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FAQ: "Exploration of the Earth's Magnetosphere"

The Answer Man

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.

AND OH!   Please note--this is a fairly long file and may take a while to load.

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
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Before you do, though, please read the instructions

46.   "Importance of auroras to society"

I am wondering what is the importance of auroras in society. I would also like to know what importance auroras have in the future. I need to know this information for an essay I am writing. I have searched everywhere but I haven't found any information on either of the two topics. Please assist me.


    I am curious--how did you come to write that essay? Your idea, or an assignment? If the latter, I wonder how other people with the same assignment are faring. By all means, send me your essay when it's done.

    How important are auroras in society? One may just as well ask the same questions about rainbows, or bolts of lightning. All these started as a supernatural mystery--evidence that nature has strange wonders we do not understand, wonders which may be manifestation of some greater and more powerful intelligence, invisible but dominating our lives.

    Gradually, they were explained, and are now viewed as evidence that nature is governed by orderly rules, and is not random or capricious. In fact, they help us learn about those rules. The fact that auroras are not completely understood shows we still have things to learn.

    You write " I have searched everywhere but I haven't found any information on either of the two topics." Where you lack information, try to think what the answer might be, and formulate your own. Any good essay comes first of all from YOU.

47.   Magnetic storms and headaches

    The geomagnetic storms we have been experiencing cause me to have severe headaches and sometimes visual field aberrations, such as a flattening of my vision and depth perception, as well as some cognitive dysfunction. I have correlated these with the strength of the storms and flares by keeping a diary and checking it against the info put out by on the daily email lists. My question is do you know of any way to counter act the effect of these ion storms on humans? I am aware of currently proposed research projects for shielding Cardiac ICUs since the storms are correlated with an increase in heart attacks. I have asked this of many doctors, both main stream and alternative practitioners. I always get a no but let me hear from you if you find out.

    Thanks for any info!


    To a scientist such effects must seem very unlikely. The magnetic field at Earth is about 0.5 Gauss, and magnetic storms change it by 1% at most.

    Have you ever undergone MRI (magnetic resonance imaging) of your head? I have. First you must remove all iron objects from your pockets or body (watches, pens etc.), lest they fly off in the strong magnetic field and stick to the magnet. You then go to a room with a big doughnut-shaped machine, and lie on a pallet which wheels you into the hole of the doughnut, until your head (or other body part being examined) is in its middle. Then it scans you for a few minutes (you may not move, it's a time exposure) with a loud racket, and finally wheels you out again.

    During those minutes your head may be exposed to 1000 gauss--maybe several thousand, I do not know the exact numbers. I never had a headache or sensed the magnetic field. But you may also ask operators of such machines (or search the web) to find more.

    Effects similar to those of magnetic storms occur frequently in Fairbanks, Alaska, during "magnetic substorms" accompanying the polar aurora. You may ask doctors in hospitals there if they find any effects. My guess is that they don't.

    So--sorry, but I must add my voice to the negative chorus. One could shield rooms (see but I doubt that a valid reason exists.

48.   Appolo Astronauts and radiation

(Or: Did our astronauts really land on the Moon?)

    On the Art Bell radio show last night I heard a fellow state that the Van Allen radiation belt's level of radiation was/is so intense... that for the 2 hours each way that our astronauts would have spent in it would have been fatal to them and therefore man being on the moon was impossible to have happened.

    He also specified that whatever shielding was present on their craft would not have spared them.

    Would you be so kind as to illuminate me about the credibility and scientific accuracy of his claims regarding the Van Allen belt's level of radiation ?


    The argument which you raised keeps coming back in my e-mail; see for instance

    The answer is always--no. The radiation is bad, but not THAT bad. Astronauts went through the belt quickly, and the dosage was minimal. Two hours were certainly not enough to kill anyone--two weeks, maybe, I am not sure (the dosage to the skin would be heavy, but to the heart or brain much less so because most of the inner belt protons do not penetrate far).

Further question: Were those astronauts safe from solar flares?

    Dear Dr. Stern,

    Thank you for your speedy response. Was there a contingency plan for X class solar flares? Would such flares have posed a danger to life for the astronauts?


    High-energy ions from large solar flares can be very dangerous to human life. They can be quite penetrating and hard to shield against; "can be," because the energies vary from event to event, and the size of a flare is only loosely related to the high-energy ion danger.

    Such flares occur infrequently, a few times each 11-year solar cycle, but if you send people to Mars, they pose a real risk (they also pose a risk on Mars itself, since the thin atmosphere there is not a very good shield).

    See .

49.   What materials does a magnet pull?

    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. Thanks!


    Iron is attracted to magnets, but why? Because near a magnet, in its "magnetic field" (roughly, its region of influence) ordinary iron turns into a temporary magnet. See
and then go to the page "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.

    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.

50.   Experimental simulation of the polar aurora

    I want to do an experiment of an aurora and all I can think to do is show the similarity on magnet shavings and a magnet. I feel this is a little lame. Any ideas on how to be more effective.

    I am in the 5th grade, live in Charleston SC and love anything that deals with the sky. Auroras seemed to be a great subject to try to prove but after hours of research I can't find any way to replicate one. any help would be appreciated. Thank you


    It would be difficult, even a crude experiment would need a vacuum pump and attached glass plumbing.

    The problem is that the aurora occurs high up, in very rarefied air, where the average spacing between molecules and atoms is large, so that an electron (like that of the aurora) has a long free run before it bumps into something. Here at ground level the "mean free path" (that is the technical term) is very short--less than the thickness of a hair.

    The closest you can come in the lab is by phenomena known as gas discharges and cathode rays.

    Air is a poor conductor of electricity, which is why power companies string their lines through the air and never worry about electricity leaking away. To conduct electricity, it needs to contain a small percentage of electrons set free from their atoms. In the dense air we breathe, these bump into molecules so quickly that they almost immediately recombine, so at any time, hardly any are present (except with very high voltages, as in lighting).

    But pump the air down to a few millionths or so of its ordinary pressure (just from memory, numbers need to be checked), and the picture changes. If the rarefied air is inside a glass tube, and you stick two wires into it (through holes with leakproof seals), then put a voltage of say 1000 volts between them, the air begins to glow. Now electrons have few collisions, and the voltage is enough to speed them up so that on colliding, not only do they not get lost, but they knock out some more electrons, so there are always enough of those to conduct a current.

    It looks like the aurora, but it isn't the same--auroral electrons go miles before colliding. Fluorescent tubes are a bit like that, too.

    Pump out more of the air, and the glow gets very dim--it's the collided atoms which emit light, and too few collisions take place (instead the electrons hit the wire at the positive end, which can get hot). This happens in X-ray tubes in the doctor's office, and in TV tubes. The glow can be deflected by a magnet which steers the electrons, but don't try it with your TV tubes because (1) their voltage is really high, (2) a magnet can defocus them permanently (3) their big shape is not good for reaching the electron beam inside and (4) you cannot see that beam anyway. Those beams are more like the aurora, though.

    About 100 years ago a Norwegian physicist named Kristian Birkeland went one step beyond, simulating the aurora by sending a beam of electrons towards a spherical magnet in a vacuum chamber. Read about him in
His experiment however is quite difficult to duplicate.

    Sorry, I cannot help you more than this. But look up cathode rays and electric discharges in rarefied gases in an encyclopaedia.

51.   Cosmic ray research using balloons

    I am dong a science project on Cosmic Rays. I found your site, or the site that some of your Research was on, and it was really interesting. I was wondering if it wouldn't be too much trouble for you if you sent me some information about when those air balloons with the photographic plates were sent into the atmosphere. I think that your work is amazing, thank you for all the help I've received from it so far. Thank you


    High-altitude balloons with photographic plates were first sent up in the 1950s and 1960s, early days of cosmic ray research. Part of the reason was not just to study cosmic rays, but also to investigate the collision of very energetic protons and ions with nuclei, which produced some interesting new short-lived particles and taught physicists about the way elementary particles were made. In those days cosmic rays were the only known source of such fast particles. In later years accelerator machines were built which produced such particles in much, much greater numbers, and those who study this sort of physics shifted to them.

    High-altitude balloons are still used, but they now lift electronic instruments which automatically sort incoming particles--and being bigger than slivers of film scanned by a microscope, they also observe many more of them. Some of them are test-versions of instruments later flown aboard satellites.

    The balloons contain helium, not air. When launched they look like long draggy bags, with a big bubble of helium on top, just enough to lift the payload in the dense atmosphere near the ground. At high altitudes, air is more arefied and much more volume is needed to lift the same weight. However, the bubble expands there and the balloon fills up to a near-spherical shape, supplying that lifting volume.

    Good luck with the project. Search Google for balloon research of cosmic rays and for primary cosmic radiation.

52.   Magnetic health products

    My wife is going to buy a mattress with built-in magnets (constant magnets with a diameter of a thumb's nail). Those things are advertised as health products. These built-in magnets should compensate for the decreasing strength of the earth's magnetic field.

    There is a lot published in the internet regarding those "magnetic products" and for me the following questions arose:

    1) If I believe those publications, the earth's magnetic field oscillates constant-periodically with 7.5 Hz. For my understanding, it's a constant field (no oscillation at all); it only decreases by time. What's your opinion?

    2) The magnets on this mattress are attached in rows (magnet distance ~10 cm). The following row is @ 30 cm distance, with all magnets in that row with reversed polarity So I have a couple of alternating N and S - pole rows from the top to the bottom. For my understanding, this doesn't reflect the earth's magnetic field and so the advertising argument, that this product does simulate/increase the earth's magnetic field once you're lying on the mattress. Does it sound logic to you?

    3) These publications above also mention that applying a magnetic field inside a spaceship avoid from any loss of calcium in the astronaut's bones, and astronauts have less depressions, and mice don't loose their coat/skin anymore !? I thought the loss of calcium is due to the gravity and not to the fact that the astronauts are further away from the earth's magnetic field. What's your comment on this and are you aware of any experiments in manned space flight with applying magnetic fields to the ship? Thanks in advance for any helpful answers/comments.


    (1)     I am not familiar with all the oscillations of the Earth's magnetic field--they exist, in various frequency ranges, but are always very small (and transient). You are right, the field is essentially constant.

    (2)     No significant magnetic effects on human body have ever been found: we do not seem to have magnetic sensors (as homing birds might have), nor do any large currents flow in the body (if they did, they might respond to magnetic forces). Have you ever taken an MRI (magnetic resonance imaging) test? I have--it's a rather noisy experience. You lie on your back on a narrow pallet which slides you in the middle of a ring-shaped magnet, and a field is switched on--I am not sure how strong, but probably a few thousand times the usual Earth's field. Before you go into the imaging room you are asked to remove all objects made of iron--key rings, watches, belts with buckles, etc.--lest they fly off and stick to the magnet. Yet while you lie on that pallet, you cannot tell the moment the magnet is switched on, or any difference when it is.

    So my advice to your wife is, save your money, or spend it instead on a comfortable conventional mattress.

    By the way, a movie is coming out, "The Core", which seems to promote the notion that the Earth's magnetic field is essential to our well-being. Haven't seen it, but from what I have read, it seems to be sci-fi, not solid science.

    (3)     I have heard the stories about astronauts, too. They seem to be flights of fancy.

    Astronauts can lose weight in space (you are right about the cause), especially in long stays on a space station, but the remedy are exercises. I wrote about this in

    The physics is discussed there in the preceding section, on mass.

53.   Geiger counters for locating lost objects.

    I am a 12 year old student and was wondering ...I read about your Geiger counter explanation and was wondering if you had any knowledge about a product that would locate missing things based on this technology? many thanks tk


    Sure, a Geiger counter can be used to locate missing objects--as long as they are radioactive. I read of prospectors using them in searching for uranium ore, at least early in the search (later more sensitive means are used). They are also used in security gates, to make sure no one is smuggling nuclear materials through them.

    Such a counters were used at the gates to the Los Alamos National Laboratory in New Mexico, where nuclear research is being conducted. One day a truck loaded with iron bars for construction work came in and the alarm started ringing. Sure enough, the iron was radioactive, and it turned out to have come from Mexico. Investigators tracked down its source and found that a medical machine containing a large radioactive source was illegally junked (the law requires all radioactive materials to be specially disposed) and ended up in a shipment of scrap iron, which was melted and turned into iron bars for construction. As far as I remember, no one was hurt that time, but inspectors had to track down all shipments of that iron, many of which ended in the US, and some buildings under construction had to be demolished and their iron removed.

    So yes, you can use Geiger counter for tracking radioactivity.

54.   Magnetic effects from other planets

Dear Dr. Stern

    Have we been able to determine if there is an induced voltage or modulation of the earth's magnetosphere at times when the other planets are at various angles to the earth?


    The short answer to your question is "no." However, you probably deserve a more detailed reply, so here it is.

    First of all, any effects of other planets existed would be more readily observed in interplanetary space outside the magnetosphere, rather than inside the magnetosphere, where they would be muffled by the stronger magnetic fields and hotter plasmas, and obscured by unrelated waves and disturbances created there.

    Second, the propagation of effects to Earth is dominated by the solar wind, flowing outward from the Sun, and to some extent by the interplanetary magnetic field. A planet upstream of Earth in the solar wind, or on the same interplanetary magnetic field line, might possibly affect us, but not otherwise.

    Upstream we have only two planets, with relatively small wakes. The one of Venus is scarcely wider than the planet, since Venus has no magnetic field. Mercury has a field, but it is weak and the planet is small, so its wake is narrow, too. I am not aware of either wake being detected near Earth, but it is possible someone has observed one. The effect on Earth, in any case, is small. About the effect of the Moon's wake, see next question below!

    Jupiter has a giant magnetosphere and a huge wake, and I vaguely recall that wake being credited for a strange observation by a space probe near the orbit of Saturn, but of course Earth is upstream of the planet and the wake will never hit it. Jupiter's magnetosphere was also at one time given credit for energetic electrons (1 MeV or so) in the Earth's magnetosphere. It was claimed that those electrons were more plentiful at times when Earth and Jupiter were linked by the same bunch of magnetic field lines. However, I have not heard that explanation recently and believe most researchers now feel they are generated in our own magnetosphere--though the process responsible remains unclear.

    So, here is your answer. It still boils down to "no."


55.   Blocking of the Solar Wind by our Moon?

    Dear Dr. Stern

    What effect does our moon have on the solar winds? Could our moon act as a shield for the earth, from the full impact of the solar storm, when it is upstream? Thank you.


    Yes, when the moon is upstream of Earth it will intercept some of the solar wind which would otherwise impinge on the front of the magnetosphere. But the effect is small, because the moon is small, too. I have never read of anyone detecting it.

    The radius of the Moon is about 1/3 that of Earth, whereas the radius of the magnetosphere abreast of Earth is about 15 Earth radii. If you take 10 Earth radii as the radius of the "front," you find the moon covers only 1/900 of the front area. A weakening of that extent in the flow of solar wind would be lost in the disorderly "noise."

    Neither do I think that the moon casts a sharp (even if small) "solar wind shadow" on the magnetosphere. At a distance of 60 Earth radii, it is about 180 moon radii away. Near the moon, on the side facing away from the Sun, the observed "shadow" is sharp, but at a distance of 90 times the width of the obstacle I expect it to be washed out, especially since the bow shock of the Earth intervenes, with its disorderly "foreshock" ahead and "magnetosheath" behind.

    Some people have claimed a correlation between positions of the Moon and observed effects on Earth. The position of the Moon does affect tides in the ocean, but one does not expect any magnetic effects.

56.   Fry or Freeze... ?


    Found your data on the earth's magnetosphere. Wanted to know how far out in space it extends. It doesn't make it to the moon, or does it?

    This new data on Mars suggests men would fry there due to cosmic I was wondering why the Apollo crews didn't...


    You wrote " found your data on the earth's magnetosphere"; but what files you were reading? The dimensions of the magnetosphere (and its shape) are discussed in


and also in question #37 on this file.

    As explained there, even though its name includes "sphere," the magnetosphere is far from spherical, and on the night side it extends well past the moon. Once a month the moon may spend about 2 days in the magnetotail region of the magnetosphere.

    Astronauts on Mars are more likely to freeze than to fry, because Mars is quite cold, being much farther away from the Sun. Cosmic radiation is a minor problem--it is fairly weak, and its danger may be small; its secondary particles produced in the atmosphere also reach us on the surface of the Earth, and though heavy nuclei among its primaries are biologically very damaging, their number is small. . The real problem are big solar outbursts, which happen (typically) several times each 11-year cycle of the Sun. They pose a danger not only during the flight there and back, but also on Mars itself, since its thin atmosphere (unlike Earth's) does not appreciably absorb high-energy solar protons.

    (from reply to another message) The astronauts may have played the odds on their flight to the Moon. They might have had contingency plans we know not of, like huddling behind fuel tanks or (on the Moon) building a temporary radiation shelter. All this remains speculation: few flare events are intense enough to imperil life, and as it turned out, none materialized during the Apollo missions.

    See also question #48 on this file and the 3 questions at

57.   The Speed of the Solar Wind

    Dr. Stern;

    I just read your Solar Wind web article (Oct 2002). I was trying to find the speed of the solar wind when I found you article, and am emailing you with that question.

    What is the speed of the solar wind with respect to the sun?


    The average solar wind moves at about 400 kilometers/second, a little more than 1/1000 of the speed of light--about 4 Earth radii per minute, about 4 days to cover the distance from the Sun to us. The magnetic field near the Sun may impede this motion; at times when the field lines linking the solar wind to its region of origin stick straight out (rather than bend around or arch back to the Sun), it gets a clear path and its velocity may rise to 700 km/s. At some other times its velocity may be only 300 km/s.

    As eclipse photos have suggested, field lines above the poles of the Sun do stick straight out, and it was predicted that this sort of fast flow, which only occurs sporadically at the orbit of the Earth (mainly around years of sunspot minimum) should be the norm above the Sun's poles. When the Ulysses spacecraft passed above the polar caps of the Sun, that prediction was confirmed.

    Go to the end of the "solar wind article" and click on the link to the master list: you'll find links to many other interesting topics.

58.   What is "Radiation"?

    What is "radiation"? And--if you "nuke" food in a microwave oven, can it become radioactive?


    The word "radiation" in common use has (unfortunately) more than one meaning. And to answer the 2nd question--no, microwave radiation and nuclear radiation (involving radioactivity) are very, very different.

    "Radiation" essentially means some effect which spreads radially in all directions, along straight lines (along the radius). Examples are light and its close relatives, infra-red (IR) radiation (emitted by hot objects) and ultra-violet (UV) radiation.

    In the middle 1800s James Clerk Maxwell concluded that light was an electromagnetic wave. That suggested such waves could be created by purely electrical means and in 1886 Heinrich Hertz did so, producing radio waves. The microwave radiation in your oven is also produced electrically, it is just a short-wave radio wave, developed in World War II for use in radar. When physicists talk of "radiation" they usually mean "electromagnetic radiation," the large family covering light, IR, UV, radio and microwaves.

    Then came the year 1895. Two discoveries stirred up the world of physics and seemed to upset Maxwell's neat scheme.

    It was already known that in a glass tube containing very rarefied air and two separated electrical contacts, if a high voltage was connected to those contacts, an electrical current flowed and a faint glow was produced. [In 1897 J.J. Thomson found the current was carried by tiny pieces of matter, electrically charged, which were named "electrons."] One of the discoveries of 1895, by the German Konrad Roentgen, was that the tube also emitted something that could fog photographic plates, even ones kept in tight dark boxes. Obviously some radiation was produced, and Roentgen named it "X-rays", "X" for "unknown."

    The same year the Frenchman Henri Becquerel found that uranium compounds also emitted some "radiation" able to darken photographic materials. It thus seemed, at least for a while, that Nature may have many other new types of radiation--in fact, some other claims were made (and later retracted), while science fiction writers had fun with stories about "death rays."

    Gradually a clear picture emerged. X-rays were part of the electromagnetic family, with very short wavelength and high penetration. Radiation emitted by radioactive materials was a mixture of 3 kinds, one of them gamma-rays, similar to X-rays, but usually with even shorter wavelength and greater penetrating ability. The other two were fast particles, atomic bullets which unlike electromagnetic radiation had mass--fast electrons (negative charge), and "alpha particles", fast positive nuclei of helium.

    Those particles should really not be called "radiation," but it is too late to educate the public, especially since many detector instruments (such as Geiger counters) respond both the particles and to X- or gamma rays. When James Van Allen's instrument on the "Explorer" satellites I and III discovered high energy particles magnetically trapped in near-Earth space, they were called the "radiation belt," and the name stuck. To reduce confusion, perhaps it is better to call fast particles emitted by radioactive substances "nuclear radiation."

    But you never find it in microwave ovens.

59. How does one Contain a Plasma?

    Dear Doctor Stern,

    Can you help me with this question? How do make a container that can hold plasma? I am 9 years old. I go to Scared Heart of Jesus. I have 4 sisters. My dad works at N.T.I.A. and is also helping me. Thank you.


Tough question! A plasma is a very hot gas, hot enough to conduct electricity. It better not touch the walls around it! If it does, it will pass some of its heat to those walls, and one of two things may happen. If it is dense and contains a lot of heat, it might melt the walls, and if it is rarefied and contains just a little heat, it will lose its heat to the walls, cool down and stop being a plasma.

    So how can you contain a plasma? Two ways have been tried.

    With a rarefied plasma, one can continually supply fresh energy and keep heating it up. This is what happens with a fluorescent lightbulb, which contains plasma. The wires leading to the lightbulb continually supply fresh electric energy, so the plasma is continually renovated, which allows it to continue carrying an electric current.

    The gas inside the lightbulb also absorbs energy from the plasma, and emits it as light, which turns out to be a very efficient way of producing light--more efficient than that of lightbulbs whose light comes from a hot wire.

    The other way is to use a strong magnetic force to keep the plasma confined: this is made possible by the electrical properties of a plasma, which determine the way it interacts with magnetism. This is the way favored by researchers trying to confine a dense plasma. The plasma is inside some container, but the magnetic "field" (region of magnetic forces) keeps it from touching the walls. The trouble is that this sort of situation can be very unstable--the plasma also creates its own magnetic field, which modifies the one keeping it trapped, and so far, just when things get interesting, it is likely to shake itself loose, slip away to the walls, cool down and be gone.

    A lot of clever physics has gone into this in the last 50 years, and scientists have managed to trap denser and denser plasmas, but even now they are a bit short of the temperature, density and trapping time which would allow them to convert hydrogen to helium (nuclear fusion) and create energy that way. The Sun does release fusion energy, but it traps the plasma in its core by its enormous gravity, which we cannot duplicate in the lab.

60.   Soviet Nuclear Explosions in Space

    Dr. David Stern

    I noticed your web page on the GSFC site, with the reference to the early bomb tests, . You mention the Soviet tests - but not the year in which they were conducted. Can you fill in the missing info?


    The Soviets conducted three high-altitude tests above the Russian arctic region, on October 22, 28 and November 1, 1962. The first two were sub megaton, the 3rd in the megaton range. Unlike the US "Starfish" explosion the preceding July, they were on field lines extending further into space, with less stable trapping, and they decayed within weeks. Interestingly, the US launched "Explorer 15" on October 27, just in time to observe the resulting radiation belts. That satellite was built in response to the unexpected damage and longevity of the "Starfish" belts. NASA engineers, under pressure, went into the "museum" display in bldg. 1 of Goddard Space Flight Center, retrieved the engineering test model of Explorer 12 and converted it into a usable satellite, all in the record time of 91 days. I wish NASA still had that agility and in-house capability!

    For further details and for references, see "The Radiation Belt and Magnetosphere" by Wilmot Hess, published by Blaisdell 1968. Chapter 5 deals with artificial radiation belts, and the discussion of the Russian tests begins on page 172. The tests are listed in a table on p. 155, while decay times of radiation are given on p.200.

61.   Can Polar Aurora be seen in Atlanta, Georgia?

    I live in Woodstock, GA, a suburban area of Atlanta. We, in this area, have had some strange occurrences this year including an earthquake. It was a small one, but since I am a native Georgian, and it was the first I've experienced--the quake got my attention. As a writer for a small publishing company, I like to record events like this as possible material for later use.

    To the point, I was told by a fellow worker (a former teacher and fellow student in a Master's writing program at Kennesaw State University) that there would be an aurora borealis visible in the Georgia night skies on May 30 and 31. I looked for other sources to predict this unusual event, but there was no mention of it anywhere else. I looked, not too hard though since I suspected an urban legend in the making. Please tell me, is it possible, ever, that an aurora borealis would appear this far south?


    It wasn't a hoax, just perhaps wishful thinking. Eruptions on the Sun occur now and then, some are quite big, and conditions may be ripe for a widespread aurora, yet often nothing happens. How often have you heard a tornado warning on the radio, but none materialized? A large flare on the Sun, a Coronal Mass Ejection (CME) heading our way--all these are necessary, but until the disturbance actually arrives, who can tell if the interplanetary magnetic field is right (it should tilt southward, not northward), how big that CME actually is, and other imponderables? The best a prediction can do is state a likelihood, never a certainty (just as with a tornado).

    Predictions were made that day: see

Some actually predicted aurora, but I have not heard about any aurora actually observed at low latitudes on that day.

    Aurora may be visible from Atlanta, but only rarely--perhaps once in a decade. On 5 November 2001 aurora was widely seen across the lower 48 states; my son in Purcellville, Virginia, saw it (I looked but saw nothing--city lights may have blotted it out). I was alerted by a message from a family in Chicago and the story is on this web collection at wnovstrm.htm . It may or may not have reached Atlanta, but a larger storm in March 1989 probably did get that far. For a picture of that storm from space (not at its greatest extent) see weather.html on this collection of web sites Reportedly, the aurora following the great flare of 1 Sept. 1859--the first ever observed--reached Havana, Cuba. About this solar event, see whcarr.html. Finally, for an overview about the polar aurora, see (also here) at aurora.htm .

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