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FAQ: "Exploration of the Earth's Magnetosphere"

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    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. Apollo 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

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    (32)   Free Energy from the Earth's Magnetic Field?

        I am a yr 12 physics student from Australia, and I was wondering if we can make an electrical current flow in a wire or piece of metal using the earth's magnetic field. Because in a similar way electrons in a wire can be forced to flow due to magnetic fields placed nearby the wire. Surely this would be a cheap and efficient form of "free energy" perhaps? I was just wondering anyhow, I hope you can provide me with an answer.


        You need understand better the difference between the effects of electric and magnetic field, say on electrons, the particle which carry electric currents in wires.     Electric fields PUSH electrons, and can give them energy--they do so to electrons in wires the way pressure drops pushes water through pipes.     Magnetic fields are more subtle. Out in space, where electrons are not bound to wires but fly through space, magnetic fields can DEFLECT them to different directions, but do not give them any energy. That is how the Earth's field can trap radiation belt particles, year after year after year: it has a great effect on their motion, but because the field needs not spend any energy on doing so, it is not weakened at all. It also means that you can get no "free energy" from this source.     A CHANGING magnetic field does create an electric field, and therefore CAN impart energy. Magnetic storms involve such changes, and can drive "Earth currents" which have been measured. They can even add (at near-polar latitudes) extra currents to the electric grid, which have been known to cause damage, because they resemble DC, and the grid is designed for AC.     For more than that, you will have to study more about electricity and magnetism.

    (33)   Relativity

    Hi, my name is Paul. I am from Birmingham, England and would just like to ask one question.

    Is there an equation to work out the effects of time while travelling at light speed? e.g If I were to travel at the speed of light to Proxima Centauri and it took me 4.23 years, how much time would pass by on Earth, etc.


    Dear Paul

    Relativity is not my field, but I believe time would stand still. Also, you could never go that fast, because an infinitely strong force would be required. But if you could, arranging to send back a radio signal when you arrived, the signal would come back 8.46 years after you have left, whereas to you the trip would be instantaneous.

    I general, time in your frame passes faster as you approach the speed of light (I am sure you can find the formula), compared to the passage of time in a frame of reference not sharing your motion. Funny things may happen as you approach that limit. Someone once told me (you can check in Birmingham) that viewed from the outside, objects falling into a black hole seem to take forever to reach the "event horizon" beyond which they cannot be tracked. This, of course, assumes the information is conveyed by light, which gets drastically red-shifted as they approach that limit. However, in the frame of the falling object itself, only a finite time will have passed.

    And as for going faster... you may already have heard the limerick:

                There was a young lady named Bright
                Who travelled much faster than light
                She set out one day
                In a relative way
                And returned the previous night.

    (34)   What is a "REM"?

        I was wondering to how much radiation you would you have to be exposed to be affected. I also was wondering what is REM and what are these funky lines outside of the earth (magnetic waves?)


        As the saying goes, no pain--no gain. To find out, you will have to do your homework, look up web sites and books. Briefly:

    --     "To how much radiation you would you have to be exposed to be affected?" Depends on your definition of "affected." Some people say ANY radiation increases your long range cancer risk, but the effect of low doses is probably small. A dose of 500 REM has a 50% chance of killing you (treatment helps, but not a lot), a dose of 100 REM probably increases your long range cancer prospects on the same order as cigarette smoking. Cosmic rays give you a lifetime dosage of about 2 REM--rock radioactivity probably a comparable amount, depending on where you live.

    --    What is REM? Stands for "roentgen equivalent man." The Roentgen started as a unit of x-ray dose measured by the number of electrons it tore from atoms, per unit mass ("man" means that every part of the body got that much radiation). Later people realized that some radiations had a greater effect on health than x-rays (for the same electron detachment) so radiation from other sources was converted to "equivalent" roentgens.

    --    Those funky lines are field lines or "lines of force." Read all about them at http://www.phy6.org/Education/wfldline.html.

    (35)   What exactly does "Radiation" Mean?

    Dear sir,

        Can you kindly help me to answer the following question: what is different between the following terms

    1. Solar radiation and solar irradiation
    2. Solar terrestrial radiation and solar extraterrestrial radiation
    3. The word radiation and irradiance
    Thank you in advance and waiting got your kind reply.


        I am no authority on language, I can only tell you what I understand in the terms you sent. Let me start with the last.

        "Radiation" is used in several ways, often causing confusion. It means a phenomenon that spreads in space (along a radius from the source, if you will), and there exist some quite different phenomena that do so.

        To a physicist, it often means "electromagnetic radiation", an electric and magnetic disturbance spreading as a wave in space. See for instance http://www.phy6.org/stargaze/Sun5wave.htm (You might also need the section preceding it http://www.phy6.org/stargaze/Sun4spec.htm)     Depending on frequency, this covers radio, microwaves, light (also infra-red and ultra-violet), X rays and gamma rays. "Solar radiation" is thus sunlight, at all wavelength.

        However, when people talk about "radioactive radiation" or "radiation belts of the Earth" they mean a stream of fast particles, electrons, ions or neutrons, which also spreads in space. Early researchers did not understand the difference and the name stuck.

        "Irradiance" as I understand it means energy deposited per unit area. The Sun beams at Earth 1.36 kilowatt per square meter, the "solar constant." I think that is the irradiance due to sunlight, or would be if the atmosphere did not scatter some of it back, and if the receiving area is perpendicular to sunlight.

       "Solar Terrestrial Radiation" I have not heard before. Solar Terrestrial Physics is the term usually applied to the particles sent out by the Sun, the magnetic fields they are embedded in and the phenomena these produce near Earth (see my web site with home page http://www.phy6.org/Education/Intro.html)

        In other words, the Sun sends us MORE than just electromagnetic radiation. I would think the term excludes the solar wind but includes higher energy stuff--see for instance "Birth of a Radiation Belt" at the end of the above web site. "Solar Extraterrestrial Radiation" I cannot even guess.

    (36)   Can anything solid be carried by the solar wind?

    Hello Mr. Stern. I have a question about solar wind. Is it possible for anything solid to be carried by solar winds? Just curious.             Jaret


        The answer is probably "no." Yes, the solar wind will exert a pressure on objects in interplanetary space, but I believe the pressure of sunlight, small as it is, is greater. It is very rarefied--about 6 atoms per cubic centimeter at the Earth's orbit.

        The solar wind does move ion tails in comets, not by colliding with them but by the magnetic field lines embedded in it--a collective effect, in which a large volume of the solar wind acts together. Dust tails however point in a slightly different direction, suggesting they are moved by sunlight pressure, and not by the solar wind. See http://www.phy6.org/stargaze/Saberr.htm

        In 1985 German scientists of the AMPTE mission exploded a charge of barium in the solar wind. The ions of barium vapor were quickly ionized by sunlight, creating an "artificial comet" of barium ions. The "comet" cloud was then quickly captured by magnetic field lines embedded in the solar wind, which made it share the flow velocity of the solar wind.

    (37)   Dimensions of the Magnetosphere

    (First of two related questions)

    Dear David Stern,

        What is the furthest recorded distance of the earth's magnetic tail, and hypothetically speaking, what would you speculate the actual maximized distance to be, if there were no spacecraft present to obtain data during the times of lowest possible solar wind conditions? During what exact part of the sun's cycle would this most likely occur, to obtain this maximized distance/length? Also with the same thought in mind, how many times its normal size would the magnetosphere bloom outwards towards the sun? What ideal factors would have to be present (in the solar system) or together to create a condition that would contribute to a total maximization of the Earth's magnetotail and what would those speculated/theoretical distances be?

        I've had strong interest in magnetic fields and tails for over 25 years. I have some theories (for which I keep to myself) concerning planetary magnetotails, but unfortunately there is not enough data on the Earth's magnetotail available. I may be totally unaware of new data that is available. Alex


        Presumably, you have read all my web pages on these matters in "Exploration of the Earth's Magnetosphere" where such matters are discussed, and I will not repeat what was stated there.

        How far does the magnetotail extend? The answer depends on both physics and our use of language: at what point do we stop calling it "magnetotail" and starting to call it "wake"? Plasma sheet field lines at IMP 8 (35 RE or Earth radii) are still firmly anchored at Earth, at both ends. However around 50-80 RE, the plasma starts streaming tailward, picking up the solar wind flow. The field lines are still terrestrial, but it's hard to say what shapes them further out. We have observations of ISEE-3 and Geotail to about 220 RE, and once long ago a spacecraft crossed the Earth-Sun line at about 1000 RE and saw some disturbance.

        We know less about the planets. With Jupiter, for a long time, space probes avoided the tail because, in order to pick extra up velocity from the planet (see "Stargazers", sect. 35), they had to exit at 90 deg. to the sun- Jupiter line. More recently, "Galileo" entered orbit around Jupiter and has done some observation. There exist some tentative observations of a drop-out of the solar wind, seen by a spacecraft near the orbit of Saturn, which might have come from the wake of Jupiter.

        The lowest possible solar wind effects occured May 11-12 1999, when the solar wind density near Earth dropped to 0.2 ions/cc (6 ions/cc is typical) and the dayside boundary of closed field lines in the Sun's direction, typically at 10.5 RE, retreated to 20-25 RE.

37-b     The Size of the Magnetosphere

Dear Professor,

    I am an Italian student of Physics and I met your very beautiful site http://www.phy6.org/earthmag/, and I have a question for you. I would like to know exactly the size of the magnetosphere, say from the Earth's surface up to the magnetopause, compressed in the day part and elongated into the magnetotail in the night part.

    I thank you very much for your kind attention!


    The average distance to the dayside "nose of the magnetosphere" ("subsolar point" in phys-speak) is about 10.5 Earth radii or some 67,000 kilometers. On the flanks, 90 degrees from there, the distance is about 15 Earth radii, and ithe flanks continue to approach a cylinder, about 25 Earth radii in radius.

    At midnight there is no clear boundary. What happens, apparently, is that past 50-80 Earth radii the solar wind infiltrates the magnetotail, so the material is mostly solar wind, but the magnetic field is still that of Earth. It goes like this for at least 220 Earth radii.

    But mind you, these are averages. When the pressure of the solar wind rises, the boundary moves inwards. It also does so if the interplanetary magnetic field slants southward, which "erodes" the magnetic field by a reconnection process. A few times each year, therefore, the boundary passes satellites in synchronous orbit, at 6.6 Earth radii. On the other hand, in 1999 an instance occurred when the solar wind was very rarefied, and the noon-side boundary went out past 20 Earth radii.

    You will find much more at "The Exploration of the Earth's magnetosphere", home page http://www.phy6.org/Education/Intro.html

    (38)   Skywriting by Aurora

        Dear David P. Stern & Mauricio Peredo,

        I want to "apply/harness" the northern lights phenomenon and turn the night sky into a giant TV screen.

        I see a satellite like device in geocentric orbit that has an electron collecting apparatus, perhaps similar to a solar panel, on the side facing the sun. These collected electrons would then be focused, and accelerated via a solar powered anode and then aimed/directed by utilizing and or manipulating the lines of magnetic force generated by the earth's core along with the magnetosphere.

        Once focused, accelerated, and directed, the electron beam could then be "shot" "Sky Illuminating Electron Gun", or SIEG at the earth's atmosphere, creating a recognizable/predetermined image by striking targeted nitrogen and oxygen atoms at various altitudes; thus providing the necessary primary colors along with a depth of field that would result in the creation of a three dimensional/holographic image(s) turning the night sky into a rainbow of choreographed light-ballerinas dancing across the heavens. Or perhaps a giant illuminated billboard for advertisement purposes. Do you too find it odd that one must deal with the devil to get to heaven, or in your case, funding for research?

        In your opinion, what would be the theoretical and practical obstacles one might face to pull this off and are these obstacles insurmountable, scientifically speaking..

        Thank you Gentlemen,


        Your proposal sounds interesting--at the very least. Still, I am greatly relieved to be able to say that it will not work, for several good reasons. Even if it did, I may add, the field line structure of the Earth's magnetic field is such that the only people able to watch your display would be inhabitants of Alaska and northern Scandinavia, Russia and Canada--not the audience to make such a project pay for itself commercially!

        In addition, however, there exist technical obstacles, listed here from the minor to the major:

    1. If an electron beam from synchronous orbit is to reach the atmosphere, it must be very narrowly focused along the direction of the magnetic field. Any electrons moving at an angle to the magnetic field direction, larger than some very small minimum, will bounce back before reaching Earth. They may hit the emitting satellite, or anyway form a belt of trapped electrons which will hit it sooner or later. It would be hard to prevent the beam from scattering while in transit. So many negative electrons, repelling each other--things may get unstable. They probably do in the natural aurora, which is why auroral beams tend to move across the sky.

    2. An electron beam from one source will only hit one spot. It is hard to paint pictures with it.

    3. The main auroral emissions--responsible for the green and red aurora--do not lend themselves to rapid changes. They are produced by atoms of oxygen in a (relatively) long-lived excited state, and are emitted at delays of the order of one second. The effect is as if you were drawing a TV picture with a long-persistence phosphor, whose glow persisted about one second. Auroras viewed through optical filters that select other emissions can see much faster changes, but for the unaided human eye the oxygen emissions cannot be removed.

    4. The energy required to create an artificial aurora can be quite large: after all, you want to create large-scale illumination, visible to an eye 100 kilometers below it! Artificial auroras have been created in the past by accelerators aboard rockets (mainly to trace magnetic field lines). The patches they produced, if at all seen, were faint. Nuclear bombs have created bright auroras, but we don't want to do that--do we?

    So--an interesting idea, but too early to coin an acronym!

    (39)   Capturing the energy of solar wind ions

        If is possible to see ion's pass through a cloud chamber, and we know they are solid particles that contain energy, why can we not develop a way to capture that energy? Truly using the power of the sun to generate electricity.

        I have an idea! If we were able to channel ions within a magnetic field with some kind of collector would this not generate electricity? Possibly developing a type of electrical absorbing gel, that when the ions pass through, collects this energy and transmits it out.

        I apologize if I do not put this idea out clearly, for I am not a scientist and only see this idea in the abstract.


    Dear Friend

        Several reasons why this will not work:

    1. The sun does not emit enough energetic particles to deliver much energy, even using an absorber with a fairly large area.

    2. The Sun emits both positive ions and negative electrons. To extract the energy of ions (which have most of it) you must screen out the electrons, an added complication.

    3. Energetic particles from the sun are stopped a long way from Earth. Even if one did capture their energy far in space, it is not clear how it might be transmitted to Earth.

    For those reasons and others, space vehicles prefer to use solar cells to capture the energy of ordinary sunlight, a much more abundant source. On Earth, too, solar cells are the usual tool.

    (40)   Radio Propagation

        Dear Mr.. Stern,

        I was involved with the Virginia Governor's School of Math, Science, and Technology this past summer, in a class on Space Physics. I am using my knowledge in this course to conduct a research project/ experimentation on Geomagnetic storms and their effect on short radio wave transmissions. I was planning on using an old radio (one my grandfather used to do his HAM radio sessions) and monitoring a singular radio station and observing the relative clarity or disturbances during each observation. I would then log onto the internet and check the levels of geomagnetic storms at www.spaceweather.com. I would then compare my observations with the geomagnetic activity and make a correlation between the two. (higher activity causes more disturbance)

        However, after reading your email, I see that solar disturbances affect radio wave transmissions more so than Geomagnetic storms. I was just wondering if you could guide me in refining my procedure so I can obtain better results. As it is now, I am unsure my project will yield valid results. Any ideas on your part would be greatly appreciated. Would it be more effective for me to change my project to measure the affect of solar disturbances (solar flares, coronal holes, CMEs) on short radio wave transmission, rather than geomagnetic storms? How can I compare and correlate the two variables if I do change?

        Thank you for bearing with me. I have still have much to learn in the realm of Solar Physics. Your help is awesome.


        This is YOUR project, so I won't do your homework for you--except refer you to a few web sites which may answer many of your questions:




        You will also find a lot about space physics on my other web sites, home page (and index page) http://www.phy6.org/stargaze/Sintro.htm

        Good luck

    (41)   Radiation Belts and Manned Space Flight

    Dear Sir:

    Would you please explain how the Van Allen Belt affected the first manned space flights. How were they protected?


        Dear Belinda

        All manned flights (except those of Apollo) have stayed below the radiation belt: the Space Shuttle, for instance, orbits at about 215 miles. The atmosphere is very rarefied there, and radiation belt particles descending to that level may well come back without encountering anything. However, such particles have thousands of Earthward excursions each day, so the only ones which are likely to survive long are those that are always confined to higher levels.

        A more subtle effect is also at work. The equations governing the motion of trapped particle indicate that each has a characteristic value of magnetic intensity, below which is cannot penetrate. Suppose a particle is reflected by the intensity existing at 215 miles. As it happens, the Earth's magnetic field--its region of magnetic forces--has some irregularities, so in some regions that intensity is only reached at 100 miles. Now and then the particle's orbit will happen to descend in that region, where it penetrates to much deeper (and denser) layers of the atmosphere, and may be quickly lost, even if elsewhere it stays at safe heights. One such notorious region exists above the southern Atlantic Ocean.

        So the radiation belt does not reach the levels where Mercury, Gemini, Soyuz and Mir used to orbit and where the Shuttle and Space Station do so now. The early Russian Sputniks failed to discover the radiation belt because they too stayed in such low orbits and Explorers 1 and 3 only detected it because they were rather poorly controlled and rose above 1500 miles.

        You will find more on my web sites, e.g. http://www.phy6.org/Education/wexp13.html

    (42)   Magnetic shields against fast-flying neutral matter

    I am writing a science fiction story in which a ship is traveling through space at a percentage of the speed of light (I was thinking about 1/3 c, but that may change depending on how the math works out). In this flight they would need a way of deflecting the particles of dust in "empty" space because hitting such items at relativistic speed would damage the ship.

        My question, then, is: will a magnetic field of sufficient strength repel non-charged particles or would they have to have to be electrons or ions with a charge to be affected? Could a magnetic field deflect say, a hydrogen atom or a helium atom that has the proper amount of electrons to be neutrally charged? And, while we're at it, would the field have to be stronger to deflect bigger particles, say something the size of a grain of sand or a pebble?

        thank you very much,


        I am afraid the answer is no--magnetic fields have no effect on uncharged dust or pebbles. Have you ever had an occasion to be examined on an MRI machine? (MRI is Magnetic Resonance Imaging--it used to be called Nuclear Magnetic Resonance, but the word "nuclear" frightened too many people, who did not realize the nuclei in question were stable well-behaved nuclei of hydrogen). You lie still on a pallet which rolls your body until the part being imaged is inside a big doughnut shaped magnet. That magnet is so strong that the attendant will collect from you anything that has iron in it--key chains etc.--lest it is snatched from you and flung towards the machine. Yet when you lie inside it (it makes an infernal racket, by the way) you cannot sense any special magnetic forces on your uncharged body.

        Even for shielding yourself from fast ions and electrons in space, it would be hard to create a magnetic field strong enough and extensive enough. For a spaceship moving at c/3, though, shielding is not that hard--all the stuff coming at it moves essentially in one direction, so one only has to provide some armor on the front side. A very, very thick armor, maybe, but only right in front, no need to cover space all around.

        Fly at c/3, you say. How are your brave astronauts going to achieve that? And how will they stop again once they reach their destination? As far as I know, the only person to address this without defying known laws of physics was Robert Forward" in "The Flight of the Dragonfly" (details there, in the appendix). His scheme is still far-far out, requiring incredible resources, but it is not impossible.

    (43)   When and where can I see "Northern Lights"?

    When and where am I most likely to see Northern Lights?


        What matters most is "where," or as they say in the real estate business--location, location, location. Your best bet is Fairbanks, Alaska; you may also see northern lights in Winnipeg, Canada, or even International Falls, Minnesota, but they could only be near the northern horizon.

        Scientists call the phenomenon the "polar aurora." Earlier it was named "aurora borealis" meaning "northern dawn" in Latin, since in Europe it was mostly seen as a glow near the northern horizon. However, it also occurs near the south pole, so "polar aurora" is now preferred. Aurora is caused by electrons energized in the Earth's magnetic environment, the magnetosphere, and guided earthward along magnetic field lines (lines of force) along which they slide, a bit like beads sliding along a wire. They move at about 1/5 the speed of light and in many ways they resemble electrons beamed at the screen inside the picture tube of a TV receiver. Where picture-tube electrons produce light when they hit the screen, auroral electrons do so when they hit the fringes of the atmosphere, about 100 kilometers (60 miles) up.

        The field lines on which this happens are the ones coming down along a circle centered near the magnetic pole ("auroral circle" or "auroral oval"), and that is where the accelerated electrons end up. The circle can expand and contract, but it usually passes near Fairbanks.

        When? Don't expect to see aurora in the Alaskan tourist season, during the summer, it just does not get dark enough. Just as near-polar winter nights are long and dark, summer days are long and bright, and even after the Sun sets, twilight persists. September has some darkness, October much more--March and April are also OK, and so is winter, if you do not mind the cold. When? The brightest auroras come from the night side of the magnetosphere, so you should see them around midnight, or later because of Alaska's time zone. If you stay in a hotel, ask the desk clerk to wake you when a good display occurs.

        What about the 11-year sunspot cycle? Occasionally, but especially in years of peak sunspot activity, the Sun sends out blobs of hot gas, which hit the magnetosphere and agitate it, causing "magnetic storms." At such times the "auroral circle" expands and reaches the lower 48 states of the US and mid-latitude Europe. The agitation also produces fine auroras, as happened on 5 November 2001, when the US was favorably located. However, since Fairbanks is located near the normal auroral circle, auroras are observed there throughout the 11-year cycle. The main factor is the slanting of interplanetary magnetic field lines (which come from the Sun)--southward slant, auroras likely, northward slant, not so much. That slant varies randomly, though satellites monitor it, and you can read their latest reports on the web.

        You find much more in the various chapters and links of "The Exploration of the Earth's Magnetosphere", http://www.phy6.org/Education/Intro.html. That collection includes sections on the aurora, magnetic field lines, the event of 5 November 2001 and one about "space weather," with links telling where to find predictions of the likelihood of aurora.

    (44)   Universal Time and Magnetic Local Time

    I am a computer science major in New Mexico. The summer before last I did research on the ionosphere and, yet, I have not found a good explanation/definition of magnetic local time (MLT) and/or universal time (UT). I used a computer language called Interactive Data Language to graph sets of data from magnetometers in the North Polar Region.. Would you explain these items and how they relate to the study of science research of the earth's magnetic fields and magnetic storms, etc.? Thank you very much!!!


        Kind of hard to work on a house when the foundation is shaky, no? Your teachers and your texts should really answer those questions, but let me try, anyway.

        How do you measure time? A good "clock" is the rotation of the Earth, and in early times this was checked by the position of the Sun. When the Sun was highest above the horizon--and exactly to the south--the time was noon, and the interval from one noon to the next was a DAY, or more precisely a solar day.

        Universal Time is essentially the time in Greenwich, England, and serves as reference time for astronomical events (e.g. the observed onset of the 1987 supernova). It's sort of a "world time" not tied to any local clock. There exists a small correction, but if the above is good enough for you, you may skip the next 2 paragraphs.

      [    As it happens, the Sun's position among the stars also varies, and that motion contributes to the length of the day (about 4 minutes--see web site). Unfortunately, that contribution varies slightly throughout the year, because the Earth's uneven motion around the Sun, etc., so the "noon-to-noon" day varies slightly in length. Astronomers use instead a MEAN SOLAR DAY averaged over the year, which is a good gauge of the Earth's rotation.

          But measuring time in mean solar days is not all. One must also decide when each day begins! Astronomers have agreed that each mean day begins with (mean) midnight at the Royal Astronomical Observatory in Greenwich, England, and time defined that way is known as UNIVERSAL TIME (UT). "Greenwich Mean Time" (GMT) is similar, but is counted from noon.]


        So, UNIVERSAL TIME essentially measures time, using the rotation of the Earth around its axis. LOCAL TIME (LT), on the other hand, measures not time but position relative to the Sun.

        Suppose you cover the Earth with a network of lines of latitude and longitude (see on the web http://www.phy6.org/stargaze/Slatlong.htm ) The at UT=0 it is midnight in Greenwich, 11 am in Russia, 6 pm in the Eastern US, and so forth; these are the LOCAL TIMES at these locations. Three hours later you add 3 hours to all these local times--and so forth, for other locations and other times. The local time at each location depends on the difference between its longitude ("meridian") and the longitude where it happens to be midnight. (You need not use time zones, but take the exact difference in longitude, for exact local time.) Your local time is 12 noon if your location faces the Sun, LT=24 (midnight) if it faces away from the Sun, and other values of LT for other relative positions of the Sun.

        One can similarly cover the Earth with lines of MAGNETIC LATITUDE and LONGITUDE, the latter converging not at the geographic poles but at the magnetic poles. As the globe rotates, we can say as before that all points on the same magnetic longitude have a certain MAGNETIC LOCAL TIME (MLT). We have MLT=12 (noon) on the magnetic meridian (magnetic line of longitude) facing the Sun, and MLT=0 (or 24) on the meridian facing the opposite direction, and so on,. It is completely similar to ordinary LT.

        Why is MLT important? Because the shape of the magnetic environment of the Earth--the magnetosphere--is determined by the solar wind, which "blows" almost exactly from the sun. So at MLT=12 a point faces in the"upwind" direction, and at MLT=24 in the "downwind" direction. Phenomena related to the action of the solar wind on the Earth's magnetic field, such as the polar aurora or daily magnetic variations, depend very much on MLT.

        Hope this makes it clear.


    (45)   Does the magnetosphere affect weather?

    I was wondering if the magnetosphere affected the Earth's weather in any way? And if so please explain in detail.



    I don't think so, we have no evidence for it. For at least two reasons, we do not expect any, either.

        First, our atmosphere is electrically neutral and does not conduct electricity (below 50 km, anyway--and weather stays below 10-20 km). Substances of this kind (except maybe permanent magnet, a very special case) do not react to magnetic fields. When a doctor puts your body inside the strong magnet of an MRI machine, you feel no difference, except maybe for the loud noise that machine makes.

        Second, weather gets its energy from heat deposited by sunlight on the surface of Earth. The energy of the magnetosphere, on the other hand, comes from the solar wind. A beam of the solar wind as wide as the magnetosphere carries only about 1/3500 as much energy as sunlight hitting Earth, and only about 1% of that is given to the magnetosphere. Of that, only a fraction reaches the upper atmosphere. It seems too little to make a difference, and it is not clear how it might move further down.

        Practically all that energy, by the way, is channeled to the auroral zone, near the poles. As was said--no clear correlation has been observed, there or elsewhere. You may not get a scientist to say "absolutely no," but "very unlikely" may be just as good.


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