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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. 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


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  1.    Are electric storms an "electromagnetic" phenomenon?

        Does electricity need an electromagnetic field to exist in? I have never heard of a report of an electrical storm in space, only on planets.


    Dear Jason

    A complete reply to your question probably requires more study on your part. Still, let me try.

        An electrical storm--one which produces lightning--essentially requires a separated electric charge: negative charge (extra electrons) in one part of the cloud, positive charge in another. This process (discussed in section 18d of "From Stargazers to Starships") involves static electric charges and no magnetism.

        Magnetism requires electric currents, that is, not just positive and negative charges, but those charges must MOVE in opposite directions. In fact, they need not be separated: in space and in most conductors of electric current, any volume contains equal amounts of positive and negative charge. It is just that positive charges move one way (or in a metal, stay fixed), negative ones move the other way. That gives current, which produce magnetic fields.

        "Electromagnetic" fields exist in such devices as electric transformer. There you find a coil which carries an alternating current, and since it is wrapped around an iron core, that creates an alternating magnetic field in the iron. By Faraday's law of induction, THAT produces an alternating ELECTRIC field in the space around the iron. If you then put a "secondary" coil in that space, it can pick up that field and generate a "secondary" current, and according to the design, it may have a higher or lower voltage, whatever is needed.

        In an electromagnetic WAVE, such links of alternating magnetic and electric fields, propagate through space. You really need math to describe this, but see also the section on electromagnetic waves and section #S-5 in "Stargazers."

        Thus electromagnetic fields can exist in space, but they are a completely different creature than electric storms

  2.    How can steady magnetic fields induce electric currents?


        your web site is really awesome, but it doesn't answer the one question that sent me looking for info on the web in the first place. Everything I have ever heard is that the only magnetic field that can induce a current is a fluctuating     The magnetic field of the earth doesn't fluctuate around, so how is that the charged particles started circling around the lines of force. And why would they spiral in towards the N/S magnetic poles? I thought that the current is supposed to run strictly at right angles to the magnetic field??

    Thanks for your help.


    Dear Diane

        As the poet wrote, "a little knowledge is a dangerous thing." There is so much involved in magnetism, in current and trapped particles, that a short message here won't do it justice! My advice is look up my web sites on the magnetosphere and the Earth's field, and take your time.

        (1) True--a fluctuating magnetic field will induce a voltage, which can drive current, while a static magnetic field just sits there. But there exists another situation important in space (and in the dynamo mechanism of sunspots and the Earth's core): a magnetic field in a fluid conducting medium which FLOWS--where part of it moves with respect to the other. See


    and sections that follow there. One such flow is the flow of the solar wind past the Earth, and apparently, this creates currents which flow into the polar ionosphere and out of it, giving us aurora as by-product. The cartoon depicting this in the first of the above files is almost certainly an oversimplification (we really don't completely know the circuit), but that apparently is the source, and the energy comes from the kinetic energy of the solar wind.

    (2) Now those auroral currents: they indeed flow ALONG magnetic field lines. You write "I thought that the current is supposed to run strictly at right angles to the magnetic field," but that is true only for the magnetic field generated by the same current itself. The field lines involved here are created by the earth's core, a much stronger source (50,000 nt, whereas the currents near Earth may generate 50 nT). The field of the currents themselves therefore only adds a little twist to those field lines. About the spiraling, etc. you will have to read elsewhere "The Exploration of the Earth's Magnetosphere." I hope you will find it interesting.

  3.    There are electromagnetic waves all around us!

        Dear Dr. Stern, I have read with great interest the following page:


    which is very informative and approachable for lay people like myself. Based on this information, I would like to ask you: do all these waves, which 'inhabit' the space around us, impact upon us humans? if so, how? has there been any research on that topic, and if so, can you direct me to some sites where I could read about this most interesting topic.

        I believe that migrating birds are one species which uses Electromagnetism to find their way around. Thus, it is not preposterous to think electromagnetism affects us, or that we use it without realising it.

        I would be very grateful if you would answer my query. I have read that You are interested in science education, and I've been trying to learn about science over the last few years. Keep up with the good work!

        Many thanks in advance.


    Dear Jocelyne

        You are right, space all around us is filled with electromagnetic waves from the entire radio spectrum--AM, FM, CB, TV, police, airplanes, satellites, radio-linked phones and computers, and more. They all overlap, and it seems almost amazing that out of all this jumble your radio, TV etc. can pick just the signal you need, and reject all the rest.

        As far as living tissue is concerned, however, all that matters is the energy carried by the waves. The electromagnetic field varies too rapidly to have direct electric or magnetic effects, and on the other hand, the photons associated with it are too weak to affect thechemistry.

        Still, the waves do deposit energy. In a microwave oven, that energy density is sufficient to cook food and boil water. Many years ago a similar principle (at lower frequency) was used in diathermy to gently heat sore parts of the human body (I don't think it's done any more). The energy density of radio waves in the space around us, though, is tiny by comparison, too small I think to have any effect.

        About 30 years ago, an interesting article appeared in "Radio Science" (I only recall part of the title, "A cabbage by any other name"). During the cold war, the Communist government of Poland maintained a powerful radio station, off limits and surrounded by a fence to keep people away from the intense radio waves. One spring day an engineer living nearby noticed a bird building a nest right on the feed horn from which the radio waves emerged, and out of curiosity, he followed it with binoculars, week after week. Eggs were laid, birds hatched, they grew and finally flew away, with no apparent bad effects. Thus it seems the effects of E-M radiation may be overrated.

        The navigation of birds, by the way, seems to rely not on electromagnetic waves but on the Earth's magnetic field.

        As for your interest in science, the file you cite is old and is just an early summary file in a collection on the magnetosphere. I encourage you to dig more deeply! For a start, look up

  4.    Best orbit for a Space Station

    Dear Dr. Stern,

        My 10th grade daughter, Erica, will be competing in a Texas Symposium this year with a project in space weather. She wants to find the "best" orbit for a space station (around earth) to maintain by balancing a few factors including "space weather". Your websites have been very helpful in getting good background info and understanding.

        However, she would like to use "real" data to help form and prove her thesis. She has found some data taken from geostationary orbits, but would like some for LEO and orbits at the Lagrange points. She knows of some satellites, like SOHO at L1, Map and I believe Wind is there now. Most of the data she has is from Wind and some from ACE. However, we can't seem to find any data for L2, L4, L5 (earth-moon). Do you know if any is available and where?

        Also, she'd like to know how much power would be needed to keep a station in these orbits, assuming some "standard" orbital perturbation. We've not been able to find that kind of data either, except a correction of 25 m/s mentioned for L1 (which we can convert to power with a few assumptions). With all the hardware in LEO and geosynchronous, one would think corrections for those would be easy to find, but we haven't succeeded yet.

    Thanks for your time,
    Erica's dad,


    Dear Erica

    Your project sounds imaginative, but when you start looking at details all sorts of questions pop up. I assume of course you are planning a space station with human crew, not an automatic satellite.

        Sending humans to space is dangerous and expensive, and for these reasons alone, the only suitable place is LEO. Out in synchronous orbit one is exposed to magnetic substorms and storms, which inject energetic particles into the environment, and both there and at more distant locations you are exposed to energetic protons from solar flares, which are even more penetrating. I have the trace of some of those, from storms around Halloween 2003, in


        In addition, raising a manned spaceship to such distances is difficult and expensive, as is resupplying it, and if a problem arises the crew cannot be quickly brought back.

        What exactly would the purpose of such a space station be? Certainly, not to observe "Space Weather. " Such observations require particle sensors, magnetometers, radio receivers and special telescopes, and these operate well without human presence--better, in fact, because once they are sent up, they can operate for years without maintenance. And the places where you best observe (say) energetic particles are generally too risky to place humans.

        About power to keep a station at Lagrangian points--you probably mean fuel, because power from solar cells is free. It depends for how long. "Wind" has been cruising near L1 and L2 for years, and I think it still has some fuel. Anyway, these are not places where you want to send human beings--supply is difficult, and a lot of shielding is required.

  5.    Is space debris electrically charged?

    Dear Sir,

    My name is Carles and I am writing you in representation of a group of students from Spain who are working in a project related with space debris.

        First of all let me congratulate you for the excellent job done with your website.

        We have a pair of questions that we think that you could answer:
            -- Is the space debris electrically charged?
            - Is it because of the Earth magnetic field or the solar wind?

    Thank you very much.


    Dear Carles

        Your question is about space debris, but the same can be asked about any object in space, even regular satellites: are they electrically charged or not?

        Most satellites in sunlight have a positive potential of a few volts, because the photons of sunlight knock electrons off their skin and give them an energy of a few electron volts. This process continues until the satellite is charged to an equivalent number of volts, and then no more electrons can leave.

        If the satellite is in the Earth's shadow, this obviously stops, but then, if the satellite (or debris) is in the outer radiation belt (for instance, in synchronous orbit), it can get charged to hundreds of volts (sometimes even over a thousand) by electrons trapped in the magnetic field of the Earth. Here is why.

        In general, the radiation belt contains an equal density N of positive ions and negative free electrons. Any departure from such "neutrality" creates strong electric forces which quickly counteract it. Trapped ions usually have a larger average energy than the electrons, but because electrons are nearly 2000 times lighter, they usually move with higher velocity--say V for electrons, v for ions. A square centimeter of the surface of a satellite or a piece of debris is hit each second by kNV electrons and kNv ions--k being a factor to take into account their distribution of angles. For both kinds of particles, N is the same and k nearly the same, so many more electrons hit, and the final voltage is something like the average electron energy in electron volts. This turns back part of the electrons and attracts ions, and equilibrium is reached when the surface receives the same rate of charging from either sign.

        This won't happen in sunlight (or if it does, it's a much smaller effect) because then enough photo-electrons are driven off to balance the arriving negative charge.

        In sunlight, especially in low Earth orbit, the repelled electrons hang around at a short distance, so that any effects of charging are confined to the immediate vicinity of the satellite. The distance depends on the density of the surrounding plasma--it's of the order of the "Debye length." When charged by radiation in the Earth shadow, a similar effect occurs, even though in synchronous orbit the Debye length can be (I think) many meters. I hope all this information is helpful

    Follow-up question:

    Your answer has reactivated our faith in the project we are working on and has encouraged us.

    Let me explain you our project: It proposes a magnetic shield for protecting satellites and/or astronauts from space debris.

        What do you think about the project?

    Thank you very much.


    I regret to say that magnetic fields will have no effect on the motion of space debris.

        Look at it this way: a satellite can orbit the Earth for years, moving all the time in the Earth's magnetic field. When calculating the orbit of such a satellite, one has to take into account, in addition to the Earth's attraction, those of the Sun, Moon and the Earth's equatorial bulge, and if the orbit is low, also of the resistance of the fringes of the atmosphere. But one never includes the magnetic field in such a calculation, because it makes no difference.

        Space debris can be a problem--see


    In low Earth orbit it is gradually brought down by air resistance, but higher orbits, especially near-circular ones, are very durable. Maybe some day a robot can be sent to collect all the dead communication satellites in synchronous orbit into one big pile, which then could be sent off to deep space, but GPS satellites will probably stay up as long as the Earth lasts. David

  6.    Magnetic induction by the Magnetosphere

    I am a mechanical engineering student in Istanbul Technical University

        I wonder if a rotating giant ring of wire in orbit (or a more rigid object such as a metal ring) surrounding all around the earth would produce electricity from the changing magnetic field of earth..


    A conducting ring on the ground could pick up induced voltages, but only at some times are the variations big enough, it's not worth the effort. Electric lines in Canada sometimes pick up voltages from variations associated with magnetic storms. The result is not extra energy but (in some events) damage to the network.
  7.    Questions about the solar corona:

                     (1) Why don't its particles separate by weight?
                     (2) What accelerates the solar wind?

        Your Web site is very useful! I'm 45 years old, with an interest in electricity in space, and still learning.

    1.     In the section on The Solar Wind, you mention that "The plasma of the corona is so hot that the Sun's gravity cannot hold it down. Instead, the upper fringes flow away in all directions, in a constant stream of particles known as the solar wind."

        Wouldn't the intense gravitation field of the Sun cause the heavier positively charged particles to fall back to the Sun, resulting in the Solar Wind containing more electrons than protons? Or would any proton falling back, attract an electron along with it?

    2.     I read on a number of Web sites that the Solar Wind accelerates away from the Sun. Is there an explanation? Eg. See




    Dear Ian

        Your first question is quite perceptive. A similar question about the Earth's atmosphere was addressed by Pannekoek in 1922 and by Rosseland in 1924 ("Electrical state of a star", Monthly Notices of Roy. Astron. Soc.84, 720-728, 1924). Here is the idea.

        In an atmosphere consisting of a gas of molecular weight M, the density falls off exponentially, at a rate which depends on M, on the temperature of the gas and on the force of gravity. In the Earth's lower atmosphere, the density falls to one half every 5 kilometers or so (or else, by a factor e = 2.71828.. about every 8 kilometers, a distance known as scale height H). High temperatures and low M increase H and spread out the atmosphere, while strong gravity decreases H and makes the atmosphere more compact. Of course, the Earth's atmosphere is really a mixture of gases, but in the first 100 kilometers, enough collisions occur to create a single scale height, some sort of average among the components.

        Above 100 km collisions quickly become rare, so different gases with different M tend to separate, as has been observed. Pannekoek and Rosseland wondered--what about free electrons in the outer layers of a star (or for Earth, in the ionosphere)? Their mass is so small, that their scale height should be much larger. Their layer should extend to great height, while the heavy oxygen ions (the main positive component) should stay well below them.

        They concluded this was not possible, because even a very slight separation of positive and negative charges would create an "ambipolar" electric field, pulling electrons down and O+ ions up. The field would effectively "add weight" to the electrons and would "buoy up" the O+ ions, until effectively each species senses the same downward force, contributed by both gravity and electric forces. With both species having the same "weight", they also will have the same scale height, and the ionosphere would stay electrically neutral, as is observed.

        The corona of the Sun behaves the same way, although the ambipolar field is probably much stronger, because of the high temperature.
        I am less certain about the second question--the solar wind is really not my field of expertise. In the Earth's lower atmosphere, temperature decreases with height, because heat from sunlight is mainly absorbed by the ground, at the bottom. It is then transported by air flows and by radiation, absorption and re-radiation (by "greenhouse gases") until it gets high enough to be radiated to space, not to return. That height defines the bottom of the stratosphere; other interesting effects occur higher up (e.g. absorption of UV by ozone), but we ignore them now. Because of the upwards flow of heat, the temperature is highest near the ground (where heat comes in) and decreases with height, up to the level where heat is given up.

        Solar physicists expected the same to hold for the Sun, also heated from below, and it is still a mystery, why so much heat is deposited in the corona, making it much hotter than the photosphere below it. Still, once you get to the corona, you would expect the temperature to gradually decrease with height, as one gets away from the source of heat and as the rising gas expands.

        Apparently this cooling is defeated by high heat conduction, because the gas is really an ionized plasma. Thus high layers are reheated from below, and an equilibrium solution, like that in the Earth's atmosphere, is not possible. The only solution Parker found in 1958 is a continued acceleration, until much of the heat is converted to kinetic energy. Mathematically the process has been compared to the acceleration of a rocket jet in the De-Laval nozzle (see section on Robert Goddard in "From Stargazers to Starships").

        I always thought the main acceleration took place in the first few solar radii--see Parker's papers. The abstract you cite claims that no, it's above 10 solar radii. I am no good judge of that, but also note that the abstract seems to refer to the solar wind above the Sun's poles, where field lines stick straight out and solar wind velocity is about double what it is near the ecliptic. Maybe the physics there is different.


  8.    Why has the aurora been so frequent lately?

    Dear Sir:

        I have experienced seeing the polar aurora in Maryland at least 3 times in 4 years. Is this unusual? If so, do you think we are experiencing more geomagnetic storms then in the past and if so, why do you think this is so?


    The polar aurora is seen in Maryland only during big magnetic storms, and these seem unpredictable, although there is evidence they are more likely during the decreasing part of the 11-year sunspot cycle, which typifies the last 4 years. See


    There was a big storm around 5 November 2001, just past the solar maximum, and I wrote about it in


    Another sequence of storms occurred around Halloween, 2003--see

    also mentioned in

    And there was another sequence of big storms, with long-lasting aftereffects, just a month ago (in November 2004)--see graph at


    I suspect these were the 3 events you observed, and the fact they all clustered around the same period of the year, between Halloween and November 10, seems to have been a coincidence. We are now around solar minimum, and observers have already noted days when no sunspots were seen at all:


    Still, big storms can happen... who knows what goes on below the surface of the Sun? About the aurora itself, I recommend you read "Secrets of the Polar Aurora" at


  9.    Was the magnetosphere involved in the hole in the ozone layer?

    Dear Dr Stern,

        Looking for an answer to my question, I landed on your web site to find out that more or less the same (!) question had popped up several times already. This concerns possibility of a correlation between ozone depletion and the earths magnetic field in combination with solar effects.

        Has the possible connection between the steady decrease of the earths magnetic field and the decrease of magnetic particles (such as ozone) at times of solar bursts been investigated?

        Could this be an alternative explanation for the chemical reduction of ozone levels as suggested by a fellow country man in 1995 (I'm not into politics)?

        The especially thin layer of ozone in the arctic during winters, on average, coincides with the tail of the magnetosphere rather than the lack of newly produced Ozone?

      --- The thin layer of ozone on the poles corresponds with the outgoing flux-lines?
      --- The "Brazilian gap" in the atmosphere coincides with a local magnetic anomaly?

        The chemical explanation is solid, but doesn't explain how gaps in the ozone layer recover at random/differing speeds. It doesn't seem to correspond to temperature. If it was just a chemical reaction, ozone would be transferred to the poles at a rate that corresponds with the intensity of the chemical reaction.

        I'd be grateful if you could shed some light on this puzzle.


    Ozone is not in my area, but I'll try to answer you as best as I can.

        Ozone is formed by ultraviolet sunlight at altitudes between 25 and 40 kilometers. To escape from Earth along a magnetic field line, an ozone molecule would have to meet several conditions:

    ---It would have to be ionized--otherwise it is not affected by the magnetic field.
    --- It has to have enough energy to escape gravity, and
    ---it must start off high enough so that collisions do not take that energy away.

        While the atmosphere is quite rarefied at 25-40 kilometers, it still is dense enough for frequent collisions between molecules; the last collisions occur around 100 km or slightly higher. I do not know if ozone molecules can survive an ionization event without breaking up, and I do not think they have enough energy to escape.

        Oxygen ions DO escape from the polar region along the magnetic field lines of the tail (which are connected to the poles all year round), as something called the "Polar Wind." These however are mainly O+ ions starting from the ionosphere, where temperatures (and energies) are higher and collisions are not frequent.

        The ozone layer is governed by a balance between processes which create ozone (from ordinary 2-atomic oxygen molecules) and the ones that remove it. In the winter polar cap, sunlight (which produces ozone) is absent, so the ozone layer is mostly governed by processes which remove ozone. For instance, if there exists enough wind to mix this layer with lower latitudes where ozone is produced even in winter, the layer will persist.

        In recent years, a "hole" was observed to form during the polar night, and that was taken as evidence that an additional removal mechanism, proposed (by Rowlands and Molina, I think) has become effective. That is catalytic breakup of ozone in the presence of chlorine, produced by the breakup of "Freon" refrigerants in the upper atmosphere, under the action of ultraviolet sunlight.

        I know of no "Brazilian gap" in the ozone layer. There does exist a "south Atlantic Anomaly" in the radiation belt, where it descends to lower levels, but that is not related.


  10.    Who Discovered Sunspots?


        I noticed the time line you have at titled "Some Dates in the Exploration of the Magnetosphere"

        Do you happen to know who first noticed Sunspots, and when that was?


    The "official" discovery of sunspots, using the newly-invented telescope, was made around 1610, independently, Galileo, the Dutchman Johann Fabricius (Latin version of Goldsmit) and the Jesuit Christopher Scheiner. These are mentioned on my web pages, e.g.
    The discovery is mentioned in all my web collections related to the Sun--see

        Earlier observations with the unaided eye have been reported--e.g. in China and Russia--because when the rising or setting Sun is viewed through a thick haze, big sunspots can be distinguished (a few years ago I even watched that way a big double sunspot). But astronomers did not pay much attention.

            The most thorough early study was made by Scheiner, who apparently introduced the safe observation method of projecting the Sun's image onto a white surface, and who published his findings in 1630 in the book "Rosa Ursina Sive Sol."

  11.    "Soda-Bottle Magnetometer"

    Dear Dr. Stern:

        I came across your web site this morning and wondered if you could answer a very basic question. My daughter (age 13) is doing a science experiment involving a homemade magnetometer. She has suspended a magnet inside a soda bottle, attached a mirror to it, and there is a laser beam that is focused on the mirror at approximately a 45 degree angle. This projects a point on a chart on the wall approximately 3 feet away.

        She/we have been randomly making recordings of the magnetometer since late November/early December. Readings are taken either 10, 15, or sometimes 30 minutes apart for a period of two-three hours at varying times of the day. Because we were aware of the activity on the sun 1/15/05, we decided to do readings 10 minutes apart yesterday for as long as we could. We began at 8:00 a.m. and, with the exception of missing here or there, continued until 12:30 a.m. It is reported that there also may be sometime of geomagnetic storm even today (

        Question: What would we see on the magnetometer that would demonstrate to us that some type of unusual magnetic activity appeared? The readings yesterday started off (on a centimeter scale of 1-27, with 15 being the center of the scale) usually are between 8-10, but have been known to be as high as 24-27. Yesterday, they hovered between 9-11, when they suddenly went up to 14-16 and stayed there for some time before eventually falling back to 12-13. Another phenomenon is that the laser beam will often be right on the line, then jump about 1-2 cm above the line or below the line. Is any of this significant?

        Thank you for your consideration. We can't find any place that answers these questions and I hope you can help. We have kept all other factors constant. We have noted if there has been any home disturbance that might affect the magnetometer and its readings (heat going in house, dryer running, etc


        What you are trying to observe is a very small effect, probably far beyond the capability of a home-made magnetometer in a soda bottle. The magnetic field at your location has an intensity of about 50,000 nano-Tesla (nT) whereas a magnetic storm may add to it about 50 nT. Even if the addition were at right angles to the prevailing magnetic force (actually, it is almost anti-parallel) it would only shift the light spot (at a distance of 1 meter) by 1 millimeter.

        That is very little, and sources of "instrumental noise" can produce equivalent results. If the suspension is an ordinary twisted yarn, the twist may absorb or lose moisture and change. Static electricity may play a role... hard to tell. I would not expect much from such crude experimentation, which must have been suggested to you by some other source.

        Some big magnetic storms occurred at the beginning of last November 2004 [letter was dated January 2005]--see
    We are currently still in their after-effects, a weakened field due (apparently) to a large reinforcement of the population of ions trapped around Earth from that time. See:
    which will tell you, at any time, about ongoing storms, and also

    As these graphs show, a few small magnetic storms occurred since then, but nothing special.

        If your daughter wants to learn more about magnetic storms and magnetism, I would recommend files of my own web material, all linked at

    The instrument she built is a crude version of one introduced by Coulomb--see section #5 in

    At one time professional magnetometers were built like that, but with many refinements, e.g. the magnet was suspended from a long quartz fiber. I believe that today's instruments are all electronic, e.g. see

    Further Correspondence:

    Thank you so much for taking the time to reply as well as for your answer. I haven't shared the links with her yet, but they will be very helpful. Your explanation on the magnetometer is very helpful as coming to a conclusion on this experiment will be difficult. It is one that we found as part of a science experiment for middle school students. I think, though, it helps to understand magnets in general as well as the concept of the earth/sun acting as magnets and affecting each other. Interestingly, the magnetometer was affected by the earthquake in Indonesia on 12/16.

        Oh, one other thing--the magnet is suspended by a cotton thread through a straw which is glued to the top of the magnet. We've kept the humidity and temperature constant in the room, but the idea of moisture being absorbed by the thread is an interesting one. I have still yet to figure out what causes the beam to vary 1-2 cm in vertical orientation and maybe that is an explanation. Thanks for the suggestion.

    Further response:

    To help your daughter understand magnets in general, please see
    Please bring that site to the attention of her teachers, too. There is much more in the collection of which this is part, also in the one of
  12.    Magnetism and Weather

    I live in Cyprus, am 16 years old and write to you to ask a question.

        I'm doing a project at school, which involves the weather, and I would like to know some basic info to help me with my work. Many of my friends are going to focus around storms, twisters etc. but I would like to find out which factors affect the weather.

        More specifically I would like to know if geomagnetic waves or electromagnetic waves affect the weather.

    If not, please I would like to know some of the factors that do.


    The answer is probably "no."

        Weather systems are driven by the Sun's heat, carried to Earth by sunlight. It is true, sunlight DOES consist of electromagnetic waves, but I don't think you were referring to light waves. Sunlight deposits heat, and both weather and climate are the result of processes by which Earth returns that heat to space. (Of course, all this heat MUST be returned, otherwise Earth would get hotter and hotter!)

        The amount of energy in that heat is huge, while the energy provided to the surface of Earth by (for example) processes in the magnetic field of Earth is small. Furthermore, magnetic variations usually affect materials which conducts electricity well, while the atmosphere in which we live is an excellent insulator.

        I wrote about weather and climate in
    and in the two sections which follow it. (Section S-5 in that group explains what makes light "electromagnetic"). You might like to look there. If perhaps you want to write about lightning, see

    I hope all these help you

  13.    Is the Polar Cusp visible to the Eye?

    Hi Doctor Stern,

    I am a retired history teacher here in Toronto, Ontario, Canada.

        I am investigating the possibility that ancient civilizations might have been able to see the polar pillar cusp with the naked eye.

        I realize that it extends up much higher than the auroral oval but am wondering if there are any circumstances that you can identify and possibly describe whereby the polar pillar cusp might have been visible to the ancients globally?

        I appreciate the time you are making to help people who don't have a background in this field understand it.


    I assume that by "polar pillar cusp" (a term I have never heard in all my career) you mean the trumpet-shaped cone of magnetic field lines (a kind of "flux tube") emanating from Earth near the noon-midnight plane and separating field lines bent sunward from those bent tailwards. See for instance

        If so, the answer is no, on several counts.

        The cusp "trumpet" is usually filled with solar wind plasma, too rarefied to be seen, about 6 protons per cubic cm. Furthermore, they could not be seen unless they hit any gas, and of course, above an altitude of (say) 500 km, the atmosphere is much too rarefied to form much of an obstacle (and if it did, the solar wind would probably be stopped by it).

        The solar wind in the cusp does hit the upper atmosphere and produces there a sort of dull red aurora, better seen by instruments than by the eye. See for instance

        Dayside cleft aurora and its ionospheric effects
    by Gordon G. Shepherd, Rev. Geophys., 17, 2017-2033, 1979.

        I doubt ancient civilizations would have noticed it, even without the visual handicap, because they could see the cusp only in mid-winter (in the summer the part of the sky where the cusp comes down is lit too brightly by sunlight). At that time the angle between the Earth magnetic axis and the Sun's direction is large, which makes the cusp migrate to higher latitudes where even fewer people can observe it. As far as I know, cusp observations from the ground have been done mainly from Svalbard (Spitzbergen).

        Besides, of course, people far north see so much aurora that this special dim kind won't register as anything unusual.

  14.    Effects of Radiation beyond the Van Allen Belts r

    I have over 10 years background in nuclear physics, especially dealing with the effects of radiation on the body. You never quite answer the questions about how were the Apollo astronauts protected from radiation beyond the Van Allen Belts. These are my issues:
    •     The Federal limit for exposure to US workers is 5 REM/year (10CFR835). Did NASA's Apollo astronauts get special permission to exceed this limit for the Apollo missions? 25 Rad = 25 REM. Their travel through the Van Allen Belts and the amount of time they spent outside the Belts would have given them doses far beyond this limit.

    •     NASA has never directly addressed how the Apollo astronauts were shielded. Lead is the only effective shielding (which was not used during these missions). The radiation levels outside the Van Allen Belt far exceed the 200-300 RADs inside the belts. Again how were they shielded? Regardless, their doses would at least have made them sick from radiation exposure to skin and organs even if it didn't kill them. None of them suffered any ill effects.

    •     Apollo astronauts were not protected from solar flares which were at their worst during this period. There is no way that the dose reports from the missions are accurate.

    Thank you for your response!


    Here are quick answers to your doubts:
    1.     I suspect that the Apollo astronauts did exceed EPA permissible doses of radiation--not by enough to pose danger to life, but above the level accepted for the population in general. Knowing NASA's concern with bureaucratic protocol, I would not be even surprised if astronauts were made to sign a waiver.

          So what? If I were an astronaut candidate, I would sign such a waiver in a heartbeat. If I am about to be launched atop a giant rocket into the vacuum of space, perhaps to the Moon, certainly facing a fiery reentry before returning home--then the risk from a moderate dose of radiation ranks far below other dangers!

    2.    The actual dosage may be 25 Rad in the inner belt, (though probably less, see below). The radiation rates in the outer belt are smaller, not larger. Lead, by the way, is not an especially good shield: it is wonderfully efficient in excluding x-rays of, say, 100 keV, but radiation in space consists of particles, fast electrons and ions. The inner belt is mostly protons of about 50 MeV and is not very penetrating--the spacecraft heat shield, tanks etc. may shield the occupants somewhat, the interior of their bodies gets less radiation, too and lying close together during the belt passage they also shield each other somewhat.

          Electrons of MeV energies in the outer belt are more penetrating, but there are not enough of them. Remember--communications and weather satellites operate in synchronous orbit, in the heart of the outer belt, year after year!

    3.     Solar flares can emit ions of 0.5 to 5 GeV, and these are bad, if intense enough. Such events however are rare, and when they occur, the dosage is usually not lethal, though the margin is smaller. For the Apollo astronauts, this was another calculated risk, and nothing happened.

          Now if you were to fly to Mars--a trip of 8.5 months each way, plus a forced wait on Mars, whose atmosphere is too thin to protect--solar eruptions could be a problem requiring attention (though not insoluble). But we are not at that stage yet.

  15.    Deflection of a beam of Electrons in the Earth's Field

        I am a sophomore in college. I am doing a physics lab about understanding the motion of a charged particle in the presence of an electric and magnetic field to estimate the mass of the electron. My question is would it be possible to use the earth's magnetic field to deflect the beam and how large a tube would you need?


    The Earth's field does bend the trajectories of electrons and ions, but not by much. Rumor has it that a long-ago plan to shoot down incoming missiles using energetic electron beams got shelved because of the uncertainty in the beam's aim, due to its deflection by the Earth's field. (There existed other problems, too.)

        In the lab, though, with short distances, you want to use a stronger magnetic field. Besides, if you use an artificial magnetic field to deflect electrons, you can switch it off and find its effect by comparing deflections with and without. You can't well switch off the Earth's field for calibration.

        You realize, of course, that the deflection depends on the ratio e/m, between the charge and the mass of the electron. You won't have the mass of the electron unless you can measure "e" independently. That is what Millikan's oil-drop experiment is for--right?

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