a plasma? If it isn't plasma that makes the fire shine--what is it?

You asked a good question, but the answer is--no, fire is not hot enough to create a plasma.

Most flames are yellow. The main reason is that flames contain little bits of burned material--bits which later form its smoke--and they get hot enough to glow. Hot solid materials always glow--for instance, the filament in a lightbulb. However ,glowing in yellow light does not require a very hot temperature.

To create a plasma takes more energy, and requires a higher temperature than the flame provides. The collisions between atoms need to be energetic enough to kick an electron completely out of the atom.

An electric arc welder drives a huge current across a narrow juncture where two pieces of metal touch, and that creates a temperature high enough to create a plasma. The surrounding air is also hot enough. After touching the two pieces can be separated, and the air continues to carry the electric current, and to heat enough to create the plasma. The metal tip glow so brightly (white light, with a lot of eye-damaging ultra-violet) that the welder can only view the work through a thick dark screen.

8. ### Do interplanetary field lines guide the solar wind back?

Don't the magnetic field lines from the Sun need to close somewhere in space? Would such closing of field lines bring back the gyrating particles back to the Sun and give rise to a "reverse solar wind"?

Magnetic field lines do not always tell charged particles where to go! Sometimes, when the particles are numerous and have a greater energy density than the magnetic field, they are in charge and flow unimpeded. The magnetic field may then be deformed, and an electric field may also arise, effects that allow the particle flow to proceed undisturbed.

• http://www.phy6.org/Education/wimfproj.htm
• http://www.phy6.org/stargaze/Simfproj.htm

Whether the magnetic field dominates the flow of a plasma, or the flow dominates the magnetic field, always depends on one question--what dominates the energy density, the plasma or the magnetic field?

If the field does, its structure is relatively rigid and it determines where particles can and cannot go. The inner radiation belt of Earth is a good example--dominated by the dipole structure of the Earth's field.

If the plasma has a higher energy density than the magnetic field, its flow is relatively undisturbed, and instead, it deforms the magnetic field to suit its motion. This is the case in the solar wind.

(What if the two are comparable? We then may get complex physics, as in the Earth's outer magnetosphere!)

Because the solar wind dominates, it drags out solar magnetic field lines (as the above web site shows), in a spiral due to the Sun's rotation, which gets more and more circular. The wind itself does not follow field lines but continues to move radially. (At the end of the above web page, however, is a story of a different population of particles, high energy particles from an eruption on the Sun, which--because their number is few--do follow the field lines.)

Ultimately the solar wind encounters interstellar plasma and magnetic fields, and undergoes a shock transition, where its density increases and its velocity drops, to less than the "Alfvén velocity", a rarefied plasma's equivalent (in some ways) to the speed of sound. That transition was crossed by Voyager 1 last December (see section #18B). Ultimately there exists another boundary, due to the interstellar magnetic field, and I am not sure what happens there, whether field lines temporarily interconnect with the outer field or get tangled up. In either case, the solar wind does not come back, though it may end up deflected to move along the interface

9. ### Magnetic connections between planets and the Sun

Where would I find a drawing of magnetic lines of force of the whole solar system showing which go in to which planets and out to which other planets, or where they go, if nowhere then to other dimension?

There can exist no "drawing of magnetic lines of force of the whole solar system" because the pattern constantly changes. The field of the Sun changes as its sources change--sunspots, etc.-- and also, it rotates past planets. Therefore planets face different fields all the time, and depending on circumstances, may or may not at any position "reconnect" their magnetic fields with the Sun's. See more on reconnection in "Exploration of the Earth's Magnetosphere," section #23 The Tail of the Magnetosphere.
10. ### The solar wind and solar escape velocity

Acording to your web site, the particles in the solar wind are leaving the Sun at about 400 km/s. That is less than the escape velocity from the surface of the Sun, which is about 600 km/s. Does it mean many of these particles will eventually fall back to the Sun? Is there any evidence of such as behaviour?

The solar wind does not start from the Sun's surface, but from the corona, and is accelerated somewhat gradually. Obviously, it has to overcome solar gravity, which I suspect is one of the conditions needed for accelerating the solar wind--maybe like a lid on a pressure cooker, holding down the hot corona until it can just barely escape.

Incidentally. NASA has been toying for years with the idea of a solar probe, approaching the Sun within 4 solar radii--following boost from a "hairpin" orbit around Jupiter (mentioned briefly in
http://www.phy6.org/stargaze/Stostars.htm

and in the page following it). It would be shielded from the Sun's intense heat by an "umbrella" of tungsten or similar material, and would study the solar wind in its source region. How can it do so with a metal barrier between it and the Sun? Simple: at closest approach is moves at about 300 km/s, so in its own frame of reference, solar wind particles (unlike sunlight) would seem to come in from the side, at an angle. They would seem to have the vector sum of their own velocity and that of the corona relative to the fast-moving probe.