The Earth And Its Magnetic Field (1969)

[Applause] [Applause] thank you [Applause] [Music] why does the compass needle hold fairly steady when a compass is turned because it's in a magnetic field the Earth's magnetic field this field exerts a force on magnetic materials such as the steel the needle of a compass is made from we can visualize a small magnetic field by sprinkling iron filings over a bar magnet covered with a sheet of plastic the filings form a pattern this pattern indicates the lines of force within the magnetic field between the North and South Poles of the magnet when magnetic fields interact we see a different kind of pattern this pattern occurs because like poles of a magnet repel and unlike poles attract a compass needle is a small bar magnet with a North and South Pole the South Pole of the magnet attracts the North Pole of the needle but because the needle is on a pivot it can turn so the compass needle will turn along the lines of force as it's moved at the other side the South Pole of a needle points to the North Pole of the magnet the lines of force connect the North and South Poles points of opposite polarity the same is true of the magnetic field of the Earth here also imaginary lines of force connect two opposite poles the north magnetic pole and the South magnetic pole but there is a difference between the positions of these magnetic poles and the geographic poles let's consider just the north Geographic and magnetic poles and a location on the globe here on the East Coast a compass placed here does not point at the North geographic pole it simply lines up with a line of force that connects the Earth's magnetic poles on Earth the north magnetic pole is defined as a pole towards which the North End of a compass needle points since unlike poles attract it must really be a South Pole but we continue to call it the north magnetic pole to avoid confusion if we extend a line in the direction the needle points an angle is formed its measure is the angular difference between true north and the direction the compass points this angle is called the angle of declination here it's 10 degrees west in this particular location the compass points 10 degrees west of the north geographic pole if we change our location to here the compass needle again lines up with a line of force but at this point the direction of the line of force happens to be the direction to the north geographic pole so there's no angle of declination or declination is zero degrees on a map we can Mark points that have the same declination they can be connected by a line called an isogonic line this line connects points having 10 degrees west declination because of irregularities in the Earth's magnetic field the isogonic lines themselves are somewhat irregular these are some of the isogonic lines that connect points of equal declination in North America this line connects points of zero declination it's called an agonic line map showing lines of declination are called isogonic Maps readings on the map show how many degrees a compass has to be corrected to point toward true north the compass needle indicates a horizontal effect of the magnetic field a declination but the Earth's magnetic field also has a vertical component using a magnetic dipping needle we can see that the magnetic field is three-dimensional the dipping needle measures the angular difference between the Earth's horizontal surface and the direction of the Earth's magnetic field at that point the angle formed is called the angle of inclination using a magnetized model of the Earth and a small dipping needle we see that at this point in the northern hemisphere the inclination is almost 90 degrees locating the north magnetic pole it's the same information that would result if the needle were above a bar magnet the main magnetic field of the earth is similar to the field of a bar magnet here is a simplified version of the lines of force of the Earth's magnetic field a magnetic dipping needle near the north magnetic pole will come to rest almost perpendicular to the Earth's surface here the inclination is almost 90 degrees near the equator the needle is almost parallel to the Earth's surface the inclination is about zero degrees and helps locate the magnetic equator we can connect points of equal inclination to form lines of equal inclination 89 degrees 88 degrees and so on the lines that connect the points are called isoclinic lines and this is also how isoclinic maps are made there is a third feature of the Earth's magnetic field this is the strength or intensity of the field let's think about a magnet again most of the iron filings are attracted to the poles of the magnet where the intensity of the magnetic field is the greatest intensity of the Earth's field is strongest at the Earth's magnetic poles too we can plot magnetic intensity of the Earth's field by connecting points of equal intensity at points near the Magnetic North Pole the intensity is 0.60 ersted the ersted is the unit of measurement of magnetic intensity intensity decreases towards the equator until it reaches a minimum of 0.25 ersted about 25 degrees south of the Equator the plotting of this kind of information is part of the work done at this magnetic observatory in Fredericksburg Virginia this is a proton Vector magnetometer a modern instrument used to measure magnetic intensity at the control console precise readings can be made of variations in the intensity of the Earth's magnetic field at this location this is an observatory type magnetometer used for measuring magnetic declination or deviation from true north this instrument an earth inductor measures magnetic inclination or dip measurements of the Earth's magnetic field are also made in many places away from permanent stations on land underwater and in the air the Earth's magnetic field is constantly being studied so that maps showing magnetic data can be brought up to date in 1903 the north magnetic pole was here its position here at present indicates a Westerly movement of three to four miles each year we also know something else about the Earth's magnetic field many places on Earth once saw volcanic activity hot lava poured from these ancient volcanoes as the lava cooled and hardened tiny pieces of magnetic material in the lava lined themselves up with a magnetic field of the earth that existed then so as the lava slowly changed into rock a geologic record of the Earth's magnetic field at the time was permanently set or Frozen geologists studying volcanic rock have discovered that the Earth's magnetic field has apparently disappeared and then reappeared with reverse polarity about nine times in the past three and a half million years scientists examining ancient rocks have also concluded that there's been a Wandering of the magnetic poles about 2 billion years ago the north magnetic pole was located in North America as it is today geologic evidence from rocks in North America indicates that it slowly moved across the North Pacific Ocean and Eastern Asia to its present location near the geographic pole what might have caused the shifting of the magnetic poles one explanation is based on the belief that the Earth's magnetism May originate in the Earth's core of molten nickel iron the rotation of the earth May set up circling Eddies in the liquid core that produce an electric current this current generates north-south magnetic lines of force the short-term variation of the poles may be due to changing patterns of motion in the core the very long-term wandering of the poles can be explained by the theory of drifting continents there are also external influences which can cause short duration modifications of the Earth's magnetic field one of the major influences is activity on the Sun for example when there are solar flares sudden temporary outbursts of electrically charged particles from the sun's surface there are Magnetic storms on Earth temporary disturbances in the Earth's magnetic field radio communication is disrupted effects like these are being studied today with space probes and artificial satellites these have been giving us a more complete picture of the Earth's magnetic field satellites and probes equipped with magnetometers have revealed that there are highly energized and potentially dangerous regions of charged particles surrounding the earth the particles are trapped in the Earth's magnetic field in donut-shaped regions of radiation these are the Van Allen radiation belts named for the man who designed the satellite equipment which resulted in their discovery it's believed that the charged particles in the outer belt come from the sun's radiation while those in the Inner Belt come mainly from excited atoms formed by the bombardment of the upper atmosphere by charged particles from the Sun the outer portions of the Earth's magnetic field are continuously buffeted by Thin streams of electrically charged particles from the Sun traveling as fast as 2 million miles per hour this solar wind the source of the Van Allen belt particles also modifies the shape of the Earth's magnetic field satellite measurements indicate that the Earth's magnetic field resembles a teardrop with a very long tail instead of being an orderly field extending almost endlessly into space the magnetic field appears to be confined in a space called the magnetosphere the magnetosphere is bounded by a thin layer of charged particles called the magnetopause toward the Sun the magnetosphere is only about forty thousand miles away from the Earth on the opposite side the solar wind draws out the magnetosphere like the tail of a comet more than three and a half million miles long the width of the Magneto pause varies from about 200 miles to 2000 miles learning about the magnetosphere and the Earth's magnetic field is important to man's further exploration and understanding of space manned spacecraft that leave Earth for the moon or the planets must be shielded from the dangerous radiation in the van Allen belts a better understanding of these belts is necessary for the safety of space Travelers further knowledge and understanding of the Earth's magnetic field will help illuminate the Earth's past and make safer man's way into the future [Music]