|
|
This workshop is titled "From Sun to the Earth." Accordingly, this talk will discuss two main question areas:
(1) What exactly is the Earth's magnetosphere, what are its interactions with the Sun and why should these be of interest to a non-scientist? And: (2) What of all that should be presented to students in middle and high school? Answering the first is relatively straightforward, but the answer is long, it's a big area. Don't expect more than a general outline and leads to where to find more. I hope you find the time and the will to follow those leads, because teachers need a deeper understanding than what students see. Answering the second is tough. The physics curriculum in high school is generally limited to one year; so even though it is jam-packed, it still misses a lot. If you add any new topic, an existing one may end up pushed out. The earth sciences class often taught in 9th grade may be more accommodating, but of course students know little physics at that stage, and the same goes for earlier grades. Some of what I am going to discuss is meant for regular high school classes in physics and earth sciences. However, these sites have an additional use. Most classes will have a few strongly motivated students, who feel held back by the pace of the class and get bored. They would like to rush ahead--but of course, the teacher cannot speed up to where the majority is no longer comfortable. Instead, they can be sent to explore on their own areas such as magnetic fields on Earth and in space, and in a moment I will explain how. Why was I asked to talk to you here? Because I have created three very extensive courses on the world-wide web, possible resources for such studies.
I am a retired magnetospheric physicist, and the first collection, completely non-mathematical, was designed to open magnetospheric physics to anyone with enough motivation and patience. It is titled "The Exploration of the Earth's Magnetosphere" and contains around 80 illustrated files. Students have a lot here they can study on their own, such as an article "Secrets of the Polar Aurora, " a very extensive discussion of the polar aurora or "northern lights." Or else, "The Birth of a Radiation Belt", describing one violent "space weather" episode which in a matter of seconds added a new radiation belt around Earth (which by the way led to the destruction of a satellite). Or else read about electrons, ions and plasmas, study their motion in magnetic fields, investigate the origin of the aurora... even browse through a long list (75 items now) of questions received from users, with answers.
A second collection is "The Great Magnet, the Earth" It discusses the magnetism of the Earth itself (also, of the Sun--there exists a connection) and is therefore useful for use in Earth Sciences class. It includes guidance for Earth science teachers who might want to use it. Slide 4--home page of "From Stargazers to Starships" The third and largest is "From Stargazers to Starships," an algebra-based course on basic astronomy, Newtonian mechanics, solar physics and spaceflight. It deserves a separate talk, at least, but since it touches on solar-terrestrial physics only peripherally, and I was asked here to tell you only about the magnetosphere and the Sun, I will leave it out for now.
The Earth acts like a giant magnet, behaving somewhat as if it had a powerful bar magnet at its center. Just as a bar magnet does have two poles where its attraction is strongest, so does the Earth, and its two magnetic poles are near the two geographic poles, which define the axis of rotation of the earth. "Near" here means about 1000 km away. The needle of a magnetic compass points towards those poles, which is how the Earth's magnetism was first noticed--probably in China, some 1000 years ago.
Now the magnetic field--the region of magnetic influence--is 3-dimensional. A compass needle must be horizontal, but similar needles with a horizontal axis, free to point up or down at any angle, showed that the magnetic force actually did not just point northward but also into the Earth (north of the equator). The closer one got to the magnetic pole, the steeper was the inclination, until at the northern magnetic pole, the needle pointed straight down. That in fact was how the north magnetic pole was located in 1831, by an expedition funded by Booth's whisky distillery, which is why the location is now called the Boothia peninsula.
Michael Faraday had the bright idea of describing magnetic fields in 3-D by imaginary lines which are everywhere lined up with the magnetic force, the direction shown by a magnet freely suspended in 3-D. (By the way, you can buy such magnetic probes, on gimbals, for the classroom--the source is listed on the web site). He called them "lines of force." Where the lines lie on a flat plane, you can outline them by iron filings, as kids do in the classroom.
We nowadays call such lines "magnetic field lines" and they are extremely important in magnetic fields in space, because they control the physics. Most space is filled with a gas (usually rather rarefied) consisting of freely floating electrons and positive ions--atoms which have lost one or more electrons, giving them a positive charge. Such a gas is called a plasma, it conducts electricity and tends to get attached to magnetic field lines.
Let me pause here for some comments on the teaching of physics. Among my books, I have somewhere a 1913 edition of the physics text by Millikan and Gale (the famous Millikan who measured the electric charge of one electron). Printed in black and white, small enough to fit a coat pocket, it covers its physics pretty well, even though its pictures of airplanes and cars will make you smile. Today's students actually seem to get less physics, because so little time is provided. Yet we now have much more physics to teach! The subject has expanded tremendously! There was no magnetospheric physics in 1913--besides electronics, nuclear energy, black holes, and so forth. How can a teacher cover it all?
To be honest--I don't know. I don't think it can be done. The most you can do is arouse the interest of your physics students, and hope they continue on their own. To do so, it is essential that they be good readers, because almost all the material is only in books. Indeed, to master any technical subject nowadays, one has to be a fluent reader, another subject where schools seem to fall behind. Without reading, a kid's future career may be on the loading dock or in a hair salon.
As part of the "new physics" added since 1913, you expect students to have some understanding of atoms, electrons, ions and plasmas. It is very important that any time a new term is introduced, it is clearly defined. Take "magnetic field." Many teachers just casually drop the term, as in "today you will learn about the magnetic field of the Earth." You will note that this was not done here, but a magnetic field was defined as "the region of magnetic influence," where magnetic forces may be observed.You should do no less in your class. Actually, "field" means more--in the 20th century, it became to be understood as "a region where space itself is modified." An "electromagnetic field" is then a region where space is modified by linked electric and magnetic forces, and it can propagate electromagnetic waves--such as radio, microwaves, light or x-rays. You will find these subjects addresses in "Exploration of the Earth's Magnetosphere" and also in sections of "Stargazers" about the Sun. You will see how clues for atoms arose from chemistry, for electrons you can tell about an interesting experiment by Edison (although there is much more!), and that free ions were found in "alpha particles" emitted by radioactive substances, although much earlier chemistry had already found ions in solutions of acids and salts in water. It is next to impossible to give kids a broad understanding of all these. But at least let them realize, that claims of science are based on observations--observations which often leave no room for alternative explanations.
As an example--how do you tell your students about plasmas? The simplest is tell them about the fluorescent tube (and make sure they do not spell it "fluorescent", as in "flour"!). "Exploration" has a section on that. In the gas inside a fluorescent lamp, there are always a few electrons and free ions, created by radioactivity and other causes. You now apply an electric voltage--and you define voltage as a "sort of electric pressure," one that drives currents through wires, just as pressure along a pipe can drive a flow of water through it. The voltage accelerates electrons and ions, they collide with atoms, knock off more electrons so that more and more are created, driving a bigger and still bigger electric current. Ultimately something will limit the current--you don't want wires to melt (or circuit breakers to turn off the flow), so a ballast coil is used (or more recently, special electronics). The plasma current then stabilizes, and as new ions are created, "old" ones recombine with their electrons, and in the process, produce light. Some of that is ultra-violet, but the whitish "fluorescent" paint inside the tube converts this into visible light, creating a very efficient la mp. Unless you give such examples, "plasma" is just a word found in the book. The magnetosphere of course is filled with a plasma of free electrons and (mainly) protons, ions of hydrogen. As noted earlier, all these are attached to magnetic field lines, a bit like beads on a wire. Unlike beads, though, they can sometimes jump from one field lines to the one next to it.
This attachment can work in two main ways. If the magnetic field is strong, it forms a rigid framework that can trap the plasma, which is what you get in the radiation belts. If the magnetic field is weak, and the plasma contains more energy-per-unit-volume than the field, then the plasma dominates. For instance, if the plasma flows, it will bodily drag the field lines with it, deforming them, and section (18A) of "Exploration" has a paper exercise on this effect.
... So field lines are important. Around 1850 or so it became evident that the polar aurora--popularly called the "northern lights"--had some sort of connection with the Earth's magnetic field. The aurora in the far north usually appears as greenish ribbons, predominantly in the east-west direction. These ribbons however consist of many transverse rays, dimming and brightening all the time, and it turned out that the rays were always parallel to the local magnetic field line.
Also, the likelihood of seeing an aurora depended on distance, not from the geographical pole but from the magnetic one. There exists an "auroral zone" where the likelihood is greatest, a circular belt about 2000-2300 km from the magnetic pole, and at greater or smaller distances, the probability quickly drops. The "lower 48 states" are likely to see aurora mainly during big magnetic storms, when electrical currents in space shift the auroral belt to lower latitudes. Still, the US is lucky here--the magnetic pole is on our side of the geographic pole, so one does not have to go as far northwards to see aurora as one does in Asia (on the opposite side) or even in Europe.
But in addition, the 11-year sunspot cycle was discovered--not by an astronomer, but by a German pharmacist. Heinrich Schwabe was an amateur who chose as his task the search for an unknown planet inside the orbit of Mercury, for which the name Vulcan was proposed. The glare of the Sun would make it hard to see such a planet (except during a total eclipse), so Schwabe tried to catch it while passing in front of the Sun's disk, the way Venus did during its recent transit. The planet would appear as a dark spot , and to tell it apart from sunspots, Schwabe started tracking sunspots, too. He never saw Vulcan--it does not exist--but after 17 years he realized the number of sunspots went up and down in a definite cycle.
That was in 1843, just a few years after the first world-wide network of magnetic observatories was established--that story is on the web, too--and these recorded events when the magnetic field simultaneously underwent a world-wide change. The change was small, typically a weakening by a fraction of 1%, but the pattern was identical, all around the world, so it clearly was something BIG. Alexander von Humboldt named those events magnetic storms, and around 1852 Sabine in Britain found that their likelihood was greatest near the peak of the sunspot cycle. Magnetic storms could also cause displays of the polar aurora far south of their usual location, so indeed, something was happening in outer space. A few years later a British astronomer, Richard Carrington, was observing a group of large and active sunspots, when suddenly a bright spot of light appeared among them. Luckily, some other astronomer also saw it, so there was no doubt, and it seemed hard not to connect it with a large magnetic storm which erupted less than 24 hours later, with aurora seen as far south as Cuba. The eruption which Carrington saw is now called a solar flare, and using special light filters, flares of all sizes can be regularly monitored; only very rarely can some be seen in unfiltered light, as in Carrington's flare. They do occur near sunspots, and are definitely associated with magnetic storms. The association was better understood after the discovery of coronal mass ejections or CMEs, huge bubbles of hot gas, blown away from the Sun. Their first extensive observations were made in 1973 from the space station Skylab.
The big plasma bubbles apparently are the cause of big magnetic storms. In addition, however, flares or associated events--scientists still argue here--also accelerate ions to fairly high energies. So high, that some can even be occasionally detected on the ground, even though the atmosphere has the shielding power of about 10 feet of concrete. To astronauts on Mars, or on the way to Mars, such radiation can border on the dangerous (depending on how big the event is). No wonder NASA is interested in what it calls "Space Weather." So magnetism, the aurora, sunspots and magnetic storms were all somehow closely connected. Sunspots, too, were intensely magnetic areas, as analysis of their light showed. What was going on? Let me now fast-forward over the next century!
A Norwegian physicist named Kristian Birkeland traced the motion of electrons in a vacuum chamber, with just enough gas left in it to light up along the path of those electrons. They seemed to be guided by magnetic field lines to the region near the poles, somewhat like the aurora. Birkeland suggested that sunspots threw out clouds of electrons, some of which reached Earth and were then guided magnetically to the polar regions, and that was how we got the aurora. But calculations showed clouds consisting just of negative electrons could not exist, electrons repelled each other too strongly. Two British scientists, Sydney Chapman and Vincent Ferraro, made the case in 1930 for clouds containing equal amounts of negative electrons and positive protons, what is nowadays called a plasma, and we now know that magnetic storms indeed start with the arrival of such a cloud. After this the process gets complicated, but one result is that a large number of positive ions is injected onto field lines near the Earth, where they get trapped. The trapped ions create a huge electrical current circling the Earth's equator (at distances between 2 and 6 Earth radii), called (naturally!) the "ring current." Another result is that some electrons, already residing on magnetic field lines of the Earth, do get accelerated downwards along those lines, hit the atmosphere and cause the light of the aurora.
Those details, however, were still unknown in 1957-8, chosen as the first "International Geophysical Year" (IGY), when the first Earth satellites were launched. Russias first 3 Sputniks, and Explorers 1 and 3, built by the team of James Van Allen at the University of Iowa. Again, I must skip many juicy details, and can only state here that the Explorers discovered the inner radiation belt, of fast protons trapped by magnetic field lines of the Earth. In the years that followed, other satellites observed the outer radiation belt--that's where the ring current is found, the long magnetotail on the night side where much of the auroral originates, and a complicated interplay of plasmas and electric currents, which we are still trying to understand.
The year 1958 was also when the solar wind was first proposed by Eugene Parker. It had been observed that the outermost atmosphere of the Sun is very hot, about a million degrees. What heats it remains a mystery--it is much, much hotter than some layers below it--but what Parker realized was that the gravity of the Sun could not possibly hold on to such a hot atmosphere. Instead, it gets blown off at a tremendous speed, though it is so rarefied that the process can go on and on for a long time. Some of the earliest evidence for it came from comet tails, as is demonstrated on this slide, of Comet Hale-Bopp of 1997. Note it has two tails, in different directions. The white one is dust, believed to be blown away from the Sun by the pressure of sunlight. Its spectrum is the same as that of the Sun. The other has been identified by its spectrum as consisting of ions. Ions are not easily pushed by sunlight, but are easily moved by the solar wind. They come at a different angle, because the solar wind only moves at about 10 times the speed of the comet, and the velocities combine to give an angle that is not exactly away from the Sun. By the time the solar wind reaches the Earth's orbit, blowing at about 400 kilometers per second, it is quite rarefied, and the Earth's magnetic field manages to keep it off, carving its own cavity in the solar wind. Tommy Gold of Cornell in 1959 named this region "magnetosphere" and we still use his term, although the shape is anything but spherical, in particular, on the night side it has a long "magnetotail" whose field lines stretch on and on.
The magnetosphere is big. On the sunward side its boundary--called the magnetopause--is about 11 Earth radii or 70,000 kilometers distant, it grows and shrinks with the velocity and density of the solar wind, which determine the pressure on the magnetopause. On the sides it reaches about 15 Earth radii--a quarter of the distance to the Moon--and on the night side a long tail extends very far, well past 200 Earth radii. Over the years satellites have explored that region and found a complicated system of electric currents, driven by the solar wind. The trapped plasma of the outer belt carries the ring current, which gets much stronger during magnetic storms. Other currents flow in and out of the auroral zone, and their flow also causes much of the aurora. During magnetic storms the field is deformed and the position of the currents is temporarily shifted, which is why aurora is then seen closer to the equator.
Some of these currents are shown in the simple 3-dimensional cutout model of the magnetosphere, which you can download from "Exploration" and xerox for your students.
You tell them, the magnetosphere is important to us, because:
--- It is an interesting region of space, dominated by electric and magnetic phenomena, not by gravity. --- It is the most accessible example of "cosmic-scale plasma" which we can study. --- It has unique links to the solar wind and to the Sun. --- It is a frontier of science: we still try to understand the polar aurora, magnetic storms and our boundary with the solar wind. ---- ----- ----- ----- -----
Mainly: Magnetism is nothing mysterious. Here is some of what we know:
--- The true nature of the magnetic force is the attraction or repulsion between electric currents. That was discovered by Oersted and by Ampére in 1820. --- The Earth is magnetic because electric currents flow in it (and the same with sunspots). There is no magnet inside it (it has a lot of iron, but it's too hot). --- Electric and magnetic phenomena are closely linked. That link not only makes electric machinery possible, but also radio, microwaves, light and x-rays, all of which are "electromagnetic waves". --- Magnetic fields have no effect on your living tissue. Magnets don't do a thing for your health (or for your car's gas mileage). To most students, and most citizens, magnetism is a complete mystery--associated with a mysterious property of specially treated iron. People still buy magnets to wear as bracelets for health, or tuck in their mattress, or attach to fuel lines of their cars to get better gas mileage. From the e-mail I get, they are also worried about the Earth's magnetic field reversing and dire effects this is said to produce, such as exposing us to radiation from space (Hollywood has not helped here!). By teaching about magnetism and showing it is actually related to electric currents and well understood, teachers can fill an important gap. For instance, they can tell the story of Oersted and repeat his experiment on a vu-graph (the sites tell how). They can then tell how magnetic fields have no significant effect on the body, even in the strong field of an MRI machine (Q&A #52 in "Exploration", also #64). Also, even if a magnetic reversal removes our magnetic shielding (as happens even now in some polar regions) we are still protected by the atmosphere (Q&Q #1, especially #1B). It helps to understand magnetism.
In conclusion, I hope you realize how big this subject is, and anything I can tell about it is sketchy and incomplete. But you will find more on the web.
One last item, of more local interest, having to do not with the magnetosphere but with spaceflight and space exploration in general. We are here within driving distance of Worcester, Mass, the home town of Dr. Robert Goddard, close enough perhaps for people to have a local interest in him. And we are about 3 weeks away from October 19, which was Dr. Goddard's personal holiday, his "Anniversary Day." On that day in 1899, Robert Goddard, age 17, sitting in a cherry tree and daydreaming (instead of pruning dead branches, as he was supposed to do) had a vision of spaceflight, and decided to dedicate himself to its realization. "I was a different boy when I climbed down from that tree" he later wrote. You might want to use the occasion to tell your students about Robert Goddard, and maybe get them more interested in space. You will find more on the web, at http://www.phy6.org/stargaze/Sgoddard.htm
(Thank you)
|
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: earthmag("at" symbol)phy6.org
Last updated 14 September 2004