GEOMAGNETISM

By

S

DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES

BAHRIA UNIVERSITY ISLAMABAD

2013

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CONTENTS

S.No TITLE PAGE

1.0

Abstract

Introduction

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05

1.1 Background 05

1.2 Objective 05

2.0 Magnetic Field 05

2.1 Earth’s Magnetic Field 05

2.2 Main Field 06

2.3 Magnetic and Geographic Poles 07

3.0 Factors Which Affect Geomagnetism 07

3.1 Curie Temperature 07

3.2 Dynamo Theory 08

4.0 Evidences of Magnetic Field 09

4.1 Solar Winds 09

4.2 Auroras 10

5.0 Conclusion 12

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LIST OF FIGURES

FIGURE TITLE PAGE

Figure: 1 Earth’s Magnetic Field 06

Figure: 2 Geomagnetic Reversal 08

Figure: 3 Dynamo Effect 09

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ABSTRACT

In our report we have started with a brief introduction covering the background of what

magnetic field is?

The different factors on which Earth’s magnetic field depends.

Then we have researched about the history of geomagnetism. In the research then we

covered the different evidences which support geomagnetism, the different studies related to it etc.

Further, we investigated rock magnetism, its types, paleomagnetism and its applications.

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1. INTRODUCTION

1.1 Background

The magnetic field is a complex part of Earth. While an understanding of the basics of

magnetism is essential, the temperatures at the core of the Earth where the field originates

require scientists to look beyond ferromagnetic properties and bar magnets for the field’s

origin. These observations, along with the current conditions of our magnetic field, have lead

some scientists to hypothesize that presently Earth is entering a period of magnetic reversal.

Which indicates that the study of geomagnetism will be very essential for scientists in the near

past as it is today?

1.2 Objective

The main objective of our report is to show that how the Earth’s magnetic field affects

geomagnetism and how in turn geomagnetism helps us in understanding the history of the

Earth, the magnetic reversals which have taken place after intervals (several hundred to

thousands of years), etc. In other words we have highlighted the affects of Earth’s magnetic

field and how it is related to geomagnetism and related phenomenon’s.

2. MAGNETIC FIELD

This magnetic field is just like the one created by a magnet, much like the electric field.

Lines originate at the North Pole and terminate at the South Pole.

2.1 Earth’s Magnetic Field

(a) Definition

Earth's magnetic field (also known as the geomagnetic field) is the magnetic field that

extends from the Earth's inner core to where it meets the solar wind, a stream of energetic

particles emanating from the Sun.

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(b) Explanation

Earth’s magnetic field is approximately the field of a magnetic dipole tilted at an angle of

11 degrees with respect to the rotational axis—as if there were a bar magnet placed at that

angle at the center of the Earth. However, unlike the field of a bar magnet, Earth's field

changes over time because it is really generated by the motion of molten iron alloys in the

Earth's outer core (the geodynamo).  The Magnetic North Pole wanders, fortunately slowly

enough that the compass is useful for navigation. At random intervals (averaging several

hundred thousand years) the Earth's field reverses (the north and south geomagnetic

poles change places with each other).

2.2 Main Field

Although the Earth’s Magnetic Field is often represented as a bar magnet, it is much more

complicated than that. The main field includes the field at the surface of the Earth, magnetic

poles, and the magnetosphere.

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Figure: 1 Earth’s Magnetic Field

2.3 Magnetic and geographic poles

Earth's geographic poles are fixed by the axis of Earth's rotation. On maps, the north and

south geographic poles are located at the congruence of lines of longitude. Earth's geographic

poles and magnetic poles are not located in the same place fact they are hundreds of miles

apart. As are all points on Earth, the northern magnetic pole is south of the northern geographic

pole (located on the polar ice cap) and is presently located near Bathurst Island in northern

Canada, approximately 1,000 mi (1,600 km) from the geographic North Pole. The southern

magnetic pole is displaced hundreds of miles away from the southern geographic pole on the

Antarctic continent for instance.

Although fixed by the axis of rotation, the geographic poles undergo slight wobble-like

displacements in a circular pattern that shift the poles approximately six meters per year.

Located on shifting polar ice, the North Pole (geographic pole) is technically defined as that

point 90° Latitude, 0° longitude (although, because all longitude lines converge at the poles,

any value of longitude can be substituted to indicate the same geographic point). Latitude

growth 1800-2012: from 1 billion to 7 billion estimated in 31.10.2011.

3. FACTORS WHICH AFFECT MAGNETISM

1. Curie temperature

2. Dynamo effect

3.1 Curie temperature

(a) Definition

The Curie temperature (Tc), or Curie point, is the temperature at which

a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is

reversible. 

(b) Explanation

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Below the Curie temperature neighboring magnetic spins are aligned parallel within ferromagnetic materials and anti-parallel in ferrimagnetic materials. As the temperature is increased towards the Curie point, the alignment (magnetization) within each domain decreases. Above the Curie temperature, the material is paramagnetic so that magnetic moments are in a completely disordered state. As shown in the figure below:

Figure: 2 Geomagnetic Reversals

3.2 Dynamo Theory

Planets do not have giant bar magnets in their cores, so what produces the magnetic field?

 Circulating electrical charges can produce a magnetic field. A theory called the magnetic

dynamo theory says that swirling motions of liquid conducting material in the planet interiors

produce the magnetic field.

In geophysics, dynamo theory proposes a mechanism by which a celestial body such as

the Earth or a star generates a magnetic field. The theory describes the process through which a

rotating, convecting, and electrically conducting fluid can maintain a magnetic field over

astronomical time scales.

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Rotation of the Earth causes a similar, though not simultaneous, rotation in the

conducting, liquid metal core which flows across the existing magnetic field. Electric currents

are induced, creating a second magnetic field. This second magnetic field reinforces the first

and a dynamo is created which sustains itself.

There are three requisites for a dynamo to operate:

(a) An electrically conductive fluid medium.

(b) Kinetic energy provided by planetary rotation.

(c) An internal energy source to drive convective motions within the fluid.

(d)

Figure: 3 Dynamo Effect

In the case of the Earth, the magnetic field is induced and constantly maintained by the

convection of liquid iron in the outer core. A requirement for the induction of field is a rotating

fluid. Rotation in the outer core is supplied by the Coriolis Effect caused by the rotation of the

Earth. The Coriolis force tends to organize fluid motions and electric currents into columns

(also see Taylor columns) aligned with the rotation axis.

4. EVIDENCES OF MAGNETIC FIELD

(a) Magnetosphere

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(b) Auroras

4.1 Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of

the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and

10 keV. The stream of particles varies in temperature and speed over time. These particles can

escape the Sun's gravity because of their high kinetic energy and the high temperature of

the corona. As the solar wind approaches a planet that has a well-developed magnetic

field (such as Earth, Jupiter and Saturn), the particles are deflected by the Lorentz force. This

region, known as the magnetosphere, causes the particles to travel around the planet rather than

bombarding the atmosphere or surface. The magnetosphere is roughly shaped like

hemisphere on the side facing the Sun, then is drawn out in a long wake on the opposite side.

The boundary of this region is called the magnetopause, and some of the particles are able to

penetrate the magnetosphere through this region by partial reconnection of the magnetic field

lines

 

4.2 Auroras

Aurora(plural: aurora or auroras) is a natural light display in the sky particularly in the

high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged

particles with atoms in the high altitude atmosphere (thermosphere).

 The charged particles originated in the magnetosphere and solar wind and, on Earth, are

directed by the Earth's magnetic field into the atmosphere.

Auroras are caused by the interaction of energetic particles (electrons and protons) from

outside the atmosphere with atoms of the upper atmosphere. Such interaction occurs in zones

surrounding the Earth's magnetic poles.

Auroras in the Northern Hemisphere are called aurora borealis, or northern lights; in the

Southern Hemisphere they are called aurora Australis, or southern lights.

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Auroras are caused by the interaction of energetic particles (electrons and protons) from

outside the atmosphere with atoms of the upper atmosphere. Such interaction occurs in zones

surrounding the Earth's magnetic poles Auroras in the Northern Hemisphere are called aurora

borealis, or northern lights; in the Southern Hemisphere they are called aurora Australia, or

southern lights.

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5. CONCLUSION

Currently, the poles are drifting rapidly and the field is steadily weakening. The

South Atlantic Anomaly also provides support for reversal. If a reversal was to occur, harmful

ionized particles from the Sun would enter Earth’s atmosphere, with varying consequences.

However, there is some debate over whether or not a reversal is in progress. The changes in

the field are within the expected limit and are simply due to the random flow of the Dynamo.

Finally, the Moon will have an adverse affect on Earth’s magnetic field over time. It is clear

that geomagnetism is a very dynamic and complex topic that includes much theory and debate

about its past, present, and future.

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