(S-3)   The Magnetic Sun  

"From Stargazers to Starships" home page and index: on disk Sintro.htm, on the web
          http://www.phy6.org/stargaze/Sintro.htm

• That magnetic forces in nature rarely involve iron, but are actually forces between electric currents.

• About sunspots and their intense magnetism, with a strength of about 0.15 Tesla (1500 gauss).

• About the 11-year sunspot cycle and its discovery.

• About "solar activity" associated with sunspots and their cycles, e.g. the abrupt brightenings known as solar flares.

• That solar activity is probably associated with the release of magnetic energy, and that such releases can propel fast plasma flows towards Earth, causing there "magnetic storms."

Terms: Sunspot, magnetic field lines, magnetic fields, sunspot cycle, solar activity, solar flare, magnetic storm, magnetic energy

Stories: The discovery of electromagnetism by Oersted and Ampére, the discovery of the sunspot cycle by Heinrich Schwabe and the discovery of solar flares by Richard Carrington.

    The teacher is advised to read those stories before the class, to help present them to students (students can also be assigned to read the material and make presentations). This material is sufficient for more than one session and the teacher should decide ahead of time, what to include and what to omit. Web pages exist on the original articles by Schwabe and by Carrington, and Oersted's experiment can be easily performed on a tabletop--see end of the above web pageon Oersted and Ampére. For a version performed with a transparent field compass on top of an opaque projector, allowing the entire class to watch it, see here.

    For a discussion of the causes of the Sun's magnetism see "The Sun's Magnetic Cycle" and the section on the dynamo process which follows it in the same web collection. For more details and for some technical references, see the section "Sunspots", part of a more advanced review.

Starting the lesson:

No area of science draws as much interest as stories of discovery, and of the unusual people who made them. This class has already covered some interesting discoveries. Which of them were associated with the names of... Aristarchus? Erathostenes? Columbus (even if it wasn't a scientific discovery)? Copernicus? Kepler? Newton?

Louis Pasteur was a French biochemist in the 19th century, whose many discoveries included a way of preserving food by heating ("pasteurization") and a procedure for saving the lives of people bitten by animals infected with rabies, which up till then meant almost certain death. Commenting on scientific discoveries, Pasteur said "chance favors the prepared mind". Discoveries often depend on luck--but luck is not enough, the mind must be prepared to exploit its opportunity.

[a student might prepare a poster with that quote, to hang in class]

Today as we discuss magnetism and magnetic phenomena on the Sun, we will discuss three discoveries in which luck had a part--but luck wasn't the only reason. (List on the board--students copy.)

1. The discovery of electromagnetism in 1820 by Hans Christian Oersted, in Denmark, and its explanation by Andre-Marie Ampere, in France.

2. The discovery of the sunspot cycle (1843, widely accepted around 1851) by Heinrich Schwabe in Germany.

3. The discovery of solar flares (1859) by Richard Carrington in England

(possible comment: Each discovery occured in a different country!. Science is truly international.)

    The discovery of the connection between electricity and magnetism

What do we know about magnetism of iron magnets?

• Like poles repel, unlike poles attract.
• The magnetic compass tends to point north-south.

[Why is plain iron attracted to a magnet? Because when iron is in the region of influence of a magnet--its "magnetic field"--it becomes temporarily magnetic itself, with the pole closest to a pole of the magnet having opposite polarity, causing it to be attracted.]

What magnetic phenomena do not involve iron, and why are they called "electromagnetic" phenomena?

• The attraction between parallel currents in the same direction, and the repulsion between them if the currents flow in opposite directions.
      The forces between the coils, etc. used in electric machinery all follow from this basic property.

                Then tell the story of Oersted and Ampére, on "how the connection between electric currents and magnetism was discovered", or else have students who have prepared the story tell the class. (The teacher might also mention that the American Association of Physics Teachers gives each year the "Oersted Award" for an outstanding contribution to the teaching of physics.)

(optional)

    The magnetic field which satellites observe in space is often different from what one would expect, based on the fields we observe on the ground.

    The reason is that large electric currents often flow through the space surrounding the Earth, and they contribute their own magnetic fields as well. The currents can flow there, because of the presence of a mixture of free electrons and free positive ions (a "plasma").

    What to you think--would such currents tend to spread out to cover as much space as possible, or would they narrow down to string-like filaments? You must give a reason.

[If no one answers:"It has to do with the forces between electric currents."]

• They might tend to narrow down. A current flowing along a wide tube may be viewed as composed of many narrower parallel currents, and we know that parallel currents attract each other.

(This tendency of currents to narrow down has been used to compress laboratory plasmas, where it is named the "pinch effect.")
            (end of optional part)

    Suppose you had a compass needle able to point in any direction in space, not just horizontally. How would the northward-pointing end of the needle point at

1. The northern magnetic pole
2. The southern magnetic pole
3. Halfway between
4. Elsewhere?

[Note: such needles are available--see end of section S-3, or click here: http://www.cochranes.co.uk/BNRVP30/edu5.htm]

• Straight down at the north magnetic pole, straight up at the southern pole, horizontally northward at the magnetic equator, northward slanting down in the northern hemisphere, northward slanting up in the southern hemisphere.
      (By the way: the early Chinese who discovered the magnetic compass claimed it pointed south.)

How would such a needle point near a straight wire carrying an electric current? (Neglect the Earth's magnetism)

• Perpendicular to the current.

What are magnetic field lines? Base your definition on the magnetic needle described above.

• They are imaginary lines giving at each point the direction in which our 3-D needle would point, if placed there.

What are magnetic field lines used for?

• They were originally used to graphically describe magnetic fields. In the rarefied plasmas of space, however, they also guide the flow of particles and currents. This is why arching solar formations above magnetic sunspots sometimes resemble the field lines of bar magnets.

  (The guiding property of field lines also makes possible the trapping around Earth of ions and electrons in the Earth's radiation belts. The motion of these particles stops and reverses before they hit the Earth, because they are also reflected from regions of stronger magnetic field, found closer to Earth.)

What is Andre-Marie Ampére remembered for?

• He explained the observations which baffled Oersted, and in doing so gave the first clear idea of what magnetism was. In his honor the unit of magnetic current is called the "Ampere."

[P.S.: He is also remembered as the man who was invited to dinner with Napoleon and forgot to go!]

The intensity of the Earth's magnetic field at the magnetic equator is about 30,000 nT (nanotesla) or 0.3 gauss. The field intensity goes down with distance r like 1/r³. If the intensity of the interplanetary magnetic field at the Earth's orbit is 5 nT (a typical value), at what distance--in Earth radii--is this matched by the Earth's field? (Needs calculator capable of extracting cube roots, or raising to the 1/3 power.)

• If at a distance of 1 Earth radius the field is 31,000 nT, at distance of r earth radii is is 30,000/r³ . If r is the distance where it drops to 5 nT, we get 30,000/r³ = 5 . Multiply both sides by r³, divide by 5 to get

Then go on to the discovery of sunspots.

When and how were sunspots discovered?

• In 1609, when telescopes were first used in astronomy. Galileo, Fabricius and Scheiner all claim credit and might be independent discoverers.

What did the discoverers see?

• They saw dark spots on the Sun.

How do we know that the Sun rotates around its axis?

• Sunspots were observed to travel across the face of the Sun in a way that suggests they rotated with it.

What is unusual about the Sun's rotation?

• The rotation period depends on latitude: the equator rotates fastest, in about 27 days. Nearer to the pole it can be 2.5 days longer.

Optional: The teacher may draw a table with the observed latitude dependence of the rotation period (in days) and let students graph it:

Question: Why is the period viewed from Earth longer by about 2 days?

• Say we track a sunspot at latitude 20°, which at a certain time faces Earth. After 25.65 days the spot again faces the same direction in space as before--in the direction of the same stars, for instance. However the Earth orbits the Sun, and has by then moved ahead in its orbit. It takes the Sun's rotation 2 more days to reach the position where the sunspot faces Earth again.

(end of optional part)

    Here tell the story of Schwabe's discovery of the sunspot cycle.

Why do we think that magnetism plays an important part in solar phenomena?

Some clues:
• From changes in the light of sunspots, it was found that they were intensely magnetic.
      (Only if the question arises: the atoms of each element emit light of characteristic, narrowly defined colors, their "spectral lines." These change subtly when emitted from a strong magnetic field.)

• During a total eclipse, the "plumes" of the corona above the poles and the "arches" above sunspot regions suggest the form of magnetic field lines near magnets.

• Eruptions on the Sun can cause (somewhat later) "magnetic storms" on Earth.

What suggests that the 11-year sunspot cycle is a magnetic phenomenon?

• Sunspots are intensely magnetic

• In each sunspot cycle, the magnetic polarity order of the leading and trailing sunspots in sunspot pairs reverses.

• Each sunspot cycle the large-scale polar magnetic field of the Sun reverses.

    [Optional: Does the Earth's own magnetic field ever reverse?

Note: This subject is extensively discussed in Magnetic Reversals and Moving Continents, part of an extensive web-collection "The Great Magnet, the Earth" whose home page is here.

Yes, it apparently does, but at very infrequent intervals. That magnetic field is caused by electric currents in the Earth's molten core, and is observed to change slowly from year to year. On rare occasions, however, the dominant pattern of currents seems to reverse direction.

    How do we know? We know because the floor of the oceans contains fissures, caused when the sea-bottom is pulled away in opposite directions, away from them. The pulling-apart is caused by forces deep inside the Earth, and a typical speed of those "plates" is about one inch a year. The most famous fissure is the one running down the entire middle of the Atlantic ocean, in a roughly north-south direction but with bends and kinks.

    As the seafloor plates are pulled away to both sides, lava comes up from the fissure and solidifies into a black rock called basalt, which become part of those plates. However, basalt is weakly magnetic, and when it solidifes it becomes magnetized in the direction of the magnetic field existing at the time. Thus each part of the seafloor records the Earth's magnetism at the time it emerged from the fissure, just as the tape of a tape recorder records the magnetism of the recording head at the time it passes near it.

    In this way, the seafloor has recorded the magnetism of the Earth during the last 10-20 million years. It turns out that the seafloor is not magnetized in a uniform direction! Rather, it is magnetized in long parallel stripes, parallel to the central fissure, and each two neighboring stripes have opposite magnetic polarity. This suggests that between the times those two stripes emerged, the Earth's magnetic field reversed direction. The last time that has happened (assuming a rate of about an inch a year) was about 700,000 years ago.]

(End of optional section)

Why are sunspots dark?

We really do not know!

   Actually, they shine quite brightly, too, but not as brightly as the surrounding non-sunspot areas. They are obviously somewhat cooler than the surrounding areas, because something slows down the flow of heat. One guesses the magnetic forces do it, but it's not clear how. Nor do we know how deep the roots of sunspots go: at one time they were believed to be quite shallow, but the matter is still debated.

---

What is a solar flare?

    A sudden brightening near prominent sunspots. Big flares are associated with acceleration of ions and electrons to high energies: the ions may reach Earth and beyond, and big outbursts may be dangerous to astronauts on the way to the Moon or Mars (or after arrival there). The electrons usually collide with atoms near the Sun and produce bursts of X-rays.

    Big flares have been also associated with fast outflows of gas from the Sun, which when it hits Earth can produce a magnetic storm, a large disturbance in the trapped radiation around Earth. The fast flows are also associated with CMEs--Coronal Mass Ejections, huge bubbles of hot gas, which are the start of such outflows, and have been observed from satellites.

    It is not clear what the relation is between flares and CMEs. Both happen so rapidly--time scale of minutes, and x-rays after seconds--that the general opinion is that they represent a release of magnetic energy from the strong field of sunspots, which has (for some reason) become unstable. Small flares and "subflares" occur frequently.

If time allows, the teacher or students who have prepared for it may
tell the story of Richard Carrington's flare observation in 1859.