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FAQ: "Exploration of the <b style="color:black;background-color:#ffff66">Earth's</b> Magnetosphere"

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62. Why no Aurora at the magnetic poles?

Dr. Stern,

My questions, which I have not been able to find an answer to, are the following:

FIRST: Since the magnetic field has continuous coverage -- there are magnetic field lines coming up out of the poles, themselves, and so forth as you go down the latitude -- why is the aurora in the shape of sheets that make a rough ring around the magnetic pole, rather than being a sort of diffuse fog of color centered on the magnetic pole?

SECOND: suppose that a planet's rotational axis is parallel to the ecliptic plane and one pole is pointing to the sun, the other pole away from the sun, and that the planet has a magnetic field roughly polar along with the rotational pole as our planet is, with the result that one magnetic pole is facing towards the sun, rather than perpendicular to the sun as is the case with earth. Would this fact cause the planet NOT to have an aurora at the sunward pole, perhaps because the solar wind hitting it would so flatten the magnetic field as to disrupt the aurora process? What about the ever-dark pole away from the sun: would it have an aurora? Thanks for your help!

Reply

Both your questions have been asked by scientists in the past, and both are quite interesting. I will try to answer them without going into too much physics, but you may have to look through "The Exploration of the Earth's magnetosphere" (see

Intro.html

The theory of such motions shows that (for energies low enough, such as those of the solar wind or of particles trapped in the magnetosphere) particles tend to spiral around field lines, and be guided by them like beads on a wire.

The light of the aurora is emitted when such particles--typically, electrons of 5-10,000 electron volts (wenpart1.html) hit the atmosphere, about 100 kilometers up. The real question is--where do those electrons come from, and what gives them their energy?

It stands to reason that if we follow auroral field lines outwards from the auroral zone, they should lead us to the source region. In the absence of any external magnetic sources (i.e., the Earth's magnetic field is a "dipole", like that of a short bar magnet), the closer one starts from the magnetic pole, the further do those lines lead us, and the field line from the pole itself would lead to infinity.

That was the reasoning of Kristian Birkeland, around 1895, based on his terrella experiment (whaur1.html). Birkeland believed auroral electrons came from the Sun (which is very distant), so the aurora should be centered around the magnetic pole, the way you argued in your message, and the way his terrella experiments showed. He had an associate, Carl Stoermer, calculate those motions theoretically, reaching the same conclusion. In Stoermer's book "The Polar Aurora" (1956; he died a few years later) that was a major unsolved mystery.

We now know two possible reasons. First, external magnetic field lines are deformed, so using a simple dipole model does not tell where they go, especially field lines near the pole. And second, auroral electrons (at least some) originate inside the magnetosphere, at distances of about 8-10 Earth radii, although they get most of their energy near Earth. If you do take a dipole and trace on Earth the footpoints of field lines from those distances, you get about the right auroral zone. Sections 25-28 of "Exploration of the Earth's Magnetosphere" have more details.

Your second question, about a "pole-on magnetic field," was topical around early 1986, when Voyager 2 approached the planet Uranus, because the rotation axis of Uranus at that time pointed just a few degrees off the direction of the Sun (see wotherms.html). The prediction was a red diffuse aurora near the sunward pole, of the kind seen near the Earth's cusps, (wmpause.html) where a weak magnetic field lets the solar wind enter (no one was sure about what might happen on the night side). However, Uranus confounded predictions when it turned out that its magnetic axis

63. When and how were positive ions discovered?

I have a question regarding how ions were discovered (around 1850s?). I searched through some undergraduate physics books, it is clearly stated that the discovery of electron is attributed to J. J. Thomson in 1897. But it is vague for ions. It was said that evidences accumulated around 1850s that ions exist, etc.

Would you please make some comments about ions? Thanks very much.

Reply

The concept of ions was introduced by Svante Arrhenius in 1884, meaning ions in a chemical solution. I briefly referred to that in http://www.phy6.org/Education/whposion.html.

I am not sure whether he also gave the name--I was told it meant "wanderers" and the Ionic tribes of early Greece were so named since they wandered into the country. (I was also told that "planets" meant "wanderers" as well; must ask someone Greek what the difference is.) Of course electrolysis was well known in 1850, but before Arrhenius, no one had a good explanation.

When Thomson discovered the electron in 1897, positive rays were already known, discovered by Goldstein in 1886 and sometimes called "kanalstrahlen" ("channel rays") because they could be extracted from a very narrow hole--channel--in the cathode (the hole couldn't be big because gas kept leaking from it, and even a narrow hole required constant pumping). J.J. Thomson studied those rays in 1907-12, measured their charge-to-mass ratio or "e/m" (students still do so in the lab) and laid the foundation to mass spectroscopy. See http://www.phy.cam.ac.uk/camphy/positiverays/positiverays1_1.htm

64. Did astronauts use articifial magnetic shields"?

Could you please tell me if you have ever heard of a supposed phenomenon called "Magnetic Deficiency Syndrome" that affected early Russian and American Astronauts?

Did NASA install "Artificial magnetic shields" to overcome the supposed condition ( Which caused bone density loss)?

The answers to these questions trouble me. I thought it was zero gravity that caused bone density loss.

Reply

Yes, I have heard about it, but only from letters like yours. Whichever way the story got started, it is false: I know of no magnetic problems in orbit, and of no artificial magnetic shields. The

magnetic field sensed in Earth orbit is very weak, and I know of no magnetic effects. Have you ever undergone an MRI scan--a medical scan using a magnetic resonance imaging (MRI) machine? You lie on a narrow pallet and are wheeled into the middle of a large magnet, with very intense field. It's a noisy procedure, but you can sense no effect due to magnetism, because to the best of my knowledge, there is nothing in the human body affected by magnetism.

Loss of bone density is still ascribed to weightlessness, and is counteracted by exercise machines. See also my web page http://www.phy6.org/stargaze/Sskylab.htm

Further comment

I wish to thank you for your reply.

There is a man that advertises on Australian T.V. most weekday mornings selling these magnetic blankets and all sorts of other rubbish. Your reply came in good time as I was about to give up. My intention is to write a dissection of his magazine called. "Biomagnetic Therapy" He claims to cure everything from Headaches to Osteoarthritis and Cancer! Bear in mind that I am not a journalist, Just someone who reads Australian Skeptic,

Reply

Personally, I find that trying to discredit health claims of magnetism does no good. People unfamiliar with physics do not realize that magnetism is well understood, and regard it as a mysterious force which can have unexpected results. Here in the US a brisk trade exists in magnetic bracelets and similar devices, and in Britain, many years ago, "Posigrade" magnets were sold, to be attached to fuel lines of cars, supposedly to increase fuel economy. All these are articles of faith, not science: write an article, if you want, but don't expect it to accomplish anything.

This brings to mind a story about the Nobel-winning Danish physicist Niels Bohr. Bohr had a cottage in the woods, and one day a visitor there noticed it had a horseshoe nailed above the door, a good-luck charm widely used in Europe.

"Professor Bohr, you are such a great scientist--do you believe a horseshoe brings good luck?" he asked.

Bohr replied:

"To tell you the truth--I don't believe in it, but I am told it works the same whether you believe or not."

65. Harvesting electrons from power lines?

My name is Steven and I'm taking a physics/electricity class in school. I'm learning all about electricity! I'm so exited! Anyways, I got to thinking about the huge power lines that are directly behind my house. If the wires only "guide" electrons along a path then could it be possible to harvest the electrons that bleed from the lines to power a TV? I thought of this idea and haven't been able to find any good answers. I want to surprise my teacher for the science fair coming up with something really cool. Have you ever heard of such an idea?

Reply

If you could do it, it would probably be illegal. It is not unheard of that people tap power lines (low voltage, not the biggies) by a secret copper wire--especially in the city, in poorly policed neighborhoods. It's against the law to take power without paying for it.

But you are safe. The air surrounding those lines is a very good insulator, and you cannot bleed electrons through it. Anyway, those lines run at very high voltage--that's to electricity like pressure to air and water. Any electricity tapped directly from them has probably enough voltage to burn out whatever you connect to it (yourself included, if you happen to put yourself in the circuit), just as connecting your bike tire to a source of 300 psi (pounds per square inch) is likely to blow it to bits.

But keep studying. Electricity is an interesting subject.

Science fair, huh? Can you wire a board with 4 switches and one lightbulb, plus battery (wiring hidden below the board), so that when the light is off, any switch turns it on, and if on, any switch turns it off? You will need two 4-way switches and two 3-way switches--get them in the hardware store. It's a simple trick but it can mystify onlookers.

66. How can the intensely hot Sun be magnetic?

I know that a magnet loses its properties at a particular "Curie temperature," but we know that there is a very strong magnetic field around the sun, where the temperature is way beyond the Curie Temperature. So what is the reason behind suns magnetism ?

Reply

Your question shows an understanding of physics... but also, that you have not read my web pages, where this is discussed in detail.

The magnetism of the Sun indeed used to be a great puzzle: not only was the Sun extremely hot, much hotter than any Curie temperature, but it was also a gas. Yet sunspots were intensely magnetic.

Sir Joseph Larmor therefore proposed in 1919 that perhaps sunspot magnetism came from electric currents, driven by a "dynamo process" by flows on the Sun (which is hot enough to conduct electricity). Later the same idea was extended to the liquid core of the Earth, which is also hotter than any Curie temperature, but again, probably contains molten iron which conducts electricity.

For more, see the sections on the dynamo process in "The Great Magnet, the Earth", home page

http://www.phy6.org/earthmag/demagint.htm

The main collection there has elementary coverage of the dynamo process, but some greater details are in "A Millennium of Geomagnetism," also linked from the home page. As you will read there, the process for the Earth has now been successfully simulated on a computer, including polarity reversals. The polarity of the Sun's magnetism also reverses with the 11-year cycle.

67. What are "geomagnetic conjugate points"?

The below page was suggested to me and I found it delightful because it is more in laymen terms.

http://www.phy6.org/Education/aurora.htm

I have searched the web for an explanation of what a "geomagnetic conjugate" is but haven't yet found anything particularly understandable. Possibly I have not tried hard enough. Do you have an explanation of this or a link to a site that I might understand? I am interested in monitoring "Natural Radio". Thank you.

Reply

Two points on Earth are "geomagnetically conjugate" if they are on opposite ends of the same field line.

As you will find by going through "Exploration of the Earth's Magnetosphere" (see site map on the file you have cited), charged particles in the magnetosphere tend to be trapped on the same field line, and therefore conjugate points may be affected by the same population, e.g. by electrons released by a high altitude nuclear bomb. A bomb at one end of a line creates aurora at the conjugate point, as noted on the web page.

Certain types of waves also tend to be guided by field lines; these are essentially radio waves, modified by the presence of plasma in a magnetic field. Known as whistlers, such waves were originally detected as a whistling sound on long telephone lines--the lines acted as antennas, and since the frequency was in the audible range (2-4 Khz, typically), the telephone converted them to sound. Owen Storey in 1953 showed they were generated by lightning at the other conjugate point. The stroke of lightning creates a sharp pulse, but like any signal, this can be resolved as a sum of frequencies (Fourier spectrum). Whistlers differ from sound in that their different frequencies travel with different velocities, along their guiding field line, so they arrive spread out over a second or so, as a descending whistle. Whistlers also get reflected by the ionosphere, so you can get several bounces of the same whistle, getting fainter with each hop.

Whistlers were the first solid information about electrons in space around Earth above the ionospheric density maximum--what is now called the plasmasphere--and were an important research tool for probing that region. Other emissions also exist, but mainly on field lines near the auroral zone. Prof. Robert Helliwell of Stanford University conducted extensive studies of these, especially using observations at Siple station in Antarctica and Roberval near Quebec, which are conjugate. The emissions are generated by unstable populations of trapped electrons, and Prof. Helliwell (whose work was continued by Umar Inan) also triggered them artificially.

68. What is the smallest magnet possible?

I read with great interest you clear and concise description of magnets and magnetism at this site:

http://www-istp.gsfc.nasa.gov/Education/Imagnet.html

and was hoping you could answer a question for me.

What is the smallest magnet that is possible ?

Reply

Maybe the electron.

Something like that was discovered in 1925 by Uhlenbeck and Goudsmit, the so-called electorn spin. They deduced it while trying to explain the spectral lines of atoms--see

http://www.phy6.org/stargaze/Sun4spec.htm

These are phenomena of individual atoms, and at that scale, physics laws change to different laws, so-called quantum physics. What on our scale is smooth and continuous (e.g. the range of orbits allowed for a satellite) becomes choppy and discrete (e.g. range of orbits allowed to an electron in an atom). By those laws, even the pictures of an electron as a rotating charged sphere, or of an electron orbiting inside an atom, are not really right. But they help our imagination get the main points right--such that electrons are magnetized.

Protons and even neutrons are also magnetized, and much more weakly. This is used in MRI magnetic imaging--see "optional excursion" for the teacher. http://www.phy6.org/stargaze/Lprecess.htm

69. Can plasma physics explain ball lightning?

Thanks for writing and maintaining such a wonderful resource! I wish more people did this for their own field of interest.

One question, though, but in a few parts:

Why can't a moving plasma stream generate its own magnetic field strong enough to contain itself?

One of the problems facing fusion in Tokamak-style plasma containment devices is the HUGE magnetic fields needed, then the instability of the plasma itself due to its own magnetic fields.

Yet, as Larmor reported, a plasma is associated with a magnetic field (I like the 'bootstrap' analogy of yours!).

A possible example from nature may be the (slightly dubious) theories I have heard about ball-lightning being 'circular lightning' caused by 'eddy currents' from a lightning strike. Some have observed ball lightning with enough energy to melt glass that it had drifted through, and boil water, and even explode with enough force to flatten a house! They seem to have enough magnetic force to rip metal objects from their fixtures, yet are stable enough to remain intact for minutes. (I confess, I even saw one one hot summer's night when 15 years old or so!)

The idea is that rather being a ball as such, it's more a donut shaped helix.

I don't know, but wouldn't the magnetic field always be at right-angles to the moving ions, and thus they would stabilize each other, as long as the ions were always moving (and thus a current was moving)?

Just wanted to ask someone that actually might know, so I can get back to writing my th