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EPS 50 – Lab 8: the Earth's Magnetic Field Chapter 2, P Name:_____________________ EPS 50 – Lab 8: The Earth's Magnetic Field Chapter 2, p. 39-41: The Seafloor as a Magnetic Tape Recorder Chapter 7, p. 213: Paleomagnetic Stratigraphy Chapter 14, p. 396-406: Earth’s Magnetic Field and the Geodynamo Introduction The Earth's magnetic field is one of the most important properties of this planet. Studies in the 1960's of the earth's magnetic field and magnetic anomalies led to the discovery of plate tectonics, which is one of the most important theories about Earth's dynamics. Earth's magnetic field also shields us from most space-derived cosmic radiation. In some ways, Earth’s magnetic field behaves in the same way that magnetic fields on ordinary magnets behave. In this lab we will be exploring some of the properties of Earth's magnetic field. Objective In the first part of this laboratory we will visualize a magnetic field by observing the alignment of iron filings around a magnet. We will then extrapolate this to Earth, which to a first approximation has a dipole magnetic field. You will explore what a dipole field is, and how Earth's magnetic field is similar and different from that of a dipole. We will then look at how we use Earth's magnetic properties as a record of tectonics and geologic time. Answers All answers should be your own, but we encourage you to discuss and check your answers with 2-3 other students. This labs is graded out of 100 points. -------------------------------------------------------------------------------------------------------------- Part 1: Magnetic Field Lines Magnetic fields are produced by moving charges. Magnetic field lines describe the structure of magnetic fields in three dimensions. They are defined as follows: if at any point on such a line we place an ideal compass needle, free to turn in any direction (unlike the usual compass needle, which stays horizontal), then the needle will always point along the field line. Field lines converge where the magnetic force is strong, and spread out where it is weak. For instance, in a compact bar magnet, which approximates a dipole, field lines spread out from one pole and converge towards the other. The magnetic force is strongest near the poles where they come together. The behavior of field lines in the Earth's magnetic field is very similar. With a group, get a bar magnet, some plastic wrap, iron filings and a compass. Place one of your bar magnets on a piece of loose-leaf paper. Place the plastic wrap over the magnet (this is so that the iron fillings do not stick directly to the magnet!). Sprinkle the iron filings from a height of about 10 cm. Continue sprinkling until a distinct pattern emerges. The iron filings fall on the plastic and align themselves with the magnetic field. 1) Draw a simplified version of the field pattern that emerges when the iron filings are 1 placed on the bar magnet. Be sure to show the overall shape of the field and the density of the field lines. (10 pts). Place your compass in your magnetic field in the spots illustrated by circles below. 2) Does the compass needle line up with the field lines? (2 pts). 2 3) Draw an arrow into each circle above to show the direction of the north pole of your compass in the field. (5 pts). 4) Where along the bar magnet is the magnetic field strongest, and how can you tell? (3 pts). Place the north end of a bar magnet about 4 cm from the south end of another bar magnet. Place a piece of plastic over the two magnets and sprinkle iron filings in the region between the magnets. 5) Draw a simplified version of the field pattern that emerged when the iron filings were placed on the bar magnets in these configurations. Only show the field lines as they appear between the bar magnets. (6 pts). 6) Use the north pole of your compass to determine the direction of the magnetic field 3 lines. Add arrows to your picture above, showing the direction of the magnetic field as determined by your compass. (4 pts). 7) How does the magnetic field line pattern differ between your single bar magnet and the double bar magnet configuration? (3 pts). 4 Part 2: Earth's Magnetic Field Left: the magnetic field of a bar magnet revealed by iron filings on paper. Right: Earth’s magnetic field viewed as if a giant bar magnet were placed at the Earth’s center and slightly inclined (11°) from the axis for rotation. A magnetic dipole field is the field produced by a north magnetic pole and a south magnetic pole in close proximity. The above figure shows an idealized dipole field for Earth. The magnetic field lines show the direction of the magnetic field and presently point from the southern geomagnetic pole to the northern geomagnetic pole. There is something surprising about this fictitious bar magnet in Earth: Its south (negative) pole lies beneath Earth’s “north magnetic pole,” or “magnetic north”! You can see why this must be so by considering that, in the absence of any significant magnetic fields other than the Earth’s, the north end of a compass needle points toward Earth’s magnetic north, which is in the general direction of the geographic North Pole. Since the magnetic field along the fictitious bar magnet’s field lines points from the north pole to the south pole, it has to be that the south pole of the Earth’s magnetic field lies beneath Earth’s north magnetic pole. The magnetic pole is not located at the same position as the North Pole. Declination is the angle between magnetic north (the direction in which a compass needle points) and true north (the direction of the north geographic pole). It is often noted on maps, and changes from year to year. 8) Draw a sketch to illustrate declination on a hypothetical map, below. (2 pts). 5 Magnetic Inclination In general, the magnetic field lines of Earth are not parallel with the surface of the planet. The angle between the magnetic field and the horizontal is called inclination, which is positive if the field lines point into the ground, and negative if the field lines point out of the ground. In the above figure, inclination is shown as i, magnetic latitude is shown as ϴ. Magnetic inclination varies from 90° (perpendicular to the surface) at the magnetic poles to 0° (parallel to the surface) at the magnetic equator. How can we characterize this relationship between the change in inclination with a change in latitude for a magnetic dipole? It can be proven theoretically (but not in this lab) that i and ϴ for an idealized dipole field are related by the following expression: tan i = 2 tan ϴ This means that for a location on an Earth with an ideal dipole field, if you know the latitude (ϴ) you can find the inclination (I), and vice versa. Refer back to your first bar magnet diagram and sketch from Part 1. Imagine that your drawing now represents the Earth. Where is the magnetic North pole? 9) From your drawing in Part 1, measure (not calculate) the inclination angle I at the 'latitudes' of 0°, 30°, 45°, 60° and 90° around the 'globe'. Record your measurements in the following table under "experimental inclination". (5 pts). 6 Dipole Field Latitude (ϴ) 0˚ 30˚ 45˚ 60˚ 90˚ Experimental Inclination (Ix) Theoretical Inclination (I) 10) Now use a calculator to find the theoretical inclinations for a dipole field at latitudes of 0°, 30°, 45°, 60° and 90°. Record your results in 'theoretical inclination row' of the table above. (5 pts). 11) Explain any significant difference between the "theoretical" and the "experimental" values. (4 pts). 12) If the Earth's magnetic field is that of an ideal dipole and if the magnetic poles are aligned with the geographic poles, what would be the expected pattern of lines of equal inclination on a world map? (5 pts). 7 Below is a world map of the magnetic inclination of the real Earth as determined by measurement- based models. This is a snapshot of the field a few years ago (2005-2006). The contour lines are 20 degrees apart, positive in the northern hemisphere, and negative in the southern hemisphere. 13) What is the current magnetic inclination of Berkeley? (2 pts). 14) Describe the similarities and differences between this map and the expected pattern for a hypothetical earth with an ideal dipole field. What do you think causes the differences between your calculations, observations, and this map? (5 pts). 8 Earth's magnetic field is slowly changing over time, on time scales that range from years to millennia. Such changes are referred to as secular variation. Secular variation was first recognized in 1634 when Gellibrand compared magnetic declination observations he had made in London with earlier observations. The observations of declination made in London over the years constitute one of the best records of secular variation. The figure below shows that declination has changed from approximately 10˚ E in the late 16th century to 25˚ W in the early 19th century before returning to a current value of about 3˚ W. All elements of the magnetic field change with time – not just the declination. For example, the total field intensity (strength) in Toronto has decreased 14% during the last 160 years. Magnetic poles are just the time-averaged direction of the magnetic field, where the effects of secular variation are (ideally) averaged out. Generally, many magnetic measurements spanning long periods of time (thousands to tens of thousands of years) are needed to derive a statistically robust magnetic pole.
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