Site Map

FAQ: "Exploration of the Earth's Magnetosphere"

Get a Straight Answer

Questions and answer--listed in the order received

Please note!

    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

Click here for a listing arranged by topic.

Items covered:

  1. Reversals of the Earth's field (4 queries)
  2. Can the Earth's field be used for spaceflight?
  3. The Sun's magnetic poles
  4. Synchronous satellites
  5. Magnetic field lines
  6. Alternate theory of the Sun and solar wind
  7. The Geiger counter (3 queries)
  8. Measuring the Earth's magnetic field
  9. The strength of the Earth's field
  10. Solar Eclipses
  11. Magnetometer for Observing Magnetic Storms
  12. Cosmic Rays

  13. Magnetic Shielding
  14. Use of solar wind for space propulsion
  15. A working model of the magnetosphere?
  16. The Van Allen Belt
  17. Magnets of different shapes
  18. On building an electromagnet
  19. Capturing the Energy of the Solar Wind
  20. About the Upcoming Solar Maximum
  21. Lining-up of Planets
  22. Radiation Hazards to Air Crews
  23. The Ozone Hole and the Magnetic Field
  24. How are Ions produced?
  25. About the "Starfish" artificial radiation belt
  26. How do Magnetic Reversals affect Animal Migrations?
  27. Which is the "True" North Magnetic Pole?
  28. Electric and Magnetic Energy
  29. Any connection between Solar Wind and Solar Flares?
  30. Ozone and the Magnetic Field
  31. What if the Radiation Belt Reached the Ground... ?

  32. Free Energy from the Earth's Magnetic Field?
  33. Relativity
  34. What is a "REM"?
  35. What exactly does "Radiation" Mean?
  36. Can anything solid be carried by the solar wind?
  37. Dimensions of the Magnetosphere (2 related questions)
  38. Skywriting by Aurora
  39. Capturing the energy of solar wind ions
  40. Radio Propagation
  41. Radiation Belts and Manned Space Flight
  42. Magnetc shielding against neutral matter?
  43. When and where can I see "Northern Lights"?
  44. Universal Time and Magnetic Local Time
  45. Does the magnetosphere affect weather?

  46. "Importance of auroras to society"
  47. Magnetic storms and headaches
  48. Appolo Astronauts and radiation
  49. What materials does a magnet pull?
  50. Experimental simulation of the polar aurora
  51. Cosmic ray research using balloons
  52. Magnetic health products
  53. Geiger counters for locating lost objects
  54. Magnetic effects from other planets
  55. Blocking of the Solar Wind by our Moon?
  56. Fry or Freeze... ?
  57. The Speed of the Solar Wind
  58. What is "Radiation"?
  59. How does one Contain a Plasma?
  60. Soviet Nuclear Explosions in Space
  61. Can Polar Aurora be seen in Atlanta, Georgia?

  62. Why no aurora at the magnetic poles?
  63. When and how were positive ions discovered?
  64. Did astronauts use articifial magnetic shields"?
  65. Harvesting electrons from power lines?
  66. How can the intensely hot Sun be magnetic?
  67. What are "geomagnetic conjugate points"?
  68. What is the smallest magnet possible?
  69. Can plasma physics explain ball lightning?
  70. Harnessing the Energy of the Aurora?
  71. Radiation Belt and Brazil
  72. Risks from stormy "Space Weather"
  73. Man-made triggering of radio emissions
  74. Does our magnetic field stop the atmosphere from getting blown away?
  75. Radius of particle gyration

  76. Are electric storms an "electromagnetic" phenomenon?
  77. How can steady magnetic fields induce electric currents?
  78. There are electromagnetic waves all around us!
  79. Best orbit for a Space Station
  80. Is space debris electrically charged?
  81. Magnetic induction by the Magnetosphere
  82. Questions about the Solar Corona:
                        (1) Why don't its particles separate by weight?
                        (2) What accelerates the solar wind?
  83. Why has the aurora been so frequent lately?
  84. Was the magnetosphere involved in the hole in the ozone layer?
  85. Who Discovered Sunspots?
  86. "Soda-Bottle Magnetometer"
  87. Magnetism and Weather
  88. Is the Polar Cusp visible to the Eye?
  89. Effects of Radiation beyond the Van Allen Belts
  90. Deflection of a beam of Electrons in the Earth's Field

  91. Space Tether
  92. Does the Earth's magnetic field rotate?
  93. Dynamo currents at Jupiter's moons
  94. A Russian space tether experiment?
  95. How come a magnetic field can block particle radiation but not light?
  96. What is a "magnetic moment"?
  97. Is fire a plasma?
  98. Do interplanetary field lines guide the solar wind back?
  99. Magnetic connections between planets and the Sun
  100. The solar wind and solar escape velocity
  101. Space tether to remove trapped radiation?
  102. Electromagnetic Waves and Electromagnetic Induction
  103. Solar wind effects on our lives
  104. Weaker global magnetic field--higher cosmic ray dosage?
  105. Sound waves on the Sun?
  106. Mapping the magnetosphere using a surface network?
  107. History of Cosmic ray Research
  108. What causes sunspots
  109. Why does Plasma Follow Field Lines?
  110. A solar wind contribution to global warming?
  111. Waves in the Magnetosphere
  112. What are "frozen" magnetic field lines?
  113. Why doesn't magnetism affect electro-magnetic waves?
  114. Eddy Currents
  115. What is the Radius of the Sun's magnetosphere?
  116. Project to show that Iron rusts faster in Fresh Water
  117. Fluorescent lightbulbs
  118. More about the Year 2012
  119. Can Space Plasma help Spacecraft Propulsion?
  120. When is Earth an Insulator and when a Conductor?
  121. Can upper atmosphere atoms join solar wind?

  122. "Radiation Remediation"?
  123. Who invented AC?
  124. Geiger Counters (1)
        (Followed by Geiger Counters (2)
  125. Fluorescent tubes lighting up near high voltage
  126. Earth's field--Magnetic or Electromagnetic?
  127. How to test a Geiger counter
  128. Gamma ray bursts
  129. Magnetic effects of our galaxy
  130. How does electromagnetic induction occur?
  131. Magnetic field of the Sun
  132. Flux Transfer Events (FTEs)
  133. Building a Terrella experiment like Birkeland's
  134. Electric field due to electromagnetic induction

If you have a relevant question of your own, you can send it to
education("at" symbol)
Before you do, though, please read the instructions


    122.   "Radiation Remediation"?

        I am an electrical engineer at Bechtel, soon to be Emerson, but not in your field (of course), my field is power systems/power distribution.

        This particular field dealing with radiation belts seems so interesting, both from a scientific viewpoint, a historic viewpoint, a military viewpoint, a health viewpoint, etc. To some degree I believe in the natural law, and always wonder if we go a little too far in our "tinkering" sometimes.

        Like for instance, did the US know that their bomb tests at the equator would actually be trapped in the atmosphere longer, and did this adversely affect human life in any way? I guess these are my real questions though.

        Do you have any specific concerns regarding the Radiation Remediation Program? Do you believe dispelling this high energy into our atmosphere will have much of an effect on weather and/or health, for the good or for the bad? My other question, with your scientific knowledge on this subject - is it at all possible to take any technical data gathered from this approach or other experiments that would have a possible application to cancer, reducing cancer, preventing cancer, treating cancer? Please advise.


        I never heard before about "Radiation remediation", which apparently refers to attempts to bring down particles trapped by the Earth's magnetism. I am not sure that is feasible (or if it is, whether the cost is justified). At the present time, it seems not to be.

        The long-term persistence of the belts caused by atomic tests was not expected--earlier tests only created short-lived belts, it was the shift in location which created the problem (the Soviets exploded bigger bombs, but in locations without long-term trapping). Anyway, it happened in 1962, and the test-ban the following year stopped all such experiments.

        I don't know what you mean by "natural law" Nature is not always benign, and if I could by some technology prevent tornadoes, or cause earthquake faults to slip slowly and gently instead of abruptly, I would be all in favor of doing so.

    123.   Who invented AC?

        Dear Dr. Stern,

        I am tired of people telling me that Tesla invented alternating current. I know it must have been known by 1833 when a handful of people were spinning magnets near electromagnets to invent dynamos and motors. Who deserves the credit?


        I am not a historian of technology but a retired space physicist. Your question is interesting, but you should also ask around and search books and web sites, to find much better authorities. What I can offer here are mostly my guesses, based on some quick searches.

        Nikola Tesla certainly did not invent AC--he was primarily the engineer who helped George Westinghouse create his AC network, while Charles Proteus Steinmetz seems to have had a similar role in General Electric. I come from central Europe and vaguely recall reading that Siemens introduced it in the mid-1800s, but it probably happened independently at several places at once (see below). Faraday could have pursued AC around 1833, or maybe Joseph Henry in later years, but the actual introduction was delayed by the simple reason that no one actually needed an electric network. There was no electric lighting, telegraphs worked off batteries, and machinery worked off steam engines--often with power distributed (e.g. in factories) by long shafts from which pulleys turned long leather belts to various machines.

        Another thing: the great advantage of AC was reducing power losses, by stepping up the voltage at the power station, transmitting it through wires and then stepping it down at the user end, the way we do now. However, early networks were in dense cities, where open-air high-voltage lines were not practical, which is why Edison could push his DC network for some time before the superiority of AC was generally accepted.

        Again, I do not know what caused the change--the motivation for the first long-distance electric power transmission--but I suspect those were electric trains, in particular, urban trolleys and subways. Here is a web site I found:
    and let me quote from it:
      "In1858, however, an unknown inventor discovered that the field magnets of a dynamo could be electrically energised by the rotation of its own armature. The next step forward in the development of electric traction was the invention, by Pacinotte, of the continuous current dynamo. This discovery was followed by the development of self-energising field magnets by three Englishmen - Wheatstone, Varley, and Ladd, by the German engineer Siemens, and also by the American inventor, Professor Farmer."

    Once urban trolleys spread, and electric lighting followed, the adoption of AC was almost automatic. Tesla's name was made popular by the press, and he was Westinghouse's point man
    but the real story of AC is one of developing a technology, by many engineers whose names are scarcely remembered.


    Dear David,
        Thank you for your reflections on AC.

        What Tesla did invent was poly-phase AC, which allowed almost as much power, at a given voltage, as DC (two-phase gives about .707 of peak).

        The first major application, after the Niagara Falls power plant, was the World's Columbian Exposition, at Chicago, in 1893. Westinghouse was able to underbid GE as their system needed much less copper

        Steinmetz brought AC to General Electric, which only peddled DC systems until they lost the fair contract. Steinmetz also came up with the theoretical and mathematical underpinnings of AC power systems and dynamos.

    124a.   Geiger Counters (1)

        I was looking at your website and I found it interesting and very useful for my studies. I was hoping that you can help me out. I have a few questions that I had in mind.
      1. What does quenching mean when applied to Geiger counters?
      2. Can a Geiger counter be used to determine the energy of the x-ray photons?
    Thank you    


        (1)    A Geiger counter (usual shape) contains a central wire at high voltage, and a passing particle starts an electric "breakdown," like a growing spark between the wire and the envelope. "Quenching" means stopping the process. You already know a particle had gone through and now want the counter (after a short "dead time") to be ready to detect more particles. Quenching is accomplished either by suitable chemicals in the gas (e.g. alcohol vapor) or electronically by quickly lowering the voltage until the discharge goes out.

        (2)    Yes, but only roughly. A Geiger counter can be triggered by electrons knocked out of molecules of the gas or the wall. The ability of substances to block X-rays (for a given thickness) depends mainly on their atomic weight--lead blocks higher energies than tin, which blocks higher ones than iron, which blocks higher ones than aluminum, etc. Testing the counter's response behind various shields approximately gives the energy, but only X-ray diffraction instruments measure it precisely.

    124b.   Geiger Counters (2)

        Thank you very much. It is very helpful towards my study. I also have another question relating to the counter.
      1. Why does the Geiger region/plateau should have a very low slope of less that 0.1% per volt?
      2. Why does the dead time correction is significant at a high count rate but not at a low count rate?


        Fifty years have passed since I studied Geiger counters, so my knowledge is rusty. As I recall, up to a certain voltage, the counter just collects ions produced locally by the incoming particle ("ionization counter"). At higher voltage those ions get multiplied by collisions near the central wire (where the electric force is strongest), in a way which depends very much on the voltage ("proportional counter"). Then suddenly, the "Geiger" regime begins, the pulses are big and do not depend on the ionization produced, because they multiply by collisions and involve the entire length of the counter. It depends very little on voltage. If you regard the counter as a capacitor, each discharge consumes a fraction of its charge, the voltage drops to where the discharge cannot be sustained, then it "refills" (with quenching preventing the discharge from restarting).

        Dead time is the time the counter is not sensitive to incoming particle, because its voltage has not recovered sufficiently. In 1958, fifty years ago, "Explorer 1" went into orbit with a counter for cosmic rays. Sometimes the rate of observed counts was reasonable, sometimes the rate was zero. We now know this happened because of the radiation belt--so many discharges occurred that the voltage was not allowed to recover. Read more in "Exploration of the Earth's Magnetosphere",
    in the talk "
    Recollections of Space Research" and also section #11.  

    125.   Fluorescent tubes lighting up near high voltage

        I am a recent college graduate and a future graduate student for MFA Photography. While working towards a certain art project I've stumbled upon your site and was wondering if you could answer a question for me. I recall having read about fluorescent tube lighting being able to light up under large power lines. I am not well versed in the area of electricity and I am currently trying to learn as much as possible. I realize some loss is possible but I'm not sure enough for the light. Any information you might have, or suggestions would be greatly appreciated.


        I have heard about this too, and you can find more at

        Be aware these are time exposure and the glow is actually quite dim. I think the cause is the electric field near a high voltage line, which ideally goes like 1/r, where r is the distance from the line. Under quiet conditions, the dry atmosphere also has a voltage drop of the order of 100 volt/meter, powered by distant thunderstorms. Here a comparable drop is powered by the voltage on the cables.

        A hundred volts for a 4-foot tube sounds a lot--but there is hardly any energy behind it. Any time a conductor of electricity (like your body) steps into such an electric field, that field is quickly nullified, and only a tiny electric charge needs to flow (which is why measuring atmospheric electricity is difficult). It is a static field.

        To extract electric power from an electric field, you need a closed circuit, and in the atmosphere the only way it can flow is through air, which is an excellent insulator. Even a tiny current in a fluorescent tube, however, causes some light emission, which may make those photographs possible. In very dry weather, if you rub polyester cloth against an unconnected fluorescent tube, you might get some interesting flashes of light, too.

    126.   Earth's field--Magnetic or Electromagnetic?

    I was just wondering if you could quickly clarify a point of debate between a friend and I.

        He was using the term "earth's electromagnetic field" whereas I argued the earth had only a magnetic field. He argued that because of the deflection of ions an electric field was created, but my guess would be that there wouldn't be a appreciable charge separation across the globe to generate a consistent electric field.

        The second point he made for including an electric field was that the magnetic pole deviated from the rotational axis of the planet, which I suppose would generate some low frequency electromagnetic radiation, but to what significance I really do not know.

        In any case, if you could please help resolve this--it would be much appreciated. Many thanks, and hopefully this didn't end up in your spam folder :)


        Physicists prefer to speak of an electromagnetic field, because by the theory of relativity, electric and magnetic fields are aspects of the same field, which may appear differently in different frames of reference. An electron at rest relative to you seems to create just an electric force. However, if you start moving, the electron is observed to be moving relatively to you, and a moving charge acts like an electric current, creating a magnetic field. What you observe depends on your motion. (And yes, there is much more!)

        In any case, the Earth DOES have electric fields associated with it, of several types. For instance, there is a fair-weather electric field in the atmosphere, due to distant thunderstorms. However, because air is a good insulator, at locations far from such storms, the electric charges maintaining the field are quickly drained away by any electric conductor, and only very slowly get replenished. One such conductor is your own body--and you don't feel any effect, because very little charge is involved.

        There also exist radio waves, natural ones (like whistlers and auroral kilometric radiation) and artificial broadcasts. Such waves involve rapidly alternating electric and magnetic fields--see for instance

    And finally, there do exist electric fields in space, associated with any streaming of plasma through a magnetic field. For instance, how do ions and electrons of the solar wind flow radially away from the sun, and not get entangled in spiral motion around field lines of the interplanetary magnetic field? It takes only a little separation of positive and negative charges to produce an electric field E whose force will cancel the magnetic force v × B (v is the streaming velocity, B magnetic field, × is a "cross product", and of course, all these are vector quantities, with direction). This also results in a certain deformation of the magnetic field, as explained in


    The important thing is that if the stream has enough momentum, it drives its way through the magnetic field, "bending it to its will" so to speak, and the electric field is its tool for doing so.

        Your friend is also right in arguing that a rotating magnetic dipole will radiate radio waves. However, the power radiated increases steeply with frequency (I think like the 4th power). A frequency of one cycle per day is extremely low, and its radiation is negligible for all purposes.


        Thanks for the detailed explanation David. I understand that electric and magnetic fields are the same thing but from different frames of reference. However from a static observer on earth, the field that is a property of earth's core is considered magnetic, correct? My argument is really that the other fields generated as a result of collisions with solar wind and ionization by radiation aren't really properties of the earth, as the earth itself isn't generating them and they are dependent on an external source for their existence.

        The radio frequencies I accept are because the earth is generating them but I had supposed they were insignificant. Also atmospheric charge etc, but it's in constant flux & doesn't seem like some property of the earth collectively rather a localized effect.

        Aside from the synonymity between the two phenomena, it still seems strange to me to say "earth's electromagnetic field". To you, is this the preferred description of the property belonging to the earth, and not all the consequential phenomena?

    127.   How to test a Geiger counter

        I was on your website And I have been trying to look up on how I can test the Geiger counter I have to make sure it still works. Is there something around the house that I can use it on to detect it to see if it works??

        Thanks for your help,


        Dear Jennifer

        Depends what you have. If you just have a Geiger counter tube, take it to the nearest physics department to be tested there, because you need too much electronic circuitry--high voltage and pulse counters--to activate it.

        If you have a Geiger counter inside a portable radiation counter--such as you can buy as surplus--again, you probably need help from someone who knows electronics. If the thing is in working order, with a dial and a needle, it should give irregular counts from cosmic radiation and radioactive traces in the ground.

        For more calibration, you need a weak radioactive source. Again, taking the instrument to the nearest university and being allowed to expose the counter to one of their sources should work . A dentist's x-ray machine may be used too, perhaps, but start with the counter covered by an iron plate, just to make sure it does not get overexposed. When I was a graduate student, I had a watch with a glow-at-night dial containing radioactive material. Supervising students in a lab, I was surprised to find that my watch was a stronger radiation source than any of the sources provided for experiments! I then had a watchmaker carefully scrape off the glowing paint.

        If the counter is not working, check the batteries and replace them if needed. If you still get no counts, check the high voltage reaching the counter, about 700-1000 volts (if you are leery about this, find someone who understands electricity). If the high voltage is present but no counts register, perhaps it needs adjusting, or else something inside is broken--perhaps (if it's an old counter) a vacuum tube is burned out. The Geiger tube may be burned out, too (take it to a university lab to test it). In any case, you need the help of a specialist.  

    128.  Gamma ray bursts

        I was wondering if you could explain to me what a gamma ray burst is? I've read various things about them but I can't really get the gist of it...I understand that some think such a burst may have affected Earth in the past...could this happen again?


        If gamma ray bursts had not been observed, I doubt anyone would have predicted them, and even now a lot of them is not understood. I wrote about them in
    and as explained there, at least two kinds exist--those in our own galaxy, thought to come from the supernova collapse of strongly magnetized stars ("magnetars"), and those coming from distant galaxies--obviously, involving much greater energy. Possibly, the latter class was emitted when large black holes formed at the center of those galaxies--see
    They may be very old, relics of the earlier universe.

        A new satellite, GLAST, is just starting to study gamma ray bursts
    with higher resolution and larger field of view than earlier ones (it was since then renamed the Fermi observatory). Astronomers hope it will provide us with new clues.

    129.   Magnetic effects of our galaxy

        Are the 11 year cycles of sunspots influenced by a spiral effect or by our galaxys magnetic field? Will the earths magnetic field be reversed by the galaxy's magnetic field?

        I heard somewhere that the solar system will be traveling through the galactic plane, would the magnetic field of the Galaxy be able to influence us there?


        The magnetic field of the galaxy does not affect either that of the Sun or of Earth, since it is pushed away by the solar wind. It can only be observed outside the heliopause. See

        Concerning the "galactic plane"--the galaxy is a big wheel-shaped collection of stars (rotating, obviously--the periods may be many millions of years) and the galactic plane is the flat surface defining the middle of the wheel. It is just a geometrical property--most stars in the galaxy are located rather close to it. If the orbit of the solar system makes a small angle with that plane, it will cross the plane twice each rotation, but I don't know when this happens, and it certainly should not make any difference.

        Plus, even at that time, the solar wind will keep out the galaxy's magnetic field.

    130.   How does electromagnetic induction occur?

        Can you please tell me "how emf is induced in a coil when magnetic flux linked with it changes". If you can, please give a comprehensive answer about the physical aspects to it.


        What do you mean by "how"? The property of induction is one of the fundamental properties of the electromagnetic field, as expressed for instance by the Faraday equation, one of 4 basic equations of electromagnetism formulated by James Clerk Maxwell.

        Nature gives us (as far as we know) 4 basic forces--gravity, electromagnetism and two kinds of nuclear force. WHY they exist in the form we find them we don't know (except, perhaps, if they were different, WE could not exist to ask the question). Electromagnetic induction is part of the package.

    131.   Magnetic field of the Sun

        Hello, I'm a graduate student at the University of Hawaii at Manoa and I had a quick question about the sun's magnetic field (I stumbled upon your page via Google.) Is the sun's magnetic field at a distance of about the earth's radius appreciable? How does this value fluctuate? Could you point me towards literature with relevant data?


        Your question is hard to answer: "distance about the Earth's radius"--from where? Or is it the radius of the Earth's orbit? And what is "appreciable"? Is the field at the Earth surface "appreciable"? It takes a carefully suspended magnetic needle to detect it--yet it is strong enough to stand off the solar wind, though it is 3000 times weaker than fields of sunspots.

        For more about the Sun's magnetic field, go to the home page
    especially sections from 16 and on. Even more is in
    and in the web collection of which it is part.

        The Sun's field is dragged out by the solar wind, so at the Earth's orbit it is strong enough to help create magnetic storms and form the planetary magnetosphere. Without the solar wind, if it decreased with distance at a rate similar to that observed in Earth's field, it would be too weak to be even observed at the Earth's orbit. As it is, it is about 1/10,000 as strong as the Earth's surface field--with ups and downs as the Sun rotates, about half the time pointing roughly sunwards, half the time pointing roughly away. See

    132.   Flux Transfer Events (FTEs)

        I happened across your web site and noticed you responded to questions regarding magnetism, sun phenomena, etc. I recently read an article about FTEs and connecting tunnels from the sun to earth.
      "A magnetic portal will open, linking Earth to the sun 93 million miles away. Tons of high-energy particles may flow through the opening before it closes again ..."

    It was reported that such events occur regularly about every 8 minutes:

      "Earth's magnetic field presses against the sun's magnetic field. Approximately every eight minutes, the two fields briefly merge or "reconnect," forming a portal through which particles can flow. The portal takes the form of a magnetic cylinder about as wide as Earth."

        Then I noticed in one of your replies to a question that "The Sun is our nearest star and the source of energy for life on Earth. It is about 150 million km away (93 million miles), a distance which sunlight covers in 8 minutes, ..."

        So, my question what is the connection/correlation or the meaning that these two facts both have in common, i.e. these 8 minute intervals? I hope you have time to reply with an e-mail!


        The "opening of a magnetic cylinder" at intervals of 8 minutes does not correspond to anything I know to exist. If you send me the article, or tell me where to find it, perhaps I could say more.

        FTE or "flux transfer event" is a certain variation of the magnetic field just ahead of the magnetopause, said to mark magnetic reconnection, but we really do not know what it is because FTE observations from single satellites does not tell enough about 3-dimesional magnetic structure. We know that reconnections (and also FTEs) occur during "southward slanting IMF"--see, also in the same collection /wtail.html and /wpcap.html (especially the illustration at the end). Southward IMF occurs randomly and may last hours, even days. Solar wind particles then enter field lines connected to Earth, but because the field lines continually deform, they usually end up in the tail, and there may continue flowing away--or sometimes, in the near parts of the tail, be convected earthward and energized, e.g. in substorms (/wsubstrm.html).

    133.   Building a Terrella experiment like Birkeland's

        I am a high school junior in Rhode Island. I have become interested in the aurora borealis (and australis) and would like to study the aurora for my high school science project. I was hoping to conduct an experiment involving different environmental conditions utilizing a process similar to the terrella experiment conducted by Kristian Birkeland. In order to do this my teacher has told me that I would need a pre-existing procedure. I was wondering if you have a plan that includes the procedure and the materials necessary to build a model for this project. Or, is there a way that I could do this on a level more suitable for high school, if I can not get access to some of the materials needed? Is there possibly a more modern model that I use? I am hoping that it is possible for me to do this.

        Can you help me access this information?


        I think this experiment is too technically challenging. You would need a vacuum chamber, sealed tight against leaks, and clean (not evolving gases that may spoil the vacuum), with electrical connections going through the wall or floor. It could be of metal with a window to look inside--Birkeland used glass chambers, but size is then limited by the need for strength to resist air pressure. A vacuum pump and a pressure gauge are needed too--Birkeland's "vacuum" was relatively modest by today's standards, with enough residual gas to trace the path of electrons

        Inside the chamber you need an electron gun like the one in a TV tube--probably a black-and white TV, which uses less voltage. I think you have to buy a new gun--opening a TV tube to air may spoil the electron gun inside, and you need a high-voltage source for it. The target sphere would have a coil inside to create a magnetic field (and a power supply outside)--painted with luminescent paint to show the arrival of the electron beam. The voltages, magnetic field etc. need to be calculated beforehand.

        I think it all would be too much.

        Strictly speaking, the orbits obtained by Birkeland apply more to cosmic rays than to the aurora. It would be easier to calculate the paths of electrons on a computer (
    like here) and trace their 2-dimensional projection on the screen. To do so you need use a calculation by Carl Stoermer, who derived the projection of the equations of motion onto a plane including the magnetic axis. The equations resemble those for a marble rolling across a "landscape" with a crescent-shaped valley along a representative field line (the scale of the map depends on energy), and a tall "mountain" inside the crescent. Outside the crescent the "land" rises again, but on the equatorial line (perpendicular to the magnetic axis) a "pass" exists, through which the marble can enter or sometimes escape.

        I am sorry, but the derivation of the orbit itself involves calculus--you need trace the solution of a differential equation. Some details are in "Charged particle motions in a magnetic field which reduce to motions in a potential", D.P. Stern, Amer. J. Phys., 43, 689-694, 1975.

    For more on the aurora, see "Exploration of the Earth's magnetosphere", especially "Secrets of the Polar Aurora."  

    134.   Electric field due to electromagnetic induction


        If changing magnetic fields create electric fields and earth's magnetic poles are shifting, does this mean that an electric field must exist perpendicular to earth's magnetic north and south? If so, does earth have electric poles? And if so, do we know where the poles are located? Thanks,


        The strength of an induced electric field depends on the rate the magnetic field changes: the gradual change of the Earth's "main" field is so slow--5 to 7% per century--that any electric field generated by it is too weak to matter, or even to be observed.

        Much more rapid variations (though of limited extent) occur in magnetic storms and substorms, and they induce secondary currents at Earth--in the ground ("earth currents"), in power grids close to the auroral zone and even in Alaska's oil pipeline. On the rare occasions when they were unusually intense, they have disrupted electric power (caused a "black-out") or damaged network equipment, but they generally only last a short time.

        The Earth's atmosphere also has an appreciable electric field , but its source is thunderstorm activity ( Because air is such an excellent electrical insulator, the currents driven by that electric field are small and unimportant. And because thunderstorm activity shifts all the time, the pattern of the Earths electric field constantly changes, too.

  Back to the main list

Click here for a listing arranged by topic.

"Exploration" home page ......... "Exploration" index

Authors and Curators: