Click here for a listing arranged by topic.
If you have a relevant question of your own, you can send it to
2. Can the Earth's field be used for spaceflight?Dear Mr. Stern:
I am an Industrial Technology teacher at a middle school and one of my students is dreaming of a space propulsion system based on magnetic repulsion of the earth's magnetic field. Could you possibly squeeze in a moment for us and provide some information on the strength of this field and how it has been measured and maybe a relative comparison? Tyson, my student, is really excited about the Internet and will be enthralled to have an answer from a NASA scientist. Perhaps you could steer him to other references as I certainly will explain to him how busy a schedule you must have. Thank you.
But there is a more fundamental reason. Magnetic poles always come in pairs, equal and opposite: if a field attracts an N pole, it repels the attached S pole. Similarly, if we generate the field by a current in a loop of wire --say, shaped like a rectangle--for each side in which the current flows in one direction, there exists a side where it flows in the opposite direction, and the magnetic field exerts opposite forces of equal strength on the two sides.
From the preceding one would guess that magnetic forces always cancel, and no net force is exerted. So how come magnets are attracted to each other, or pins to a magnet (same thing, really, since each pin in the magnetic field turns into a small magnet)?
The answer is that the forces on the N and S poles (or on the opposing currents) are not exactly equal, if one pole, or one wire, is closer to the source of the field than the other. This can be put into a mathematical formulation and the bottom line is that a suitably oriented magnet may be attracted by a magnetic field, moving towards the greatest strength of that field. But the force is proportional to the rate at which the field changes with distance, which in the case of the Earth, is very small.
The idea of magnetism as anti-gravity has come up before. Your student may look up "Gulliver's Travels" by Swift, where in the third voyage, in a spoof on science and learned societies, Gulliver arrives at an island floating in the air, held there by the repulsion of a large magnet. Swift even gives an explanation, except it's all gibberish gobbledygook, as befits a book of satire. (I won't cite here the name of Swift's island, since too many people in Texas speak Spanish!)
3. The Sun's Magnetic PolesDear Mr. Stern:
I have a question about the sun that I was hoping you might be able to answer for me. A friend of mine recently returned from a new-age conference where it was presented that the magnetic poles of the sun were about to reverse, and cause a number of changes.
The idea of the sun having magnetic poles seemed counter to what I remember learning about the sun, and your web page seems to dispel the idea that the sun has actual poles. My guess is that the presenter was taking a dose of creative license with the 11 year cycle of sunspot activity.
Is it true then, that:
1.) There are no magnetic poles on the sun.
The Sun's most concentrated magnetic fields are of course in sunspots, but people have long suspected there might also exist polar fields, because during a total eclipse of the Sun one often sees streamers coming out from the polar regions, looking very much like the pattern of iron filings near the poles of a magnet.
But there was no good way of measuring such diffuse magnetic fields: the field of sunspots affects the light emitted from them ("Zeeman splitting") but the effect elsewhere is very weak. Then in the 1950s (if memory serves me) the Babcocks pushed the technique to its limits and found the polar field. This revealed the reversal of the polar magnetic field and suggested this field was somehow coupled to that of sunspots (which also reverse each cycle--they come in pairs, and the leading spot, in the direction of the Sun's rotation, has north or south polarity, in alternate cycles), a sort of a cumulative effect of the distant field of many spots. Theories exist by Horace Babcock and Robert Leighton, though they are somewhat qualitative.
The fact the magnetic field lines at the poles stick straight out means they do not hinder the escape of the solar wind in any way, and indeed the Ulysses spacecraft which recently passed above the Sun's poles confirmed (as was predicted) that the solar wind there is faster. There seems to exist no great effect of the reversal on Earth, though one might expect a bit more magnetic storminess when the polarity is opposite to that of the Earth.
For more on the Sun, see:
4 Synchronous SatellitesDear Dr. Stern:
I have been told, and read, that in order for a satellite to remain in a fixed position relative to the earth, it must be in a synchronous orbit, and that this type of orbit is best for communication purposes. All of the other orbits I have read about are used by satellites with goals other than communication. Satellites in orbits other than synchronous are not fixed in position relative to the earth.
This being the case, it seems to me that all satellite dishes for reception of TV signals would face the equator - south east, south west, or somewhere between. My observation of satellite dishes does not seem to support this concept. Many dishes do have a southerly inclination, but others do not. Further, here, where we are close to the equator, it seems to me that in order to focus on a satellite, the dish should be aimed high - higher, at least, than one located in the USA or Canada where the angle between the earth and the satellite would be smaller than here in Barbados. Many dishes here seem to follow a line of sight that is barely above the horizon.
So, my question - are all communication satellites in fixed orbits above the equator, and if so, why don't all reception dishes face the same way?
As I said at the outset, I know that I am being brash in writing you, and I would appreciate your indulgence. Over the past year or so, I have asked at least a half dozen engineers for an explanation, and received little more than a blank stare in response. Access to other resources here is not always easy. If you are unable to take the time for an explanation, perhaps you would direct me to a source that could satisfy my curiosity.
If you do reply, please bear in mind that I am an accountant, not an astronomer.
It is true, however, that not all satellites tracked from Barbados are at the longitude of the island. To receive phone calls from Europe, say, it could be that a satellite is tracked which is orbiting at the longitude of Europe, and then the dish should point towards the southeast.
I do not know how you reached my name; maybe you were directed by a search engine to the file.
I hope you are aware that this is only one part in a much larger exposition, the Exploration of the Earth's Magnetosphere, dealing with the Earth's magnetic environment, with its home page at
You might look it up. Also try:
dealing with satellites keeping a fixed position (or staying close to one) relative to the Earth in its orbit around the Sun.
5. Magnetic Field LinesMr. Stern,
I am enjoying your presentation on magnetospheres very much and I am finding it most interesting and informative. However, I have one question that I have not found an answer to yet.:
If the earth's magnetic lines of force are in fact "lines" upon which electrons and protons can collect like "beads on a wire", what is the spacing between these lines say, at the altitude of the recent Tethered Satellite experiment?
Some co-workers and myself have had a rather heated discussion on this matter (i.e. whether the magnetic lines are "lines" or a "field"). We would be most greatful if you would enlighten us about these magnetic lines of force around our planet just a little more than you have in your presentation on the Goddard Home Page on the WEB.
Magnetic field lines are defined as lines that point everywhere along the magnetic force (in a fluid, a complete analogy is given by flow lines or "streamlines"). They can be described by formulas, in terms of quantities known as Euler potentials or Clebsch functions.
But there also exist intuitive properties: particles threaded by a common
field line, tend to share that field line later on as well. Say we have 10
ions numbered 1... 10 sitting on a common spot on the Sun, and therefore
sharing there a field line, and destined to come out in the solar wind
one day apart. The Sun rotates, so make a drawing with a circle representing
the Sun and 9 radial lines coming out about 15 degrees apart. After 10 days,
particle #1 is 2.5 inches along the first line, particle #2 2.25 inches on
the 2nd one, and so on, down to particle #10 still on the surface: the line
conecting the particles is a spiral, so we expect interplanetary field lines
to have a spiral shape, and we derived this from intuitive concepts alone
(though the same thing can be derived from formulas).
The spacing between field lines is not meaningful (though some engineers speak of "density of magnetic field lines" to describe a quantity commonly known as "flux density.") Suppose you draw two field lines of the Earth, reaching Earth 1 meter or 1 foot apart. Each can have electrons or ions trapped around it. The meaningful question is what is the radius of the circle these electrons or ions describe around their guiding line, and that depends on their energy, and how strong the field is (the circle gets larger in the weak fields far from Earth), but it is generally much more than 1 foot or 1 meter. No problem: densities are so low that such ions or electrons rarely collide, and their orbits can easily overlap. The radius of gyration of auroral electrons can be 100 meters, which is why auroral "curtains"are so thin. On the other hand, solar wind ions entering near the "nose" of the magnetosphere have radii of the order of 500 kilometers, or (say) 350 miles, because the field there is much weaker, and that is therefore the order of the expected thickness of the magnetopause, the boundary between the solar wind and the magnetosphere.
6. The Solar WindTo David P. Stern:
Hello David, I liked the WWW article on The Solar Wind-History. In the article it said:
"Not everyone accepted Parker's theory of the solar wind when it was first published in 1958, and it was debated until observations confirmed it."Have you ever heard of the book OAHSPE by John Ballou Newbrough copyright 1882? Oahspe means Earth, Sky, and Spirit. Oahspe contains much scientific information that was discovered many years later, such as the earth's magnetosphere, Van Allen Radiation Belts, the SOLAR WIND, the origin of stars in nebula, the configuration of galaxies, the beginning substance of life(DNA) in nebula, the cycle and age of galaxies and the stars they contain, interstellar and intergalactic Unseen matter(Dark matter). Oahspe says the Sun has a vortex that streches throughout the solar system. Scientist have some knowledge of the Sun's vortex, they call it the "Solar Wind'. .... (most of the letter omitted)
Tell me what you think about it. Please send me a reply email. Hope to hear from you soon.
The solar wind, by the way, is not a spiral: it flows straight out. Only the Sun's magnetic field lines are spiral, because the sun rotates: they tend to act like continuous strings, and since their "roots" rotate with the Sun, they get twisted into spirals.
7a. Explaining the Geiger Counter
Hi, I am a 10th grader ...I came across a web site ... in my search for how a geiger counter meter works. I was hoping you could give me a relatively simple yet good explanation of how it works (preferably, really simple). I'd appreciate it very much.
Imagine a fast ion or electron going through the tube. On its way it hits atoms of the gas in the tube and ionizes them--knocks off electrons and leaves positive ions. Usually, such electrons recombine soon. But if there is a voltage difference (electric field) in the tube, before they can do so, the electrons will start moving towards the positive wire and the ions towards the negative walls.
As they move, they gain energy. This is particularly true near the wire, where the electric field is concentrated and its force is strong. (One can draw electrical field lines just like magnetic field lines, and near the wire they bunch together, like magnetic field lines near the poles of a magnet.)
If the energy gained by the average electron is enough to knock out additional electrons from atoms of the gas with which it collides, the number of electrons will multiply. As the electrons move towards the wire and the field gets strong, this process grows quickly: one electron frees up two, two release four, and by the time the wire is reached, many more electrons arrive than were released by the initiating particle, enough to draw a measurable current and create a signal in the circuitry attached to the counter.
There is much more, of course, e.g. ultraviolet light which spreads the process away from the wire as well, which could cause the current to continue without stopping even after all the initial electrons (and the additional electrons caused by them) have reached the wire. Special gas filling takes care of that.
The counter is usually charged by just a trickle of current from a high voltage source, so that the current taken by the discharge is easily measured. If too many particles pass through the tube, too much current is drawn, the voltage drops and the discharges get smaller, until the electronic circuits supposed to count them don't do so any more. I think that's what happened on Explorer 1.
7b. Building a Geiger CounterHello!
I found your web-site in the internet. But I have still two questions.
1. Which gas is suitable?
2. Which pressure is in the metal tube(vacuum?)?
Please can yuo send me a wiring diagram from a geiger counter.
I hope all this is useful. Sometimes simple questions lead to complicated answers!
7c. Who was Hans Geiger?Hi
My name is Matt and I am a High School student in Corona, CA. I was recently assigned to do a report on a scientist. I found Hans Geiger interesting, and I picked my topic to be about his work.
You might also find some about his work in the beginning chapters of The Making of the Atomic Bomb" by Richard Rhodes and "From X-rays to Quarks" by Emilio Segre (look him up in the indexes).
Later he devised the "Geiger counter" instrument and helped deduce laws of radioactive decay.
Hello Dr. Stern,
Please answer to my following questions. My father couldn't help me. I am very happy now to know you.
Your answers will be greatly appreciated. These questions bother me every night when I lie in bed.
Your message was very much appreciated. You must be fairly young, or else you might have asked not your father but your physics teacher or professor. Yet what you ask are demonstrate thought and understanding). I hope you will continue to pursue your science interests, because you seem to have what it takes. I will try to answer, but please remember, I am no expert.
8. Measuring the Earth's Magnetic FieldI am doing a sixth year studies project on magnetism in and was delighted to find the question and reply page with topics similar to what I had thought of studying.
I was wondering if there was a practical method for measuring the strength and direction of the Earth's magnetic field at different geographical locations. Any help or inspiration would be greatly appreciated.
In any case, the electronic gizmos nowadays used in space are too complicated for a quick discussion, so let me instead describe earlier, simpler methods.
The direction of the magnetic field is of course given by the compass needle: but that is just the horizontal part of the force, Actually the magnetic force also points i n t o the Earth (or out of it, in the southern hemisphere).
To find the angle at which the force points down ("dip angle") people used a needle similar to a compass needle, but on a horizontal axis, allowing it to swing in the various directions to which the hands of a wall clock might point.
That is a bit harder to arrange than a compass needle: if one end of such a needle points at an angle downwards, how is one to know whether the magnetic force is responsible, and not, say, that the needle is not quite balanced on its pivot, but that one end is slightly heavier and therefore points downwards? To avoid this problem one starts with an unmagnetized needle, balances it very carefully, and only then magnetizes it. When in 1831 the expedition of John Ross searched for the north magnetic pole, it carried along a dip needle, and when it pointed straight down (while the regular magnetic needle showed no preference for any direction), that was it .
Measuring the strength of the field is harder. Take a thin long bar magnet and hang it by a thin thread, then wait until it points north-south. After it does, push one tip slightly left or right and let go: it will swing back to north-south, but will overshoot to the other side, then turn back to the right direction, swinging back and forth like a pendulum, gradually quieting down to point steadily. The average length of each swing depends on two things: the strength of the bar magnet and the strength of the magnetic force. With a stopwatch, measure 20 swings or so and figure out how long each swing takes.
Then put a small compass needle on a table, and put the small magnet nearby, in such a position that it tries to line up the compass to point east-west.The small magnet and the Earth's magnetic force obviously compete fordetermining which way the needle points, and by looking at the actual angle of the needle, and its distance from the small magnet, we again get an observation that depends on how strong are (1) the small magnet and (2) the magnetic attraction of the Earth. Using these two observations and some calculation, the physicist can find both these unknown quantities.
This method was proposed by Carl Friedrich Gauss in Germany around 1835. It obviously won't work on an orbiting satellite--but how measurements are made there is another story altogether.
9. The Strength of the Earth's Magnetic FieldCould you please send me any information regarding the current field strength of the earths electromagnetic field? My data is current as of 1975 which is by far outdated. My reading from that time were 30,000 gammas at the equator. If possible could you please send information on the current decay of the earth's magnetic field.
Any information would be greatly appreciated.
Some of these models also include the annual change of the field (but not in the above files). You might like to search the web using (say) the Altavista or Yahoo search engine, on the term IGRF.
If you just want maps of the field, for instance those describing, the variation of its strength over the globe, try
Clicking on the map of horizontal intensity in Mercator projection (use GIF format, which web browsers read) will show you that the horizontal intensity around the equator varies quite a bit. but 30,000 gamma (or nanotesla, same thing) is a reasonable value. A central dipole field would be horizontal at the equator; the average total intensity is somewhat larger (32500 is a good average) but that may be due to by non-dipole components.
The field has been weakening since Carl Friedrich Gauss measured it around 1836, by about 5% per century, recently accelerating to 7%/century. The total energy of the field however is nearly constant, as shown by the late Ned Benton. This means that the field is not really weakening, only reshuffling its energy, reducing the "main dipole" (=north-south bar-magnet pattern, declining as noted by about 7% per century) and reinforcing the more complicated parts.
These tend to contribute a weaker field, because the magnetism originates in the Earth's core, about half an Earth-radius down: all magnetic fields at the surface are weaker than those in the core, because of the distance, but the more complicated fields decrease faster.
Whether the main dipole will reverse in about 1300 years is anyone's guess. Geological evidence suggests it has happened in the past, but odds are against it, because the mean frequency of such reversals in the past seems to be about once in 500,000 years.
10. Solar EclipsesI'm working on a science project about the solar eclipse.
My first question is, how can you figure out exactly when the next eclipse will come?
The next question is: What are the main theories about the incredible heat of the corona? How can it be so warm when it's so far away from the sun's center?
Predicting eclipses is relatively straightforward, you just need to know the motion of the Sun and Moon across the sky, and when they occupy the same area, you get an eclipse.
By now we have pretty good formulas for the orbital motion of the Earth around the Sun (which determines where the Sun is in the sky) and of the moon around the Earth, and can predict eclipses quite accurately. The journal "Sky and Telescope" usually carries accurate maps of where the eclipse can be seen (if the sky isn't cloudy) and the times when it should happen. The journal also maintains an eclipse page on the web, at:
The heat of the corona is still a great mystery. I can describe to you one theory, but it is probably not the right answer.
When you walk along a beach, you usually see fairly large waves, breaking on the seashore. If you take a boat past those waves, you are likely to find that in deeper water the waves almost disappear, or anyway are much smaller. Why?
It happens because a traveling wave carries a certain amount of energy, which causes the water in the wave to rise and fall again. As the wave moves into shallower water near the shore, the same energy now moves a smaller amount of water: if energy is conserved, the motion must be bigger, which is why waves become higher. Finally, the water is not deep enough for the wave to keep going, and the wave breaks, giving up its energy all at once to irregular swirling of the water.
Some scientists have speculated that waves, perhaps similar to sound waves, rise from the surface of the Sun into the corona. They carry much less energy than sunlight, but as they rise, the density of the gas around them quickly decreases, until finally they reach a height at which not enough gas is left to carry the wave: it then gives up its energy, and since that energy is given to the surrounding gas, and there is very little such gas left at those heights, that remaining gas gets very hot.
That at least was the theory some time ago: however, scientists now know what sort of waves can move in the atmosphere of the Sun, and they say that such waves get reflected back downwards before they reach high enough for this process to happen. So we really don't know.
11. Magnetometer for Observing Magnetic StormsI am in contact with a doctor in a hospital and we would like to build a magnetometer that measures geomagnetic storms, a normal magnetometer, but cheap. The doctor made studies that show how patients react to differences in the geomagnetic field. He says that it is possible to help them feel better, and to help hospital personal by informing them about the geomagnetic status, allowing them to anticipate the patient's pains...
I have experiences with microprocessor-driven machines, to build and program them, but the problem of geomagnetic detector is new for me.
In more detail:
12. Cosmic RaysAfter reading your cosmic rays report, my friend and I decided that I would like to do a science project on cosmic rays.
However, we are only eighth grade students, and therefore do not have much background on the subject. It would be much appreciated if you could provide us with some background on cosmic rays, and perhaps with a science project we could perform.
I would recommend that you and your friend use the "index" file and reach from there the following files, in the order listed here:
Electrons, Positive Ions, Energy, Energetic Particles, The Geiger Counter, Cosmic Rays, High Energy Particles, Solar Energetic Particles.
Copy them on paper, if you can. Of course, if some items are not comp- letely clear, you will need to look up other sections as well.
In general, our study of cosmic rays can be divided into two phases. The first started in 1912, with the discovery that some unknown radiation, similar to the one emitted by radioactive materials, was reaching Earth from space. Gradually, it was identified--first as electrically charged particles, by the fact the Earth's magnetic field excluded some of it from near the equator (around 1922). Then it was found that the particles had positive charge--because the field affected unequally those coming from the east and from the west (around 1936). Finally, around 1947, photographic plates in balloons at high altitudes recorded tracks of individual particles, finding they were familiar ions--mostly hydrogen, some helium, and a scattering of heavier stuff, not too different from the composition of the Sun.
Scientists also found out much about the fragments produced when these particles hit the atmosphere (what we get on the ground is almost entirely fragments) and measured the particles' energy distribution. It turned out that some of them had phenomenal energies, raising the question of what process could provide them. But looking for the source of the rays proved elusive: it was like trying to observe the Sun in a heavy fog. In a fog sunlight gets scattered until it comes evenly from all directions, leaving no clue about where the Sun actually is. Similarly, cosmic rays seem to be thoroughly scattered in space, arriving equally from all directions. It is true that it is hard to bend the path of particles with such high energies, but the energies meet their match in the great distances of space: even a weak magnetic field can bend the path of a cosmic ray proton, if it acts gradually over cosmic-scale distances.
So these days the emphasis is on tracing the source of x-rays and gamma-rays, high-energy relatives of visible light, which move in straight lines no matter what happens and therefore tell where they come from. It takes a high-energy particle to produce a high-energy gamma-ray, so observing the sources of such rays tells us where in the universe high-energy particles are plentiful, and perhaps these are cosmic rays near their sources. The catch is that (1) these gamma rays do not penetrate the atmosphere well, so the observation must be done from satellites, and (2), their intensity is much weaker than that of cosmic rays. Still, we have been looking, and discovering (e.g. see the story of gamma ray bursts in "Exploration"). Currently NASA has a gamma-ray observatory in orbit, doing a nice job. This area of research goes by the name of high energy astrophysics.
I don't know how much beyond this an eighth-grader can go. The "Resources" section lists some files you might look up, e.g. on high energy astrophysics, and a book "Moments in the Life of a Scientist" by Bruno Rossi, Cambridge 1990
Bruno Rossi was a pioneer of cosmic ray research and this is his own story. He began in Italy as part of the talented group which included Enrico Fermi and Emilio Segre, and died a few years ago as a much-honored professor at the Massachussetts Institute of Technology.
Another book, by a pioneer of x-ray astronomy which covers that aspect as well as the rest of astronomy, is "The Astronomer's Universe" by Herbert Friedman, W.W. Norton, 1990.
Finally, your own country of Canada has contributed significantly to the study of cosmic rays. Perhaps the science museum in Ottawa can help you.