1. GEOMAGNETISM
By
S
DEPARTMENT OF EARTH AND ENVIRONMENTAL
SCIENCES
BAHRIA UNIVERSITY ISLAMABAD
2013
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2. CONTENTS
S.No
TITLE
PAGE
Abstract
1.0
1.1
1.2
2.0
2.1
2.2
2.3
3.0
3.1
3.2
4.0
4.1
4.2
5.0
04
Introduction
Background
Objective
Magnetic Field
Earth’s Magnetic Field
Main Field
Magnetic and Geographic Poles
Factors Which Affect Geomagnetism
Curie Temperature
Dynamo Theory
Evidences of Magnetic Field
Solar Winds
Auroras
Conclusion
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05
05
05
05
06
07
07
07
08
09
09
10
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LIST OF FIGURES
FIGURE
Figure: 1
TITLE
Earth’s Magnetic Field
PAGE
06
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3. Figure: 2
Figure: 3
Geomagnetic Reversal
Dynamo Effect
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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.
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4. 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.
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
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5. 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.
(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
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6. 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.
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
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7. 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
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:
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8. 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.
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:
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9. (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
(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
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10. 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.
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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11. 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.
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12. 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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