Site Map

FAQ: "Exploration of the Earth's Magnetosphere"

Get a Straight Answer

Questions and answer--listed in the order received

Please note!

This is just a selection--

either

or else

Click here for a listing arranged by topic.

Items covered:

If you have a relevant question of your own, you can send it to
education("at" symbol)phy6.org
Before you do, though, please read the instructions

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 was inclined by 59° to its axis of rotation. As a result, the Uranus magnetosphere varies rather rapidly. It does hold some trapped radiation, and there is evidence of a weak aurora, located closer to the equator than Earth's.

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 thesis, like I should.

Reply

A magnetic field of the kind you discuss is theoretically possible, but I think it needs an outside pressure to hold it together.

I once calculated such a field, which involves the same mathematics as Hill's spherical vortex in fluid dynamics. I know this is mathematically far beyond your level, but let me just say that the result is given (including a term B1 which may be omitted) in problem 13, p. 17193, Journal of Geophysical Research, 1994. After deriving it I found that Harold Grad did so first, around 1958. It has field lines somewhat like the Earth's, confined in a sphere.

However, I think such a plasma needs an external pressure to keep it from dispersing, so it might not be a good model for ball lightning. In fact, I vaguely recall a general theorem in mechanics, called the virial theorem, which states, roughly, that an independent mechanical system will not hold together unless the sum-total of attractive forces exceeds that of disruptive forces. "Sum-total" here is taken over the space occupied by the system (e.g. interstellar cloud, planet etc.), with different regions weighed in some specific way, and the primary attractive force is usually gravity, while pressure and magnetic forces are disruptive.

The Earth's magnetosphere can be stable, since the main magnetic source is in the Earth's core, where gravity holds it together. Plasma in containment machines is kept in place by auxiliary coils, embedded in the machine (though I am not familiar with details). However a system which has only plasma (producing pressure) and magnetic fields will not stay together.

Ball lightning remains a mystery, its appearances are too brief and unpredictable for any meaningful study. I have not heard about it producing strong magnetic fields, although ordinary lightning certainly does. If ball lightning occurs immediately afterwards, magnetic effects may perhaps come from the primary stroke.

70. Harnessing the energy of the Aurora?

Hello!

We had read about the magnetic storms and we had an idea, and hoped that you'd be able to shed some light. We thought wouldn't it be terrific if we (humans) could harness the energy from the aurora to provide an unlimited source of energy, eliminating our reliance on foreign oil, etc.? Crazy idea?

Our question is, what prevents us from doing this? Is it too powerful to harness? Do we lack the technology to harness it and if we had the technology, what would it require? Why or why not could this be a possibility? Thank you for any comments!

Reply

Nice thought, but it won't work.

Actually what you want to tap into is not the aurora but the large electric currents which accelerate the aurora and provide its energy--so called Birkeland currents which flow into the ionosphere from space and out again. They can carry several millions of amperes at something like 50,000 volts, certainly a lot of power. See 25. Electric Currents from Space.

They are variable and erratic, but that is not their worst problem. First, the conducting layer of the ionosphere is separated from us by about 120 kilometers of air (70 miles). Over that distance the density of this air varies by a factor of a few million, but most of it is like air near the ground, a good insulator. Power companies string their cables through air with only negligible power leaking away. And anyway, no extension cord can reach so far. So we are electrically separated from the source.

Secondly, it is a very spread-out and diffuse current. To draw any serious amount of energy, one would have to connect to a huge area, many, many square miles. We are only beginning to learn to extract power from sunlight, using solar cells, and their main problem is that sunlight is spread out so thinly. Auroral currents, and the aurora itself, are spread out much more thinly than that.

About the aurora... see "Secrets of the Polar Aurora".

71. Radiation Belt and Brazil

Dear Dr. Stern

(from Brazil)

There are many years I look forward info and details not only about the Van.Allen belt but about the probable effects on race characteristics of persons born and living in the areas where such belt lacks, as part of Brazil. Have you ever had interests connected to those effects? If not I apologize myself for daring contacting you

Reply

The Van Allen belt does not seem to have any effect on people on Earth. It does have a small magnetic field, but that is so small as to be lost among other fluctuations (and magnetic fields have no confirmed biological influence, anyway).

By the way, Brazil is probably CLOSER to the belt than most other countries, closer than the USA. It is near the magnetic equator, and the magnetic intensity around that equator is not constant but varies, because of irregularities in the structure of the field. Near Brazil that magnetic field is at its weakest. The Van Allen belt is centered above the magnetic equator, but it reaches lower down is where the field is weaker.

Now the ozone layer, which protects us by absorbing ultra-violet sunlight... that is different. It is thinner near the equator, meaning sunlight there contains more ultra-violet. People living in that part of the world have developed dark skin, as protection against such light.

72. Risks from stormy "Space Weather"

Hi Dr. Stern:

I hope you can help me. I was just checking out your webpage and a question regarding the earth's Van Allen belts and solar flares/solar winds. I read that the earth's magnetic field has actually weakened by about 7% and field's actual total energy measured is less by 14% (since 1829). What is the impact of this weakening on the Van Allen Belts and the earth's Magnetosphere?

If solar flare activity increases (e.g. second-biggest geomagnetic storm ever measured hit the earth about a week ago) and the earth's magnetic field weakens, what impacts would we observe inside the atmosphere? Higher radiation exposure for folks on planes? Greater disruptions with electrical grids and radio transmissions? What's projected in the long term?

Can you recommend any websites that "a non-scientist lay person" might be able to read up on this. I guess the late August solar flare activity had nothing to do with the New York blackout (it occurred 2 weeks earlier in August).

Reply

When discussing risks and dangers from radiation in space, you should really distinguish two kinds of radiation:

(1) Trapped radiation is governed by the geomagnetic field. If you are below the belt (as in the international space station) or elsewhere outside its intense part, you should have nothing to worry about. It could well be that the belt is now weaker than in the time of Gauss, 160 years ago, but that does not really change the preceding statement. These ions have about 50 MeV.

(2) Solar flares release unpredictable blasts of particles of higher energy, often 500 MeV and up to 10 GeV. In this case, people on the ground are still safe, because the atmosphere has enough thickness to stop the particles, equivalent to something like 4 meters of concrete. See the end of
http://www.phy6.org/Education/wsolpart.html
If you are in a spacecraft on your way to Mars, that can be dangerous. In Ben Bova's book "Mars" this does happen, and astronauts have to hide in a protected area--behind fuel tanks, probably.

On Earth, we have an additional shield, the Earth's magnetism, which will deflect all but the highest energies from regions at equatorial and middle latitudes. Jetliners crossing the polar region may perhaps find it useful to fly a little deeper in the atmosphere when the sun emits high-energy particles, and I heard the Concorde carried a radiation alarm.

The magnetic field would have completely protected the space station in its originally planned orbit, inclined 29 degrees to the equator (latitude of Cape Canaveral). As it happened, this was later increased to about twice as much, to enable Russian launch sites to resupply the station (which turned out quite important after the "Columbia" disaster). Twice each orbit, therefore, the station has relatively weak magnetic protection, near its closest approach to the magnetic poles. I heard a rumor that during the 3 big flare events at the end of October 2003 the astronauts did in fact hide, but that is strictly hearsay which I cannot confirm. The even bigger flare on November 4 did not produce such a radiation surge.

I am not sure about disruption of power grids, but I think it arises when the auroral electrojets shift to lower latitudes during storms. There are two large electric currents flowing along the auroral zone towards midnight, associated with the polar aurora (or more precisely, with the electric currents which produce big aurora; see in "Exploration of the Earth's Magnetosphere.") Like any electric currents, they produce a magnetic field which can be observed on the ground, and which changes fairly irregularly.

When they move equatorwards, into more inhabited regions (and out of them again), the changing magnetic field induces electric currents in the high voltage networks there. The induction is slow, so the transformers of the grid, configured to impede currents of 60 or 50 cycles/second, see essentially a DC current, to which they offer it no significant impedance, allowing it to grow big. Such a current can burn out transformers, unless appropriate circuit breakers are tripped in time. I don't know how serious that is: burn-outs happened in 1989, but as far as I know, not recently.

I am not an expert in disruption of radio. Flares emit X-rays, which modify the ionosphere, adding ionization deeper down. It then can absorb certain frequencies, but I am not sure whether, say, cell phones are affected, or ships and airplanes. I think the frequencies used by communication satellites are high enough to be immune, and of course a lot of land traffic these days uses optical cable. I am not sure about GPS.

That's about all I know on this subject. Let me know if you find anything more, or anything contrary to what I know.

73. Man-made triggering of radio emissions

I have read that Dr Robert Helliwell and John Katsufrakis of Stanford's Radio Science Laboratory demonstrated that VLF radio waves can cause oscillations in the magnetosphere. Specifically, they used a 20 km antenna and a 5 kHz transmitter in the Antarctic and found that the magnetosphere could be modulated to cause high energy particles to cascade into our atmosphere. By turning the signal on or off they could start or stop the energy flow.

Question 1: Is this true and if so, what is happening?

Question 2: What are the practical implications of this discovery aside from the obvious weather-control possibilities? Has any progress been made on any applications of this knowledge?

Thanks for your wonderful availability!

Reply

These experiments are well known. Helliwell had a big wire antenna on top of the snow at Siple station in Antarctica (I believe that station no longer exists), and detectors at the other end at Roberval, north of Quebec. He indeed stimulated emissions, in frequency ranges close to those of atmospheric whistlers.

But don't get your hopes up for weather control or great energy release. The reason he got such results was that the trapped electrons which produced those waves were already in unstable distributions and ready to let go. It's a bit like those mortar shells rangers in Colorado and elsewhere fire into steep snowbanks, to release avalanches before anyone walks into them. Those shells do not create avalanches, they only trigger them off.

74. Does our magnetic field stop the atmosphere from getting blown away?

Thank you for such an in depth and informative Web Site. I have a question about the force of the solar wind against the magnetosphere. If the Earth's magnetosphere turned off (magnetic fields no longer existed on Earth), how long could life on the Earth survive? Eventually the solar wind would blow the atmosphere away.

Reply

I don't have an exact answer, and it all depends on what you mean by "eventually." If the process happens it is probably too slow to make much difference.

Venus has no magnetic field, its gravity is less than ours and being closer to the Sun, it experiences a much stronger solar wind, yet it retains a dense atmosphere. True, because its atmosphere is so dense, it can suffer appreciable loss without much change.

I don't know what the loss rate is, but it could be calculated from observations downwind from Venus, and maybe someone has done so. I have found a long article (below) but it does not seem to give an answer
http://www-ssc.igpp.ucla.edu/personnel/russell/papers/interact_solwind/
Another web page suggests little change in the atmosphere:
http://pauldunn.dynip.com/solarsystem/Venus_B.html

75. Radius of particle gyration

Hi Dr. Stern,

You have a very useful web site!

I'm trying to estimate parameters of a 50 keV electron's helical motion in the aurora at about 100 km, assuming no collisions. (I'm just going to calculate the radius, and distance traveled in one orbit.)

The detail I can't seem to find is, very roughly, the Earth's magnetic field at that height above the poles. I'm guessing its not much weaker than it is at the Earth's surface, but I'm not sure.

And is it reasonable to guess the perpendicular and parallel components of velocity are roughly equal (or can be for some typical particles)?

Reply

To save you some work I looked up "Introduction to the Physics of Space" by Olbert and Rossi, 1970. They give the gyration radius in cm as

R = (144 / B)

R = (3.37 / B)

where B is the field in Gauss (1 Gauss is the same as 100,000 nanotesla or nT), √ means square root and W is the kinetic energy in ev (electron volts) of the part of the motion perpendicular to the field line. The field above the auroral region may be around 50,000 nT or half a gauss (negligible difference from ground level) so you get about 15 meters.

An energy of 50 keV is rather high for the aurora; 5 keV may be closer, the typical range is 1 to 15 keV. And yes--auroral electrons arrive about evenly distributed in all directions (from above--not from below!) so your assumption of equal parallel and perpendicular components is reasonable.

Back to the main list

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

Authors and Curators:

Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: education("at" symbol)phy6.org

Co-author: Dr. Mauricio Peredo
Last updated: November 9, 2003
Re-formatted 9-28-2004