Magnetic Fields, Chapter 5


Theories

Earth's magnetic field extends far beyond the orbit of the moon and millions of kilometers into interplanetary space. It probably would have the same shape as the field of a bar magnet except the pressure of the solar wind drastically changes its form.

Until recently, no one knew how the magnetic field was produced. Scientists thought that an internal dynamo near the center of the planet generated the current that produced the magnetic field near the core of the planet. Then somehow the magnetic field was extended from the outer core through the planet creating the North and South poles.  They also thought the system was sustaining itself by using fuel from the Earth's core.

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Now we know that the magnetic field would not exist at all if the dynamo had to depend on earth’s iron core to fuel the system. The fuel within the earth would have burned out long ago because it takes millions of tons of energy to produce and sustain the magnetic field.

Earth's magnetic field covers every square foot of the planet with its lines of force, which loop thousands of miles away from Earth’s magnetic axis and returns in a great closed oval. For hundreds of years we have used this magnetic field along with a compass to find our way on land, in the air, and by sea.

Earth’s magnetic Field Theories

The theory of Earth’s magnetic field has changed over the years. At first scientists thought a bar magnet was in the center of the planet and the compass needle pointed to the poles of that magnet. After a while they realized that hot metal could not hold magnetism because high heat destroys it, so another theory evolved. The new theory put forth the notion that a dynamo was in the outer core of the planet, which produced a much weaker magnetic field. This theory reasoned that the weak magnet field in the planet’s outer core interacts with the ionosphere and creates Earth’s present magnetic field and dynamo system. This theory would account for the field’s strength, its continuous fuel supply, and give it a connection to the magnetic fields of other planets. This would mean that Earth’s magnetic field and other magnetic fields would be getting most of their fuel from the solar wind and account for the needed energy.

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This theory is questionable because some planets with magnetic fields, do not have an iron core available to generate enough current to produce even a weak magnetic field. That line of thought began to change because of Dr. Van Allen’s radiation belts. Research now shows that there is no dynamo within the Earth’s outer core because it would not have the fuel it needs to sustain itself and that not all planets have a liquid iron core that would be necessary to produce a magnetic field.

The Magnetic Axis

The magnetic axis is the point where the lines of force come together and travels through the center of the planet. The axis is the strongest part of earth’s magnetic field because the lines come together to form the axis. The axis attaches itself to the core of the planet (earth) because of its iron properties.  This interaction between the axis and the core seems to be a natural attraction. Since the axis travels through the core, we get the impression that an internal dynamo generates the magnetic axis which expands out to form the magnetic field.


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In other planets, the magnetic axis does not travel through the center of the planet because they don’t have iron cores to attract it. Earth’s core has the properties necessary to attract the axis to its center, but it does not have the means to produce the magnetic field. Other planets do not have the means to attract the axis to the center, that is why the magnetic axis is off center in those planets.

Some scientists have estimated that the magnetic field would have decayed away after about 20,000 years in a closed dynamo system such as Earth’s outer and inner core. Their estimations were based on the size and electrical conductivity needed to run a dynamo large enough to create earth’s magnetic field. Since we know that the temperature of Earth’s core is too high to sustain magnetism, it cannot produce magnetism.  Also, an internal dynamo would have no way to replenish its fuel supply. That is why we looked for another explanation of how the magnetic field is produced.

The North and South Poles

The poles that a compass needle points to, are not poles at all. They are the points on the surface of the planet where the magnetic axes enters and exits the planet.  Since the ends of a permanent bar magnet are called poles, the points on the ground where the magnetic axis is located are called poles.  Even though there is no magnet running through the planet, we still use the terms North Pole and South Pole to describe the locations where the compass needles point.

Magnetic Poles and the Equator

The magnetic poles are located at the top and bottom of the planet.  The North Pole is above the equator at the top of the planet, and the South Pole is below the equator at the bottom of the planet.  The magnetic lines of force converge and form the magnetic axis, which in turn creates the poles. The North Pole is the point where the magnetic lines of force enter the planet.  The South Pole is where the magnetic lines of force leave the planet.  When represented by a straight line, the magnetic axis runs between the North Pole and the South Pole and extends out into space. The magnetic equator is always perpendicular to the magnetic axis and centered between the two poles.

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The Needle Pivots Downward


Within a dynamo system, a compass needle will line up parallel with the magnetic axis and perpendicular to the direction of current flow.  When the needle is positioned so that it can pivot up or down, the north-seeking end will point straight down when it is placed over the point on earth where the magnetic lines of force enter the planet.  This means that the north-pointing end of the needle will point straight down at the North Pole, and the south-pointing end will point straight down at the South Pole.  When the compass needle points downward, it will be keeping its position, which is parallel with the magnetic axis and perpendicular to the direction of current flowing within the dynamo system.

The North Pole is a South Pole

The compass needle points to the North Pole because it is attracted to what would be a huge magnet buried beneath the planet.  Since the northern end of the needle points toward the North Pole, it is attracted by a pole that is opposite to itself because opposite poles attract each other and like poles repel each other. To recognize the law, like poles repel and opposite poles attract, we have to acknowledge that a south pole attracts the north-pointing compass needle. This means that earth’s North Pole is really a South Pole, and the South Pole is really a North Pole.

The Location of the Magnetic Field

Earth’s magnetic field is situated so that the magnetic lines of force completely cover the planet and extend out into space. When the lines of force converge and run through the planet, they form the magnetic axis near the center of the planet. This magnetic axis expands into individual lines of force when it leaves the planet. The lines of force loop out into space and return at the other end of the planet.  This is what allows the magnetic field to cover every inch of the planet. The magnetic equator is East and West of the North and South poles and run around the planet. It is centered on and is perpendicular to the magnetic axis. When standing at the magnetic equator the compass needle will point North and South.

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The Geographic Poles and the Equator

The other set of poles on earth is called geographic poles.  They are also North and South poles.  The magnetic poles are related to the electric current moving around the planet, but the geographic poles are related to the planet’s rotation.  Since the planet is constantly rotating, it is said to rotate on its geographic axis.  This axis of rotation runs through the center of the planet just like the magnetic axis does.  The geographic North Pole is at the top of the axis of rotation and the geographic South Pole is at the bottom of the axis of rotation. 

The geographic equator is centered on and is perpendicular to the axis of rotation.  The geographic equator is normally referred to simply as the equator, and is the region of Earth that receives most of the sun’s rays throughout the year.  The North and South Poles receive the least amount of the sun’s rays throughout the year.

Separate Axes

The poles of the magnetic axis and the poles of the geographic axis can be studied independently of each other.  However, if the magnetic axis and geographic axis were aligned together, the magnetic equator and the geographic equators would match up. This would bring Earth’s planetary dynamo system into perfect alignment. However, the force of gravity, the planet’s rotations, and the electromagnetic forces throw the system out of alignment and keep the axis of rotation tilted away from the magnetic axis. The same forces cause the axis-of-rotation of other planets to tilt away from their magnetic axes. Normally we say that the magnetic axis is tilted away from the axis of rotation.  However, changing this statement makes a big difference in what we observe when we look at the planet and magnetic fields and how they interact. It also makes a difference in the outcome of those interactions. If it weren’t for the outside forces acting on and tilting the planet, the dynamo system would remain perfectly aligned.  Each planet in the solar system would be rotating on both its magnetic axis and its axis-of-rotation.

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The Electromagnetic Field

A magnetic field that is produced by current through a looped wire will affect a compass needle the same way that earth’s magnetic field influences the needle. When a compass is placed next to a loop of current-carrying wire, the needle will line up perpendicular to the direction of current flow and parallel with the magnetic axis.  The loop of wire will produce a magnetic axis and a magnetic equator.  The North and South poles will be centered on the convergent lines that form the magnetic axis.  A magnetic North and South pole will be established by the converging and diverging lines of force of the magnetic axis. This shows that there are only two known ways a magnetic field can be created: with the movement of electric current or with a bar magnet.

Since there is no permanent bar magnet running through the center of the planet, earth’s magnetic field has to be produced by the planets electric field. Earth’s magnetic field is created by the current flowing in the radiation belts, not the hot, liquid outer core. When flowing current produces a magnetic field, the magnetic field is called an electromagnetic field.  Earth’s magnetic field is an electromagnetic field, which cannot be produced from a bar magnetic.

Since an electromagnetic field is produced by electric current, and it takes a magnetic field to produce electric current, the magnetic field and electric fields are bound together. The two fields depend on each other for existence and growth. We know that they exist together because an electromagnetic field will always be present wherever we find moving charged particles. The charged particles move in the Van Allan radiation belts as ring current and give Earth both its electric field and its magnetic field.

Electric Current and the Magnetic Field

Current is produced when electrons are forced to move from atom to atom in a conductor.  (A conductor is usually a copper or silver wire that is used to carry electric current).  Electric current is used to run electric appliances such as refrigerators, stoves, fans, dishwashers, toasters, and can-openers.  It also operates equipment such as stereos, radios, CD players, cameras, computers, and arcade games.

A bar magnet will produce a magnetic field without the apparent flow of electric current, but there will always be a flow of electric current with an electromagnetic field.  As stated earlier, earth’s magnetic field is produced by the flow of electric current not by a bar magnet. 

There are two types of current flow. The current from an alternator is called alternating current (AC) because the flow changes directions or alternates. The current from a battery is called direct current (DC) and constantly flows in one direction.  The constant, steady flow of electric current through a conductor from a battery will affect a compass needle because it produces a magnetic field which remains at a constant strength and direction.  When a compass needle is placed within the magnetic field, the needle will always line up perpendicular to the direction of the current flow and parallel with the magnetic axis. 

Electromotive Force and Alternating Current

Electric current moves through a conductor just as water current moves through a river.  When an electromotive force, which is produced by some type of dynamo system is applied to a copper wire, current will flow through the wire.  When a steady electromotive force is applied, current will be pushed in one direction as direct current.  If the force is reversed or is applied to the other end of the wire, current will be pushed in the opposite direction.  When current is forced to constantly change directions, alternating current is produced.


Our Solar System, Chapter 2
Magnetosphere, Chapter 3
Radiation Belts, Chapter 4
Magnetic Fields, Chapter 5
Earth’s Dynamo, Chapter 6
Dynamo Systems, Chapter 7
The Sun as a Dynamo, Chapter 8
Auroras Chapter 9
Space Weather Chapter 10
References

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