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Magnetism and the
Sun
Audience | Goals | Magnetism | Magnetic Fields | Magnetic Force |
Stored Energy in Magnetic Fields | Magnetism & the Sun | Further
Exploration | Bibliography
Audience:
This activity is targeted towards high school students who have taken or
are taking pre-calculus.
Goals:
Through this activity, students will learn the basic principles of magnetism
and how they apply to the Sun.
Magnetism
Why study magnetism?
Magnetism is important in the study of the Sun, since it plays a key role in
the dynamics of its surface. It is in part responsible for eruptions called
coronal mass ejections that release high amounts of energy into space. If
the radiation from these eruptions reach Earth, they can damage satellites,
interrupt electronic communications, and even bring down electrical
power grids. Like meteorologists who study thermodynamics in an attempt
to predict the weather, solar physicists study magnetism on the Sun in
hopes of understanding the "space weather" created by the Sun.
What is magnetism?
Magnetism is phenomenon that arises out of the movement of electric
charge, a fundamental property of matter. It creates a magnetic force, a
"push" or "pull", on objects with moving electric charge.
Magnetism can also be demonstrated with a pair of magnets. Although
currents, or a moving flow of electrons, are not present within them,
magnets do in fact have moving electric charge at the atomic level. The
electrons within the magnets are thought to spin in the same direction,
resulting in a magnetic field. As a result of this self created field, each
magnet has a polarity, or two poles, north and south. When a pole of a
magnet is brought to the same pole of the other, the two magnets repel.
When the poles are different, the magnets attract. As a pair of magnets can
show, the movement of electric charge is the driving force of magnetism,
which we will continue to study in further depth.
Magnetic fields
One way to describe magnetism is through magnetic fields. A magnetic
field defines the magnetic force, the "push" or "pull," felt by a particle
independent of its charge and velocity (the speed and direction of the
particle) due to the presence of other moving charges. The stronger the
field, the stronger the magnetic force felt by the particle. Likewise, the
weaker the field, the weaker the magnetic force.
Magnetic fields can be visualized using magnetic field lines. These lines
are curves where:
1. At every given point the tangent (the line that intersects the curve
only at the given point for an infinitesimal distance) is in the same
direction as the magnetic field. If one were to place a compass in a
magnetic field, the needle would point tangent to the magnetic
field line.
2. The magnetic field magnitude is proportional to the density of the
lines. The closer together the field lines, the stronger the magnetic
field. The more spread out they are, the weaker the magnetic field.
Magnetic field lines on the Sun
The image above is from a computer simulation of how the Sun's corona,
or outer atmosphere, is heated by its "magnetic carpet." The loops that
extend into the corona are magnetic field lines that join the north and
south poles in the "magnetic carpet".
Observing magnetic fields experiment: The magnetic fields of a magnet
can be indirectly observed using iron filings.
Equipment:
bar magnets
a piece of cardboard much larger than a bar magnet
iron filings
a table
Assembly:
1. Place a bar magnet on the table
2. Place the cardboard on top of the magnet.
3. Scatter the iron filings evenly over the cardboard.
Procedure:
1. Observe the iron filings. Are they pointed in any certain direction?
Where do the filings concentrate, near regions of weaker or
stronger magnetic field?
2. Using the pattern formed by the iron filings as a guide, draw a
magnetic field line diagram for the given magnet.
3. In your magnetic field line diagram, make note of where the field
lines are spread apart or concentrated. Using a pair of magnets,
determine which parts of a magnet are stronger or weaker.
Establish a relationship between the magnetic forces you felt using
the pair of magnets and the concentration of field lines in your
diagram.
What's Going On?
The magnetic field created by the bar magnet induces a magnetic field in
the iron filings. The iron filings become little magnets. Because the iron
filings are rod-shaped, they have a tendency to form their poles in a
lengthwise direction. As a result, they point in the same direction of
applied magnetic field of the bar magnet, just as a compass needle would
do when brought next to a magnet.
Magnetograms
Magnetic fields can also be visualized as
magnetograms, which are used to make observations
of the Sun. Magnetograms are visual representations of
the polarity and the strength of magnetic fields that
point directly away or towards the observer. Black
regions have the strongest southward field (the field
points away from the observer and into the Sun), and
white regions have the strongest northward field (the field points towards
the observer and away from the Sun). Gray areas have little or no
magnetic field. One such instrument that generates magnetograms from its
observations is the Michelson Doppler Imager (MDI) aboard the SOHO
satellite. Daily magnetograms from this instrument are available.
Magnetic force
Using the idea of fields that we explored in the previous section, the
magnetic force exerted on a given particle can be expressed as follows in
SI units:
Fmag = qvBsin (1)
Where q is the amount of charge, v the velocity of the particle, B the
magnetic field, and  the angle formed between the velocity and magnetic
field directions.
If you are familiar with vectors and vector math, the magnetic force
equation can be expressed in a more elegant form in SI units as:
Fmag = qv x B
In circuits, electrons, which have a negative charge, are moving through
the wires. This flow of electrons, a unit charge per unit second, is called
current. As a result of this current, magnetic fields are created around
wires. Hans Christian Oersted (1777-1851) stumbled upon this
phenomenon after a lecture that he had given. He observed that if a
compass was placed parallel to a wire with flowing current, the compass
needle would deflect. Perhaps the reason why magnetic force was not
detected in wires until Oersted is because of the large amount of current
needed to create a measurable magnetic force. The relative weakness of
magnetic force, compared to electric force that arises out of the presence
of electric charge, can be seen in the next experiment with a wire coil and
magnet.
Force between a wire coil and magnet: The fact that moving charges
create magnetic force can be seen in a wire coil with current flowing
through it and a magnet.
Equipment:
Thick magnet wire for the coil (around #20 gauge)
Thin magnet wire that is light and flexible (around #36
gauge)
A Magnet
Power supply
Stands (to hold the coil)
Thread (to suspend the coil from the stand)
Wires with alligator clips
Graph paper
A flat surface such as a textbook (should be able to hold the
graph paper vertical)
Assembly:
1. Wrap the thick magnet wire around a cylindrical
object such as a pen and count the number of turns
made. Make thirty to forty turns. Be sure that the
wire is wrapped evenly. Record the number of
turns.
2. Trim the ends of the wire so that they are about the
same length. Burn off the enamel on the ends by
passing them through a lighted match. Another way
to remove the enamel is by scrapping it off using a
pair of scissors or pliers.
3. Carefully remove the coil from the cylindrical
object so that its shape is preserved.
4. Tie one end of the string around the middle of the
coil so that when suspended, it hangs horizontally.
Tie the other end to the stand, suspending the coil.
5. Remove the enamel on the ends of two thin magnet
wires in the same way that enamel was removed for
the thick wire. Connect an end of a thin wire to one
end of the coil. Do the same to the other coil end
with the other thin wire. This allows the coil swing
easily without being restricted by heavy wires.
6. Connect a wire with alligator clips to one thin wire.
Do the same with the other thin wire. Make sure
that the thin wire is long enough, so that the metal
of the alligator clips does not interfere with the
magnet when it is brought up to the coil.
7. Connect the other ends of the wires with alligator
clips to the power supply, forming a circuit.
8. Tape the graph paper to the flat surface and stand
the surface up behind the coil. With a pencil, place a
mark on the graph paper directly behind the right
side of the coil. This allows us to measure the
distance the coil has moved when a magnet is
placed next to it.
Procedure:
1. Turn on the power supply. Turn the current up to
one ampere or the highest setting if one ampere
cannot be reached. A current will flow through the
coil, resulting in a magnetic field.
2. Bring a magnet close to the right end of the coil.
Does the coil repel or attract?
3. Orient the magnet so that the coil is attracted to the
magnet. Being careful to see that the thin wires are
not pulling the coil, pull the coil to the right from its
initial position using the magnet. Place a mark on
the graph paper at the location of the right side of
coil where the coil falls away. Also write the
amount of current being provided by the power
supply next the mark.
4. Decrease the amount of current provided by the
power supply. Repeat steps two and three. Continue
decreasing the current until a satisfactory number of
measurements are made.
5. If you have extra magnet wire, make another coil,
this time with a fewer number of turns and conduct
the same experiment (steps one through four) as
before.
What's Going On?
According to Equation 1, Fmag = qvBsin, magnetic force
arises out of the movement of electrons in the coil (the qv
part) and the application of a magnetic field supplied by the
magnet (the B part). The movement of charge through the
coil, which also gives rise to the coil's own magnetic field,
and the magnetic field of the magnet is why a force is felt
between the two object. As for the need to coil the wire,
coiling allows more charge to flow through the wire in a
smaller volume, thus increasing magnetic force. If we were
to use a straight wire, a magnetic force would still be
present, but it would be so slight that it could not be
observed easily. The coil falls away at a certain point
because gravity is pulling down on it, and the magnetic
force between the coil and magnet is great enough to
overcome the opposing gravitational force.
Calculations:
Using the data obtained from the experiment, show that the
magnetic force is directly proportional to the amount of
current provided (which is related to qv in Equation 1).
Also, if more than one coil was made, establish a
relationship between the number of coils and the strength
of the magnetic force.
Applying this idea to the Sun, we see that magnetic fields and force arise
out of the movement of charges through its highly conductive plasma, just
as magnetic fields and force arose out of the movement of current through
the coil.
Stored energy in magnetic fields
The final aspect of magnetism that is necessary to have a basic
understanding of the dynamics on the surface of the Sun is the idea that
magnetic fields can store energy. The energy stored in a magnetic field is
essentially the total amount of work required to assemble a system of
moving charges. Stored energy in magnetic fields can be illustrated in the
following experiment with a pair of magnets.
Magnet Acrobatics:
Equipment:
Two magnets
Procedure:
1. Place a magnet on the table and hold it in place with
one hand.
2. Orient the other magnet above the one on the table
such that each pole faces the same pole on the other
magnet.
3. Bring the top magnet down onto the other one on
the table. The magnets should repel each other as
you do this. Hold the stacked magnets together.
4. Let go of the magnets, being sure to move your
hands out of the way. The magnets will perform
their acrobatics.
What's Going On?
When the two magnets are brought together, a force must
be exerted for a given distance, i.e. work is being done to
bring these two magnets together. Energy in the form of
moving magnets is released when the one lets go of them.
This release of energy causes them to jump.
Magnetism on the Sun
Combining our understanding of the basic features of magnetism,
specifically magnetic fields, magnetic force, and the storage of energy in
magnetic fields, we can now focus on magnetism particular to the Sun.
Magnetic reconnection
One important aspect of magnetism on the Sun is magnetic
reconnection, which can be better understood if we first
examine what plasma is. Plasma is a state of matter
occurring at high temperatures where electrons are not
bound to the nucleus. As a result, ions and electrons are
free to move about the material. The free movement of
charges makes plasma highly conductive, thereby causing
magnetic field lines to be "frozen" into the plasma.
In reconnection, fluid motions in plasma bring together two
"frozen" and oppositely directed magnetic field lines. These
field lines then reconnect into a lower energy state. As we
found out in the Magnet Acrobatics activity, magnetic
fields can store energy. Energy is stored in reconnection
when the "frozen" field lines become distorted as a result of
fluid motion. Reconnection reduces the amount of
distortion, which in turn causes energy to be released. This
can be illustrated in the following activity with rubber
bands.
Reconnecting Rubber Bands - Since magnetic reconnection
occurs in plasmas, which cannot be feasibly produced in a
high school lab, we will have to be content with an activity
using rubber bands to model magnetic reconnection. This
activity requires two people.
Equipment:
A flat rubber band
Two binder clips
A marker
A pair of scissors
Procedure:
1. Have your partner stretch out the
rubber band into a rectangle using
the index finger and thumb of both
hands as corners.
2. Use a marker to draw four arrows on
each side of the rubber band
rectangle. The arrows should point in
the direction of a closed path traced
along the rubber band. The rubber
band now represents magnetic field
lines.
3. Choose two opposing sides of the
rectangle and pinch them together.
This is simulating the distortion of
magnetic fields that occurs before
reconnection.
4. Hold the two opposing sides together
with both binder clips.
5. Take a pair of scissors and cut the
two "magnetic field lines" between
the binder clips. This is simulating
reconnection of the field lines.
6. Note that energy from the rubber
bands is released, which also
happens during magnetic
reconnection.
Magnetic reconnection is important to solar physicists since
they think that it in part is the cause of flares and coronal
mass ejections (CMEs). It is also thought to be a possible
heat source for the corona, which is unexpectedly hotter
than the surface layers below it.
Sunspots
Magnetism is also responsible for the formation of
sunspots. Sunspots are small areas on the Sun that
appear dark because of their relative low
temperature. This low temperature is thought to be
caused by magnetic fields. The magnetic field
inhibits convection, or the distribution of heat,
resulting in a cool sunspot (Tayler 29).
Features accompanying sunspots (Papagiannis 112)
Faculae (or photospheric faculae)
are bright areas above the
photosphere that appear near
sunspots. Although they occur
everywhere on the Sun, faculae can
only be seen on the limb, or edge of
the Sun. This is a result of limb
darkening and the different
temperatures of the faculae. Limb
darkening occurs because only the
shallow layers of the Sun that are
cooler and less bright can be seen at the limb. Consequently, the limb
appears darker than the rest of the Sun. Likewise, only the upper or
shallow layers of the faculae can be seen at the limb. Since the upper
layers of the faculae are several hundred degrees hotter than the
photosphere (whereas its lower layers are cooler) and consequently
brighter than the rest of the
darkened limb, the faculae is
visible.
Flares are eruptions of particles
and radiation on the surface of the
Sun. They are the result of a
buildup in activity around sunspots. Lasting up to an hour, they release
massive amounts of energy equivalent to a billion megatons of dynamite
(Hathaway par.1). The energy that they release is thought to be from
magnetic reconnnection.
Prominences are arch-like structures in
the corona and are seen at the limb of the
Sun. They are called filaments when they
are projected onto the disk of the Sun.
These structures are supported by long
lasting magnetic field formations in
quiescent prominences. Active
prominences occur over sunspots,
whereas quiescent prominences appear
where there are no sunspots or with decaying sunspot groups.
Coronal mass ejections
One of the most significant
consequences of magnetism is the
coronal mass ejection (CME), a
large scale and sudden expulsion
of magnetized plasma from the
Sun's corona, or outer atmosphere
(Wagner 267). CMEs are so large
that they can occupy as much as a
quarter of the solar limb. Despite
their size, CMEs were not
discovered until the 1970s, in part
because the Earth's atmosphere
prevented instruments from
detecting their presence and the
erroneous attribution of geomagnetic storms primarily to solar flares.
CMEs can send so many charged particles at high speeds to the Earth that
they penetrate the magnetosphere, a cavity created by the Earth's own
magnetic field that deflects particles from the solar wind. These
geomagnetic storms caused by CMEs can disrupt communications &
navigation systems, satellites and power grids. Like flares, CMEs derive
their energy from magnetic fields. The twisting and distortion of magnetic
field lines stores energy in the field which is later released when a CME
forms.
The flare "myth"
Before modern observations proved otherwise, the majority of scientists
believed that the geomagnetic storms experienced on Earth were a result
of solar flares. As a result, they based their theories on a strong causal
relationship between flares and geomagnetic stores. However, starting
with the discovery of coronal mass eruptions (which are called coronal
mass ejections today) in the spacecraft observations of OSO-7 and Skylab,
it is clear that this relationship is wrong (Kahler 114, Gosling par.3).
Coronal mass ejections rather than flares are the causes of energy release
and geomagnetic storms. Unlike what was also previously thought, flares
are not even required for coronal mass ejections to take place.
Further Exploration:
Similar sites:
http://ippex.pppl.gov/ippex/
http://FusEdWeb.pppl.gov/
Bibliography:
Gosling, J. T. "New Findings Challenge Beliefs about Solar-Terrestrial Physics." 28
December 1993. 27 July 1999 <http://www.agu.org/sci_soc/gosling2.html>.
Hathaway, David. "Solar Flares." 20 November 1998. 27 July 1999
<http://wwwssl.msfc.nasa.gov/ssl/pad/solar/flares.htm>.
Kahler, S. W. "Solar Flares and Coronal Mass Ejections." 1992. Annual Review of
Astronomy and Astrophysics. Ed. Geoffrey Burbidge, David Layzer, and John G.
PhillipsVol. 30. Palo Alto, CA: Annual Reviews Inc., 1992. 113-141.
Papagiannis, Michael D. Space Physics and Space Astronomy. London: Gordon and
Breach Science Publishers, 1972.
Tayler, Roger J. The Sun as a Star. Cambridge: Cambridge UP, 1997.
Wagner, William J. "Coronal Mass Ejections." 1984. Annual Review of Astronomy and
Astrophysics. Ed. Geoffrey Burbidge, David Layzer, and John G. PhillipsVol. 22. Palo
Alto, CA: Annual Reviews Inc., 1984. 267-289.
Audience | Goals | Magnetism | Magnetic Fields | Magnetic Force | Stored Energy in
Magnetic Fields | Magnetism & the Sun | Further Exploration | Bibliography
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Last revised by Eugene on August 18, 1999
http://solar-center.stanford.edu/magnetism/full.html