Items covered:
- Reversals of the Earth's field (4 queries)
- Can the Earth's field be used for spaceflight?
- The Sun's magnetic poles
- Synchronous satellites
- Magnetic field lines
- Alternate theory of the Sun and solar wind
- The Geiger counter (2 queries)
- Measuring the Earth's magnetic field
- The strength of the Earth's field
- Solar Eclipses
- Magnetometer for Observing Magnetic Storms
- Cosmic Rays
- Magnetic Shielding
- Use of solar wind for space propulsion
- A working model of the magnetosphere?
- The Van Allen Belt
- Magnets of different shapes
- On building an electromagnet
If you have a relevant question of your own, you can send it to
u5dps@lepvax.gsfc.nasa.gov.
Before you do, though, please read the
instructions
QUESTION
I need to know as much as possible about the reversal of the magnetic
field:
- how it was noticed
- who discovered the reversal
- how long ago did it reverse
- how many times did it reverse
- more information about the radiation from the sun if the magnetic
field reverses.
REPLY
ABOUT GEOMAGNETIC REVERSALS: This is a huge subject and I cannot do quick justice to it: look up in the index volume of the Britannica under: Geomagnetism, Plate tectonics, Reversals of the Earth's magnetic field.
HOW IT WAS NOTICED: When lava pours from a volcano, it solidifies to a black rock called basalt. Basalt is slightly magnetic, and it takes on the direction
of the surrounding magnetic field at the time it solidifies. Scientists
examined lavas for their magnetism early in this century (I believe) to see
how consistent the direction of ancient magnetic fields was with the direction
we observe now (would compasses point in the same direction?). The directions
generally agreed, but there existed reversals of directions which suggested
that there were times in the past when the poles were roughly interchanged.
No one knew what to make of it. Some suggested "polar wandering", that
the whole surface of the Earth slid around the interior like a loose shell.
WHO DISCOVERED: I don't remember. Check a book by Allan Cox, a collection
of historic articles.
But a big change happened in 1963. People noted that while rocks on Earth
were magnetized in a disordered way, the sea bottom was magnetized in long
strips. Larry Morley (whose article was regarded so speculative that journals
would not publish it) and then Matthews and Vine (who managed to publish)
suggested that molten rock was spreading out like a conveyer belt from volcanic
cracks in the middle of the ocean floor, e.g. the one in the middle of the
Atlantic (Azores islands sit on it). Or rather like 2 belts, one moving towards
Europe, one towards America, carrying on them the continental plates, so that
Europe and America gradually drift apart. As each belt comes out of the crack,
its lava solidifes to basalt, causing it to become magnetized, and when the field
reverses, its magnetization reverses too. So the bottom of the ocean records
the field like the tape of a tape recorder, containing perhaps 50 million years
of record.
HOW LONG AGO: about 700,000 years, according to the "tape recorder"
HOW MANY TIMES: Many, about half a million years apart on the average.
RADIATION FROM THE SUN: Sunlight of course is undisturbed. High-energy
protons from the Sun are usually diverted by the magnetic field. During
the reversal the field probably does not disappear, but becomes complex
and weaker, and protons can more easily reach the atmosphere, as they do now
within 1000 miles or so of the magnetic pole. On the ground it makes
no difference because the thick atmosphere shields us very well, and none
of the protons penetrates far into it.
David P. Stern
Reversal of magnetosphere
We have been studying the magnetosphere and the Van Allen radiation
belts in a high school physical science class. It has been brought to
our attention that the magnetic poles of the earth reverse on an average
of about every 500,000 years. The last change was about 700,000 years
ago, so it would appear that we are long overdue.
What are the implications of this? How significant would the
fluctuation of the magnetic field during such a change be on our
protection from solar wind?
Ricky
Dear Ricky
Only yesterday a similar question was submitted, so as a shortcut a copy of it [next item below] and its answer are attached below.
Some people worry that during magnetic reversals the Earth
would receive a higher dosage of high-energy ions and electrons
("radiation" in common terms), which might affect us and any living
creatures on Earth. This is not so. Even today, the magnetic shield
is not effective near the magnetic poles, yet the radiation received
there on the ground is only slightly higher than anywhere else. The
reason is that our main shield against such particles is not the magnetic field of the Earth but the atmosphere, equivalent to some
10 feet of concrete.
In any case, during reversal the magnetic field does not go away,
it only gets weaker and develops several more magnetic poles,
at unpredictable locations.
QUESTION
Could you tell me when the earth's magnetic poles will change, and what
will happen when it does? Will it happen fast (seconds) or slowly?
Thank you!
Sarah
Dear Sarah
No one knows when the next field reversal will occur: in the past,
they have occured on the average about once in 700,000 years. The change, whenever it occurs, will be gradual and the field will not drop to zero in between--doing so would mean that the magnetic energy of the Earth was somehow converted or dissipated, and all processes we know for this tend to run on scales of thousands of year, if not more.
Right now the main (dipole) field is getting weaker at a rate of
about 7% per century, and if you draw a straight line through the points
you find it reversing between 1000 and 2000 years from now. It might happen,
although the trend may well change. The energy of the field, however, has
hardly changed. What seems to have happened is that the more complicated
parts of the field (equivalent to several magnets in different directions)
have got stronger while the main two-pole ("dipole") field lost strength.
The complex field is somewhat weaker (it drops off faster with distance
from the source, which is the core of the Earth), but we should not
expect the field to be ever greatly weakened.
The polar field of the Sun seems to reverse every 11 years or so,
taking about a year or more. But the Sun's magnetism is different,
it has foci right on the surface, in sunspots.
Hope this answers it.
David Stern
Earth's magnetic field weakening--leading to a pole shift?
I am just a tax paying citizen, interested in astromony all of my life.
I am very interested in the physics of our earth which I believe is
related to astronomy as it is our home and a part of this solar system.
My question is: Is the earths magnetic field weakening, heading to
zero point? With this, is the base pulse frequency of the earth
speeding up causing the magnetic fields to fluctuate so that it
interferes with the pilots navigational equipment, so that
the navigational charts have to be redrawn periodically and the
air strips renumbered? Are the magnetic poles fluctuating? My
experience is that they are. I have a quality, liquid filled compass
secured to my desk. It has been very still now for the past month but
the six weeks or so prior to that, there were consistant fluctuations
in its direction, up to as much as 2 1/2 degrees, always to the west.
My understanding is: I have seen photographs of the sun taken from
satellites, showing the sun going through major activity. Repolarizing
itself? Causing the earth to repolarize itself? Going through a natural
cycle as it has many times in the past with pole shifts? On a scale
from 1 to 10, with 1 being the weakest and 10 the strongest, 2,000 years ago
it was a 10, today it is a 1. Is it heading for a zero point when a pole shift
will occure? The closer it gets to the zero point, the more fluctuations
will occur?
Are the change in the magnetic frequencies causing at times a confusion
in migratory animals? Causing cells to mutate, changing the DNA pattern
within the cell? Causing certain strains of bacteria such as staph
infections to become resistant to our antibiotics and causing new
viruses to appear that we have never seen before, being able to survive in
a new magnetic frequency?
I believe these are very facinating times in which we live. The science
of all of this intrigues me to no end. I have some taped interviews
of scientists and geologists relating to this subject and I read all that
I can get my hands on, on the subject also. Your straightforward comments
and answers will be most welcomed to help me to understand more, what
is taking place. Thank you so very much.
Michael
Dear Michael
A question on reversals appears in a list of questions and answers, at
http://www-spof.gsfc.nasa.gov/Education/FAQ.html
and two more are due to be posted there soon. Also, if you look up "Exploration of the Earth's Magnetosphere" you will see that the reason the Earth has
a magnetic field is not any polarization, but electric currents flowing in the
Earth's core. You will also find there a great deal of material on the Sun's magnetic field and its relation to sunspots and their cycle.
Now to your questions.
Is the Earth's field getting weaker? Yes and no. That field is often
viewed as being a two-pole ("dipole") structure similar to that of a
small bar-magnet at the center of the Earth, inclined by about 11 degrees
to the rotation axis of the Earth, so that the magnetic poles are not the
same as the geographic ones. But the actual situation is more complicated,
and magnetic charts note the fact by mapping deviations between magnetic
north and the direction to the magnetic pole, which fit no simple pattern.
Why? Because the magnetic field is actually more complicated, and it
contains additional fields, of more complex nature. All this originates
in the Earth's core, about half the radius of the Earth. If we could go
to the surface of the core, all the complicated parts would be much
bigger. But they weaken more rapidly with distance, so at the surface
of the Earth they are already quite weak, while the "dipole" part
stands out more (in addition of actually BEING the biggest chunk of the
field).
Are you still with me?
The magnetic field of the Earth changes all the time, and yes, magnetic
charts have to be redrawn from time to time (this was first found in
1641, by an Englishman named Gellibrand). And yes, in the century
and a half since the first careful mapping of the Earth's field, the
dipole has become weaker by about 8% (the rate may have speeded up in
1970). If you draw a straight line through the points, you will find
that perhaps 1200 years from now, the line goes through zero.
Extending straight lines too far beyond the present, however, is risky
business, as noted by no less a scientific authority than Mark Twain.
In "Life on the Mississippi" Twain noted that the Mississippi river was
getting progressively shorter (mainly by floods--and people--creating
shortcuts through bends in the river) and he wrote:
"Now, if I wanted to be one of those scientific people, and "let on"
to prove what had occured in the remote past by what had occured in
a given time in the recent past, or what will occur in the far future
by what has occured in late years, what an opportunity is here! ...
Please observe:
In the space of one hundred and seventy six years the lower Mississippi
has shortened itself two hundred and forty-two miles. That is an average
over a mile and a third per year. Therefore, any calm person, who is not
blind or idiotic, can see that in the lower Oolitic Silurian Period,
just a million years ago next November, the lower Mississippi was upward
of one million three hundred thousand miles long, and stuck out over the
Gulf of Mexico like a fishing rod. And by the same token any person can
see that seven hundred and forty years from now the lower Mississippi
will be only a mile and three quarters long, and Cairo and New Orleans
will have joined their streets together, and will be plodding comfortably
along under a single mayor... There is something fascinating about science.
One gets such wholesale returns of conjecture out of such a trifling
investment in fact."
It is not impossible that the magnetic field will go through zero
1200 years from now, but (judging by the past record of reversals) not likely.
In any case, the field is not going away: when one uses observations on the
surface to reconstruct fields at the core, one finds that while the dipole
field is getting weaker, the complicated parts are getting stronger, and
the total magnetic energy does not change, within our observational
accuracy. That's why I wrote "yes and no."
I don't know about migrating animals (they may have magnetic organs,
sort of built-in compasses), but there seem to exist no magnetic effects
on DNA, resistance to antibiotics and so on; those changes seem more
related to chemistry.
Finally, be cautious with compass readings in your house. Houses do
contain electric currents and machinery, and these may affect the readings
of a magnetic compass. On NASA's satellites the magnetic sensor usually
sits at the end of a long boom, to keep it away from interfering electric
currents in the satellite's circuits.
Keep up your interest in science!
David
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QUESTION
student question on earth magnetic strength (from Texas)
Dear Mr. Stern:
I am an Industrial Technology teacher at a middle school
and one of my students is dreaming of a space propulsion system based on
magnetic repulsion of the earth's magnetic field. Could you possibly
squeeze in a moment for us and provide some information on the strength of this field
and how it has been measured and maybe a relative comparison? Tyson, my
student, is really excited about the Internet and will be enthralled to
have an answer from a NASA scientist. Perhaps you could steer him to
other references as I certainly will explain to him how busy a schedule
you must have. Thank you.
REPLY
I am afraid it won't work. First of all, the magnetic field is very
weak. Compared to fields in electric machinery, where appreciable forces are
exerted, it is a few thousand times weaker.
But there is a more fundamental reason. Magnetic poles always come in pairs, equal and opposite: if a field attracts an N pole, it repels the attached
S pole. Similarly, if we generate the field by a current in a loop of wire
--say, shaped like a rectangle--for each side in which the current flows
in one direction, there exists a side where it flows in the opposite direction,
and the magnetic field exerts opposite forces of equal strength on the two
sides.
From the preceding one would guess that magnetic forces always cancel,
and no net force is exerted. So how come magnets are attracted to each
other, or pins to a magnet (same thing, really, since each pin in the magnetic
field turns into a small magnet)?
The answer is that the forces on the N and S poles (or on the opposing
currents) are not exactly equal, if one pole, or one wire, is closer to the
source of the field than the other. This can be put into a mathematical
formulation and the bottom line is that a suitably oriented magnet may be
attracted by a magnetic field, moving towards the greatest strength of that
field. But the force is proportional to the rate at which the field changes with distance, which in the case of the Earth, is very small.
The idea of magnetism as anti-gravity has come up before. Your student may
look up "Gulliver's Travels" by Swift, where in the third voyage, in a spoof
on science and learned societies, Gulliver arrives at an island floating
in the air, held there by the repulsion of a large magnet. Swift even
gives an explanation, except it's all gibberish gobbledygook, as befits a book
of satire. (I won't cite here the name of Swift's island, since too many
people in Texas speak Spanish!)
David Stern
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QUESTION
Question about the sun
Dear Mr. Stern:
I have a question about the sun that I was hoping you might be able to
answer for me. A friend of mine recently returned from a new-age conference
where it was presented that the magnetic poles of the sun were about to
reverse, and cause a number of changes.
The idea of the sun having magnetic poles seemed counter to what I remember
learning about the sun, and your web page seems to dispell the idea that the
sun has actual poles. My guess is that the presenter was taking a dose of
creative license with the 11 year cycle of sunspot activity.
Is it true then, that:
1.) There are no magnetic poles on the sun.
2.) Is the change in sunspots related at all to a reverse of polarity of
magnetic fields?
Thank you.
If you can provide reference to a college-level text as a reference, it would
be appreciated.
REPLY
Actually, your friend was right: the Sun does have polar
fields, and they do seem to reverse their polarity around sunspot minimum.
The Sun's most concentrated magnetic fields are of course in sunspots, but
people have long suspected there might also exist polar fields, because during
a total eclipse of the Sun one often sees streamers coming out from the polar
regions, looking very much like the pattern of iron filings near the poles
of a magnet.
But there was no good way of measuring such diffuse magnetic fields: the
field of sunspots affects the light emitted from them ("Zeeman splitting")
but the effect elsewhere is very weak. Then in the 1950s (if memory serves
me) the Babcocks pushed the technique to its limits and found the polar field.
This revealed the reversal of the polar magnetic field and suggested this field
was somehow coupled to that of sunspots (which also reverse each cycle--they
come in pairs, and the leading spot, in the direction of the Sun's rotation,
has north or south polarity, in alternate cycles), a sort of a cumulative
effect of the distant field of many spots. Theories exist by Horace
Babcock and Robert Leighton, though they are somewhat qualitative.
The fact the magnetic field lines at the poles stick straight out means they
do not hinder the escape of the solar wind in any way, and indeed the Ulysses
spacecraft which recently passed above the Sun's poles confirmed (as was
predicted) that the solar wind there is faster. There seems to exist no great
effect of the reversal on Earth, though one might expect a bit more magnetic
storminess when the polarity is opposite to that of the Earth.
For more on the Sun, see:
Yours,
David Stern
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QUESTION
Synchronous Satellites (query from Barbados)
Dear Dr. Stern:
I have been told, and read, that in order for a satellite to remain in a
fixed position relative to the earth, it must be in synchronous orbit, and
that this type of orbit is the best for communication purposes. All of the
other orbits I have read about provide means for satellites to meet needs
other than for communication. Satellites in orbits other than synchronous
are not fixed in position relative to the earth.
This being the case, it seems to me that all satellite dishes for reception
of TV signals would face the equator - south east, south west, or somewhere
between. My observation of satellite dishes does not seem to support this
concept. Many dishes do have a southerly inclination, but others do not.
Further, here, where we are close to the equator, it seems to me that in
order to focus on a satellite, the dish should be aimed high - higher, at
least, than one located in the USA or Canada where the angle between the
earth and the satellite would be smaller than here in Barbados. Many dishes
here seem to follow a line of sight that is barely above the horizon.
So, my question - are all communication satellites in fixed orbit above the
equator, and if so, why don't all reception dishes face the same way?
As I said at the outset, I know that I am being brash in writing you, and I
would appreciate your indulgence. Over the past year or so, I have asked at
least a half dozen engineers for an explanation, and received little other
than a blank stare in response. Access to other resources here is not
always easy. If you are unable to take the time for an explanation, perhaps
you would direct me to a source that could satisfy my curiosity.
If you do reply, please bear in mind that I am an accountant, not an astronomer.
REPLY
I am very glad to hear from you, to find someone as far
away as Barbados interested in satellites, but I have no good answer to
your question. It is absolutely true that all commercial communication
satellites orbit above the equator, at a distance of 6.6 Earth radii,
with a 24-hour period, which keeps them above the same station. You know
probably that the space shuttle and other low-altitude spacecraft
complete one orbit in about 90 minutes: the further out you go, the longer
it takes, until you reach the moon, which takes one month. So it stands
to reason that somewhere in that range the orbital period is exactly 24 hours.
If a 24 hour orbit is inclined to the equator, the satellite does not stay
above the same spot but wanders back and forth: so it must be an equatorial
orbit. Only then can the antenna on the ground be fixed in one position.
It is true, however, that not all satellites tracked from Barbados are
at the longitude of the island. To receive phone calls from Europe, say, it could be that a satellite is tracked which is orbiting at the longitude of Europe, and then the dish should point towards the southeast.
I do not know how you reached my name; maybe you were directed by a search engine to the file.
http://www-spof.gsfc.nasa.gov/Education/wsynch.html
I hope you are aware that this is only one part in a much larger exposition, the Exploration of the Earth's Magnetosphere, dealing with the Earth's magnetic environment, with its home page at
http://www-spof.gsfc.nasa.gov/Education/Intro.html
You might look it up. Also try:
http://www-spof.gsfc.nasa.gov/Education/wlagran.html
dealing with satellites keeping a fixed position (or staying close to one)
relative to the Earth in its orbit around the Sun.
David P. Stern
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QUESTION
Magnetic Field Lines
Mr. Stern,
I am enjoying your presentation on magnetospheres very much and I am
finding it most interesting and informative. However, I have one
question that I have not found an answer to yet and it is:
If the earth's magnetic lines of force are in fact "lines" upon which
electrons and protons can collect like "beads on a wire", what is the
spacing between these lines say, at the altitude of the recent Tethered
Satellite experiment?
Some co-workers and myself have had a rather heated discussion on this
matter (i.e. whether the magnetic lines are "lines" or a "field"). We
would be most greatful if you would enlighten us about these magnetic
lines of force around our planet just a little more than you have in
your presentation on the Goddard Home Page on the WEB.
REPLY
Be very careful here! There is the magnetic field--space modified by
magnetic forces, so to speak--and there are magnetic field lines, which are
a mathematical description of that field. They are no more tangible than
lines of latitude and longitude on Earth, and one never asks how close THOSE
are--you can draw any number of such lines, depends how tightly you are
willing to space them.
The structure of magnetic fields can be described by lines that point
everywhere along the magnetic force (in a fluid, a complete analogy is
given by flow lines or "streamlines"). This can be described by formulas,
in terms of quantities known as Euler potentials or Clebsch functions.
But there also exist intuitive properties: particles threaded by a common
field line, tend to share that field line later on as well. Say we have 10
ions numbered 1... 10 sitting on a common spot on the Sun, and therefore
sharing there a field line, and destined to come out in the solar wind
one day apart. The Sun rotates, so make a drawing with a circle representing
the Sun and 9 radial lines coming out about 15 degrees apart. After 10 days,
particle #1 is 2.5 inches along the first line, particle #2 2.25 inches on
the 2nd one, and so on, down to particle #10 still on the surface: the line
conecting the particles is a spiral, so we expect interplanetary field lines
to have a spiral shape, and we derived this from intuitive concepts alone
(though the same thing can be derived from formulas).
The spacing between field lines is not meaningful. Suppose you draw two
field lines of the Earth, reaching Earth 1 meter or 1 foot apart. Each can
have electrons or ions trapped around it. The meaningful question is what is
the radius of the circle these electrons or ions describe around their guiding
line, and that depends on their energy, and how strong the field is (the circle
gets larger in the weak fields far from Earth), but it is generally much more
than 1 foot or 1 meter. No problem: densities are so low that such ions or
electrons rarely collide, and their orbits can easily overlap. The radius of
gyration of auroral electrons can be 100 meters, which is why auroral "curtains"are so thin. On the other hand, solar wind ions entering near the "nose" of the magnetosphere have radii of the order of 500 kilometers, or (say) 350 miles, because the field there is much weaker, and that is therefore the order of the expected thickness of the magnetopause, the boundary between the solar wind and the magnetosphere.
David P. Stern
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QUESTION
Solar Wind
To David P. Stern:
Hello David, I liked the WWW article on The Solar Wind-History. In the
article it said:
"Not everyone accepted Parker's theory of the solar wind when it was first
published in 1958, and it was debated until observations confirmed it."
Have you ever heard of the book OAHSPE by John Ballou Newbrough copyright 1882? Oahspe means Earth, Sky, and Spirit. Oahspe contains much scientific information that was discovered many years later, such as the earth's magnetosphere, Van Allen Radiation Belts, the SOLAR WIND, the origin of stars in nebula, the configuration of galaxies, the beginning substance of
life(DNA) in nebula, the cycle and age of galaxies and the stars they
contain, interstellar and intergalactic Unseen matter(Dark matter). Oahspe
says the Sun has a vortex that streches throughout the solar system.
Scientist have some knowledge of the Sun's vortex, they call it the "Solar
Wind'.
(most of the letter omitted)
Tell me what you think about it. Please send me a reply email. Hope to hear from you soon.
REPLY
I am sorry to have to disagree with you, but just coming out with a
statement which is later verified is not a prediction, unless it is supported
by cogent arguments, which distinguish it from similar predictions which
were not verified. That's the essence of the scientific method: what we believe
to be true is based on a tight interlocking web of observed evidence. The first
evidence for the solar wind came from comet tails, whose behavior had been
explained by sunlight pressure: it took a lot of physics to understand why
this process worked on dust tails, but not on ion tails. Parker's theory
was based on the high temperature of the corona, discovered in 1939 or so,
and again involved some intricate physics.
The solar wind, by the way, is not a spiral: it flows straight out. Only
the Sun's magnetic field lines are spiral, because the sun rotates: they
tend to act like continuous strings, and since their "roots" rotate with
the Sun, they get twisted into spirals.
David Stern
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QUESTION
The Geiger Counter
Hi, I am a 10th grader ...I came across a web site ... in my search for how a
geiger counter meter works. I was hoping you could give me a relatively simple
yet good explanation of how it works (actually really simple). I'd appreciate it
very much.
Thank you.
REPLY
And I thought "Exploration" did give a simple explanation!
Imagine a fast ion or electron going through the tube. On its way it hits atoms
of the gas in the tube and ionizes them--knocks off electrons and leaves
positive ions. Usually, such electrons recombine soon. But if there is a
voltage difference (electric field) in the tube, before they can do so, the
electrons will start moving towards the positive wire and the ions towards the
negative walls.
As they move, they gain energy. This is particularly true near the wire,
where the electric field is concentrated and its force is strong. (One can draw
electrical field lines just like magnetic field lines, and near the wire they
bunch together, like magnetic field lines near the poles of a magnet.)
If the energy gained by the average electron is enough to knock out additional
electrons from atoms of the gas with which it collides, the number of
electrons will multiply. As the electrons move towards the wire
and the field gets strong, this process grows quickly: one electron makes two,
two make four, and by the time the wire is reached, many more electrons arrive
than were released by the initiating particle, enough to draw a measurable
current and create a signal in the circuitry attached to the counter.
There is much more, of course, e.g. ultraviolet light which spreads the
process away from the wire as well, which could cause the current to continue
without stopping even after all the initial electrons (and the additional
electrons caused by them) have reached the wire. Special gas filling takes care
of that.
The counter is usually charged by just a trickle of current from a high
voltage source, so that the current taken by the discharge is easily measured.
If too many particles pass through the tube, too much current is drawn, the
voltage drops and the discharges get smaller, until the electronic circuits
supposed to count them don't do so any more. I think that's what happened on
Explorer 1.
David Stern
QUESTION
Building a Geiger Counter
Hello!
I found your web-site in the internet. But I have still two questions.
1. Which gas is suitable?
2. Which pressure is in the metal tube(vacuum?)?
Please can yuo send me a wiring diagram from a geiger counter.
Best regards
Matthias
Dear Matthias
I did my thesis work with Geiger
counters, but that was 40 years ago and my memory of that time is
sketchy. If you have a local university with a physics department,
it (or at the very least, its library) could provide you with much
better and much more up-to-date information. Here is what I remember
- You should understand, of course, that if the Geiger counter
is a metal tube, the end plugs must be glass, with a tight
metal-to-glass seal, because the central wire must be insulated
from the tube. Or else, the whole thing is inside glass, and
the tube nowhere touches the wire.
- When a particle passes inside, creates ions and initiates a discharge,
you don't want the discharge to get too big. So if the tube is
grounded, the central wire had a small capacitor between it and
the ground, charged through a large resistor from a high voltage
supply (about 1000 volts--depends on the counter). Then when the
counter discharge, only the electric charge on the capacitor is
involved, and afterwards the counter is "dead" (insensitive) for
a small fraction of a second, while the capacitor refills.
- The trick with the counter, if I remember, was for it not to
go into a continuous discharge. The filling, at a pressure of a
few tens of milibars, was either alcohol vapor or halogen.
Alcohol stopped the discharge by itself, but was gradually
broken up by the discharges, so that the counters had a certain
lifetime. Halogen--I don't remember what it was--chlorine?--
lasted longer but needed an external electric circuit to give
it a large negative pulse and shut it off, after each discharge.
In either case, the voltage was critical--too big, it went into a
continuous discharge, too small, the pulses were small, too, in
what is known as the "proportional counter" regime. Today many
people prefer proportional counters, since the electronics can
amplify the pulses very well. But 40 years ago it was good to
have a detector whose output pulse was big enough without any
amplification. The pulse was carried out through a small
capacitor, which let the pulses through to whatever counting
circuit was used, but kept out the high voltage.
I hope all this is useful. Sometimes simple questions lead to complicated
answers!
Yours
David P. Stern
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QUESTION
Measuring Earth's magnetic field
I am doing a sixth year studies project on magnetism in and was delighted to
find the question and reply page with topics similar to what I had thought of
studying.
I was wondering if there was a practical method for measuring the strength and
direction of the Earth's magnetic field at different geographical locations.
Any help or inspiration would be greatly appreciated.
REPLY
Is your "sixth year" in school or 6th year in college? It is not easy
to tailor an answer to fit either level!
In any case, the electronic gizmos nowadays used in space are too complicated
for a quick discussion, so let me instead describe earlier, simpler methods.
The direction of the magnetic field is of course given by the compass needle:
but that is just the horizontal part of the force, Actually the magnetic force
also points i n t o the Earth (or out of it, in the southern hemisphere).
To find the angle at which the force points down ("dip angle") people used a
needle similar to a compass needle, but on a horizontal axis, allowing it to
swing in the various directions to which the hands of a wall clock might point.
That is a bit harder to arrange than a compass needle: if one end of such a
needle points at an angle downwards, how is one to know that the magnetic force
is responsible, and not, say, that the needle is not quite balanced on its
pivot, but that one end is slightly heavier and therefore points downwards? To
avoid this problem one starts with an unmagnetized needle, balances it very
carefully, and only then magnetizes it. When in 1831 the expedition of John
Ross searched for the north magnetic pole, it carried along a dip needle, and
when it pointed straight down (while the regular magnetic needle showed no
preference for any direction), that was it .
Measuring the strength of the field is harder. Take a thin long bar magnet
and hang it by a thin thread, then wait until it points north-south. After it
does, push one tip slightly left or right and let go: it will swing back to
north-south, but will overshoot to the other side, then turn back to the right
direction, swinging back and forth like a pendulum, gradually quieting down to point steadily. The average length of each swing depends on two things: the
strength of the bar magnet and the strength of the magnetic force. With a
stopwatch, measure 20 swings or so and figure out how long each swing takes.
Then put a small compass needle on a table, and put the small magnet nearby,
in such a position that it tries to line up the compass to point east-west.The
small magnet and the Earth's magnetic force obviously compete fordetermining which way the needle points, and by looking at the actual angle of the needle, and its distance from the small magnet, we again get an observation that depends on how strong are (1) the small magnet and (2) the magnetic attraction of the Earth. Using these two observations and some calculation, the physicist can find both these unknown quantities.
This method was proposed by Carl Friedrich Gauss in Germany around 1835. It
obviously won't work on an orbiting satellite--but how measurements are made here is another story altogether.
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QUESTION
The Strength of the Earth's Magnetic Field
I was writing to inquire on a couple of subjects. Seeing that you have access to
better information than I do I was wondering if you could update some
information that I have. Could you please send me any information regarding the current field strength of the earths electromagnetic field. My data is current as of 1975 which is by far outdated. My reading from that time were 30,000 gammas at the equator. If possible could you please send information on the current decay of the earth's magnetic field.
Any information would be greatly appreciated.
REPLY
I am not sure at what type of information you need, or to what
use you put it. The most complete information on the Earth's internal magnetic field is in form of a set of coefficients, to be plugged into a mathematical representation--the so-called spherical harmonic expansion. The coefficients generally used are the so-called IGRF set (International Geomagnetic Reference Field) chosen by a committee every 5-10 years and based on the "best available" observations. You can find them on the world-wide web at
http://fdd.gsfc.nasa.gov/IGRF.html
Some of these models also include the annual change of the field (but not in the above files). You might like to search the web using (say) the Altavista or
Yahoo search engine, on the term IGRF.
If you just want maps of the field, for instance those describing, the
variation of its strength over the globe, try
http://swdcdb.kugi.kyoto-u.ac.jp/igrf/index-j.html
The text seems to be in Japanese, for on my computer it does not give anything readable, but the maps are in English. Clicking on the first will show you that the magnetic intensity around the equator varies quite a bit. but 30,000 gamma (or nanotesla, same thing) is a reasonable value.
The field has been weakening since Carl Friedrich Gauss measured it around
1836, by about 5% per century, recently accelerating to 7-8%/cent.The total
energy of the field however is nearly constant, as shown by the late Ned Benton. This means that the field is not really weakening, only reshuffling its energy, reducing the "main dipole" (=north-south bar-magnet pattern, declining as noted by about 7% per century) and reinforcing the more complicated parts.
These tend to contribute a weaker field, because the magnetism originates in
the Earth's core, about half an Earth-radius down: all magnetic fields at the
surface are weaker than those in the core, because of the distance, but the more complicated fields decrease faster.
Whether the main dipole will reverse in about 1300 years is anyone's guess.
Geological evidence suggests it has happened in the past, but odds are against
it, because the mean frequency of such reversals in the past seems to be about
once in 700,000 years.
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QUESTION
Solar Eclipses
I'm working on a science project about the solar eclipse.
My first question is, how can you figure out exactly when the next
eclipse will come?
The next question is: What are the main theories about the incredible
heat of the corona? How can it be so warm when it's so far away from
the sun's center?
REPLY
Dear Lars
Predicting eclipses is relatively straightforward, you just need
to know the motion of the Sun and Moon across the sky, and when they
occupy the same area, you get an eclipse.
By now we have pretty good formulas for the orbital motion of the Earth
around the Sun (which determines where the Sun is in the sky) and of the moon
around the Earth, and can predict eclipses quite accurately. The journal
"Sky and Telescope" usually carries accurate maps of where the eclipse can
be seen (if the sky isn't cloudy) and the times when it should happen. The
journal also maintains an eclipse page on the web, at:
http://www.skypub.com/eclipses/eclipses.shtml
The heat of the corona is still a great mystery. I can describe to you
one theory, but it is probably not the right answer.
When you walk along a beach, you usually see fairly large waves, breaking
on the seashore. If you take a boat past those waves, you are likely to
find that in deeper water the waves almost disappear, or anyway are much
smaller. Why?
It happens because a traveling wave carries a certain amount of energy,
which causes the water in the wave to rise and fall again. As the wave moves
into shallower water near the shore, the same energy now moves a smaller
amount of water: if energy is conserved, the motion must be bigger, which is
why waves become higher. Finally, the water is not deep enough for the
wave to keep going, and the wave breaks, giving up its energy all at
once to irregular swirling of the water.
Some scientists have speculated that waves, perhaps similar to sound
waves, rise from the surface of the Sun into the corona. They carry much
less energy than sunlight, but as they rise, the density of the gas
around them quickly decreases, until finally they reach a height at which
not enough gas is left to carry the wave: it then gives up its energy,
and since that energy is given to the surrounding gas, and there is very
little such gas left at those heights, that remaining gas gets very hot.
That at least was the theory some time ago: however, scientists now know
what sort of waves can move in the atmosphere of the Sun, and they say
that such waves get reflected back downwards before they reach high
enough for this process to happen. So we really don't know.
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QUESTION
Magnetometer for Observing Magnetic Storm
I have contact to one doctor in hospital and we would like to
build a magnetometer that measures geomagnetic storms, a normal
magnetometer, but cheap. The doctor made studies that show how patients
react to differences in the geomagnetic field. He says that it is possible to
help them to feel better, and to help hospital personal to understand
geomagnetic status and to make predictions of the patient's pains...
I have experiences with microprocessor-driven machines, to build and
programming them, but the problem of geomagnetic detector is new for me.
REPLY
I hate to discourage someone as enthusiastic as you, but I would
recommend to reconsider your plan, for several reasons
- Building a magnetic observatory is technically difficult, and so is
running it.
- Information on geomagnetic storms is probably easily obtained from
some convenient magnetic observatory, even for free over the
internet.
- I doubt very, very much in any effect of geomagnetic storms on
people. No one has ever proved anything in that direction, and
it would be very difficult to imagine a mechanism by which such
an effect could arise.
In more detail:
- The most practical magnetometer is a fluxgate instrument, using a
core of a ferrite with very precise saturation level. If an external
magnetic field exists, it will penetrate the core, and magnetize it
in some direction, and the core is then a bit quicker to saturate
in that direction than in the opposite one; modern instruments use
a ring shaped core. Sensitive electronics can detect the difference
in saturation.
Obviously, you need 3 such instruments to detect the components
of the magnetic field in the 3 directions of space--say x, y and z
(storms mainly affect the north-south component). But geomagnetic
storms are very small--the field may change by 0.2% only--so one must
be able to tell it apart from all sorts of other disturbances, e.g.
magnetic observations must be made away from electrified streetcars
and railroads, and from other electric machinery. And other sources
of disturbance ("daily variation" for instance, due to tides
in the ionosphere) must be subtracted.
- There exists a world-wide network of magnetic observatories which do
this sort of work routinely. Try contacting (name of local insti-
tution)--I think they have one. Or else try the data
center in Japan:
http://swdcdb.kugi.kyoto-u.ac.jp/wdc.Sec3.html
For geomagnetic data:
http://swdcdb.kugi.kyoto-u.ac.jp/magqldir/
And for real-time data of the Kakioka observatory:
http://swdcdb.kugi.kyoto-u.ac.jp/magqldir/q/KAKrtoday.html
Other data are also available there, but many of them are from
near-polar observatories, sensitive to auroral currents. Kakioka is
fairly close to the equator and therefore more suitable for
magnetic storms. To be sure, it is half a world away from where
you are--but magnetic storms are world-wide.
- I doubt the magnetic field can affect pain in patients. You cannot feel
the field, even less so a variation of 0.2% in it. One can easily
imitate the field using coils and see whether patients can sense
anything, if you want.
By the way, if you want to build a magnetometer, there exist other
uses, and you can buy commercial instruments for them, e.g. finding
boundary stakes in surveying, stopping theft of library books (marked
with magnets), screening for guns at airports, finding submarines
under water, etc.
Best wishes
David Stern
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QUESTION
Cosmic Rays
After reading your cosmic rays report, my friend and I decided that I
would like to do a science project on cosmic rays.
However, we are only eighth grade students, and therefore do not have much
background on the subject. It would be much appreciated if you could
provide us with some background on cosmic rays, and perhaps with a science
project we could perform.
REPLY
Actually, you can find quiet a bit of material about cosmic
rays in "Exploration," including material you need to understand more
advanced discussions of cosmic rays.
I would recommend that you and your friend use the "index" file and
reach from there the following files, in the order listed here:
Electrons, Positive Ions, Energy, Energetic Particles, The Geiger
Counter, Cosmic Rays, High Energy Particles, Solar Energetic Particles.
Copy them on paper, if you can. Of course, if some items are not comp-
letely clear, you will need to look up other sections as well.
In general, our study of cosmic rays can be divided into two phases.
The first started in 1912, with the discovery that some unknown radiation,
similar to the one emitted by radioactive materials, was reaching Earth from
space. Gradually, it was identified--first as electrically charged
particles, by the fact the Earth's magnetic field excluded some of it
from near the equator (around 1922). Then it was found that the particles
had positive charge--because the field affected unequally those coming from
the east and from the west (around 1936). Finally, around 1947,
photographic plates in balloons at high altitudes recorded tracks of
individual particles, finding they were familiar ions--mostly hydrogen,
some helium, and a scattering of heavier stuff, not too different from
the composition of the Sun.
Scientists also found out much about the fragments produced when
these particles hit the atmosphere (what we get on the ground is almost
entirely fragments) and measured the particles' energy distribution. It
turned out that some of them had phenomenal energies, raising the question of
what process could provide them. But looking for the source of the rays proved
elusive, it was like trying to observe the Sun in a heavy fog. In a fog
sunlight gets scattered until it comes evenly from all directions, leaving no
clue about where the Sun actually is. Similarly, cosmic rays seem to
be thoroughly scattered in space, arriving equally from all directions. It
is true that it is hard to bend the path of particles with such high
energies, but the energies meet their match in the great distances of space:
even a weak magnetic field can bend the path of a cosmic ray proton, if
it acts gradually over cosmic-scale distances.
So these days the emphasis is on tracing the source of x-rays and gamma-rays,
high-energy relatives of visible light, which move in straight lines no matter
what happens and therefore tell where they come from. It takes a high-energy
particle to produce a high-energy gamma-ray, so observing the sources of such
rays tells us where in the universe high-energy particles are plentiful, and
perhaps these are cosmic rays near their sources. The catch is that (1) these
gamma rays do not penetrate the atmosphere well, so the observation must be
done from satellites, and (2), their intensity is much weaker than that of
cosmic rays. Still, we have been looking, and discovering (e.g. see the story
of gamma ray bursts in "Exploration"). Currently NASA has a gamma-ray
observatory in orbit, doing a nice job. This area of research goes by the
name of high energy astrophysics.
I don't know how much beyond this an eighth-grader can go. The "Resources" section lists some files you might look up, e.g. on high energy astrophysics, and a book "Moments in the Life of a Scientist" by Bruno Rossi, Cambridge 1990
Bruno Rossi was a pioneer of cosmic ray research and this is his own story.
He began in Italy as part of the talented group which included Enrico Fermi
and Emilio Segre, and died a few years ago as a much-honored professor at
the Massachussetts Institute of Technology.
Another book, by a pioneer of x-ray astronomy which covers that aspect
as well as the rest of astronomy, is "The Astronomer's Universe" by
Herbert Friedman, W.W. Norton, 1990.
Finally, your own country of Canada has contributed significantly to the
study of cosmic rays. Perhaps the science museum in Ottawa can help you.
Good luck!
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QUESTION
Magnetic Shielding
I got a question here about induced magnetism and magnetic shielding.
I understand that we can screen out magnetic field from a region by
wrapping a piece of soft iron around the region. However, I also
understand that soft iron can easily receive induced magnetism when placed
near a permanent magnet.
So now my question is that:
How is it possible to shield a region that is around a permanent by
using a piece of soft iron as this piece of soft iron will eventually
get induced magnetization and have the ability to attract any magnetic
material that is nearby.
Ong
REPLY:
Magnetic shielding is not my speciality and you might get a better
answer from an engineer familiar with magnetic design, but I will try.
Soft iron--especially the kind used in shielding (mumetal, etc.) does
not take permanent magnetization. Steel does, but even there, the
magnetic intensity must be high enough for that to occur.
In shielding (e.g. a video tube) you wrap a sheet of soft iron around
the shielded object, and the magnetic field lines which would have closed
through the interior are diverted and close through the shield instead.
Therefore any magnetic field that existed in the interior is greatly
weakened. The field inside the iron sheet is stronger, but that is no
problem--in fact, that is what we wanted to do, take the magnetic field
from the inside volume and put it elsewhere (you can't just get rid of it,
for all magnetic field lines have to close somewhere).
I hope this answers your question
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QUESTION
Using the Solar Wind for Space Propulsion
I am a student working on a science fair project which deals
with possible use of solar wind in space travel. My hypothesis is that
ions in solar wind can be gathered and used as propulsion for long-term
spaceflights. In theory, would this work?
Thank you. SEG
Dear SEG
Something like this has been considered, but using the pressure of
sunlight rather than that of the solar wind (I have not calculated it,
but it seems the former is much bigger). The idea is to spread a huge
sail, say of mylar with a reflecting coating (the kind that is used
to darken glass walls in office building) and have it face the sun,
so that sunlight bounces back. It is analogous to the way the wind
pushes a sailing ship and in fact, this has been called a "solar sail."
None has been tried so far. I believe there was even a story by
Arthur Clark on that idea.
If you wish to study this further, look up
http://www-spof.gsfc.nasa.gov/stargaze/Solsail.htm which is part of a sister-site "From Stargazers to Starships." You will find additional links there.
By the way, sunlight pressure and solar wind pressure combine to
push comet tails away from the sun--sunlight the part of the tail
consisting of dust, the solar wind that part which consists of ions.
The two tails are sometimes distinct, and people who watched the bright comet Hale-Bopp through binoculars could see both. Good luck with your project!
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QUESTION
A Working Model of the Magnetosphere?
I am an 8th
grade science teacher in Texas. I am trying to find
out if anyone has ever tried to make a working model
of a sphere capable of generating a magnetophere. I
feel that the testing of such a model in space could
provide some insight into geologically problems such
as why the earth's magnetic field periodically reverses itself. Please
let me know if anyone has
ever made such a model to test in space and what
has been learned from such models. Thank you -- Jim
Dear Jim
Your short questions requires a very long answer!
Briefly: Yes, people have devised working models of the magneto-
sphere, but while such models have provided clues on magnetic fields
in space, they tell nothing about reversals of the Earth's field.
For that you have to probe the inside of the Earth, on which whatever
happens in space has little effect. However, there also exists
progress in that direction.
Back to your question: The Earth's magnetism affects surrounding space in interesting ways. But surrounding space has very little effect on it--things would be just the same if the Earth were a hunk of magnetized iron. Look up under "Terrella" (in the index here) and you might see why ideas like yours are nearly 400 years old!
Yes, it has been suggested that the astronauts on the shuttle
stick out a large magnet and see how it reacts with the ionized
gas through which its orbital motion carries it (more about that
gas, in "Exploration", it is called "plasma'). It's hard to control
such an experiment, hard to put measuring instruments around it, so
scientists do it in the lab--put a magnetized ball ("terrella") in
a vacuum tank, blow a puff of plasma at it, and measure what happens.
Prof. Hafez U-Rahman at the University of California, Riverside,
has such a tank and has experimented with it.
But the source of field reversals is not above our heads, rather
it is beneath our feet. Why is the Earth a magnet, you may ask? It
could in principle have a huge iron magnet somewhere in its middle,
but that does not work: any magnetic material loses its magnetism when
heated to red heat, and the interior of the Earth is much hotter than
that, in fact, earthquake waves tell us that in the middle is a molten
core (inside that is a solid inner core, but at many thousands of
degrees).
So the Earth's magnetism is not produced by magnetized iron, but rather,
by electric currents (see "magnetism" in "Exploration"). Those currents
are produced by a "dynamo process" (see again, "Exploration") in the
flowing hot metal (we think) of the Earth's interior. The Earth's
magnetic field changes slowly, so that magnetic charts have to be
redrawn every few decades, and that is apparently because the pattern
of the currents shifts.
Recently, the process has been successfully simulated by a computer.
And yes, the poles sometimes reverse. Oh, and did you know that the
polar field of the Sun reverses every sunspot cycle, every 11 years or so?
David Stern
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QUESTION
The Van Allen Belt
Dear sir, I wonder if you could tell me exactly what the VAN ALLEN BELT is
and how much radiation does it contain, ie how many rems of radiation are
there out there?
Plus, what protection would organic life need to be protected from this
radiation
REPLY
The radiation belts are regions of high-energy particles, mainly
protons and electrons, held captive by the magnetic influence of the
Earth. They have two main sources. A small but very intense "inner belt"
(some call it "The Van Allen Belt" because it was discovered in 1958 by
James Van Allen of the University of Iowa) is trapped within 4000 miles or
or so of the Earth's surface. It consists mainly a high-energy protons
(10-50 MeV) and is a by-product of the cosmic radiation, a thin drizzle
of very fast protons and other nuclei which apparently fill all our galaxy.
In addition there exist electrons and protons (and also oxygen particles
from the upper atmosphere) given moderate energies (say 1-100 keV; 1 MeV
= 1000 keV) by processes inside the domain of the Earth's magnetic field.
Some of these electrons produce the polar aurora ("northern lights") when
they hit the upper atmosphere, but many get trapped, and among those
protons and positive particles have most of the energy .
I looked up a typical satellite passing the radiation belts (elliptic
orbit, 200 miles to 20000 miles) and the radiation dosage per year is
about 2500 rem, assuming one is shielded by 1 gr/cm-square of aluminum
(about 1/8" thick plate) almost all of it while passing the inner belt. But
there is no danger. The way the particles move in the magnetic field prevents
them from hitting the atmosphere, and even if they are scattered so
their orbit does intersect the ground, the atmosphere absorbs them long
before they get very far. Even the space station would be safe, because
the orbits usually stop above it--any particles dipping deeper down
are lost much faster than they can be replenished.
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QUESTION
Magnets of Different Shapes
I'm curious as to what effect the shape of a magnet has on the nature of
the magnetic field patterns generated. For example, would there be any
significant or even noticeable difference between a cylindrical bar
magnet with fillet 'radiused' edges and one without the fillets?
Instead of your run-of-the mill bar magnet or cylindrical bar magnet
with flat ends, what can be expected in a spherical magnet, an oval/oblate
spheroid ('jelly bean' shape), a cylindrical bar magnet w/hemispherical
divots scooped out of the ends?
Thanks, -- Rhamis
Dear Rhamis
The shaping of magnetic fields is a complicated art, with
formulas and computer codes. In general engineers separate the source
of the magnetic field--an electromagnet or a piece of magnetized iron--
from the "pole pieces" which shape the field, which are usually made of
soft iron or special alloys and are fitted to the ends of the magnet.
The virtue of soft iron is to confine the magnetic field lines inside it.
So if a magnet has conical pole pieces, tapering to a sharp point, the
iron will try to keep the field lines inside itself even though (as one
approaches the tip) the cross section becomes smaller and smaller. When
the lines finally emerge near the tip (the must emerge somewhere), they form
a tight small bundle, and therefore the magnetic field is much stronger
there--though in a much smaller area--than it would be in the absence of
any pole pieces.
I don't know about spherical and elliptical magnets, but formulas
probably exist for them. Some intricately shaped magnetic fields are
used inside research accelerators which speed up protons and electrons
to very high energies, to keep the beam confined to its vacuum tube,
to focus the beam's particles and to push them out of the machine onto
the target area. As I said, it is a whole science unto itself.
David Stern Back to the main list
QUESTION
Building an electromagnet
My name is Jon and I am a 6th grader. I have an
invention using magnetism to prevent cars from being stolen and to keep them
from bumping into each other. I tried making an electromagnet with a 9Volt
battery, but it wasn't very strong. Can you tell me how to make a stronger
magnet? Can I use a larger battery or real electricity? Thank you,
Dear Jon
I don't know what your invention is, what the magnet is supposed to do.
If you want it to close an electric circuit, you are essentially building
a device known as a relay. You can probably get old relays from a radio
repair shop, or any place which has junked electric devices (cars have
relays, too). Or ask your science teacher for help.
Building electromagnets without calculating and measuring is not simple:
you must match the size of the wire and its length to the source of
current (manufacturers of relays do so, of course). In particular be
cautious about using house current (you call it "real electricity",
but anything you use is real electricity). A small battery is limited
in what it can do--usually, not much. House current is backed by big
power stations, which can pour a LOT of "juice" into whatever you attach.
If your wire is short and thick, it will try to draw a big electric current:
a battery will be unable to provide it, but the power station can and will, enough electricity to perhaps melt a wire and cause a fire, or at
least blow the fuses of trip the circuit breakers which are meant to
protect houses against just this.
Also, house current is backed by a relatively high "electric pressure"
(voltage) and can cause a nasty shock. Finally, even if you got the
magnet working on this, it would hum and jitter, because houses have an
"alternating current", which goes down to zero and up again more than
100 times each second. If you ever heard an electric device humming
(old fluorescent lights somethimes do), that is the reason.
So my advice--stick to batteries, get a relay (you can also disassemble
it and use just its magnet, if that's what you want), and most important,
read and learn. You are just at the very beginning of an interesting
adventure.
David Stern
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