the earth magnetic’s field

7/31/2019 THE EARTH MAGNETICS FIELD

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THE EARTH

MAGNETICS FIELD

M.Sc. Programme of Earth Sciences

Institute of Technology Bandung

2005


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Overview

I. Introduction

II. The Field of Uniformly Magnetized Sphere

III. The Origin of The Earths Magnetic FieldIV. The Earths Magnetic Field At The Core-

Mantle Boundary

V. Reversals of The Earths Magnetic Field


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Magnetics Field Component

I. Introduction


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Magnetics Field Component

H= F cos I

Z= F sin I

Tan I= Z/H

X= H cos D

Y= H sin D

Tan D= Y/X

F2 = H2 + Z2 = X2 + Y2 + Z2


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World map showing contours of equal

inclination(isogonics) in degrees for 1980. Contours

interval 5o


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World map showing of equal inclination (isoclinics)

in degrees for 1980. Contours interval 10o

for -40 I 40o and 5o for I 40o


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Two Types of Temporal Changes

of Magnetic Field Over The Earth

1. Transient Fluctuation

- No enduring changes in the

earth fields

- Arises from causes outsidethe earth

2. Secular Changes

- Due to causes within the earth

- Over a long period of time


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Secular Change of Declination and Inclination at

London, Boston and Baltimore


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World Map showing contours of rate in

intensity (Isopors) of the vertical componen Z

for 1987. Contour interval 20 nT per year


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II. The Field of A Uniformly

Magnetized Sphere

William Gilbert, before 1600 revealed that :

- The earth behaved substantially

as uniformly magnetized sphere

-Its magnetic field being due to

causes within the earth


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Gauss, 1839 revealed that :

The field of a uniformly magnetizedsphere which is the same as that

of a dipole at its centre is an

excellent first approximation to the

Earths magnetic field


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Magnetized Sphere

The variation of physical parameters over sphere are most easilyexpressed using Spherical Harmonic Analyis.They are used in the

treatment of gravity,seismology, heat transfer and magnetic field. No

detailed of such a representation will be attempted here. It must be

stressed that spherical harmonic analysis is just a mathematical

convinence for describing global variations and does not necessarily have

any physical significance


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Magnetized Sphere

F= (H2+Z2)1/2=

o.m(1+3cos2 )1/2/4 r3

Intensity measurement are the

function of latitude


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self-exciting dynamo in the Earths molten outercore (Bullard, 1949)

fluid motions (convection cells) inducemagnetic fields(magnetohydrodynamics)

molten Fe and Ni, excellent conductors

III. Origin of The Earth Magnetics Field


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IV. The Earth Magnetic Field

At The Core Mantle Boundary (CMB)

Measurement of the field at the surface can be

extrapolated to the CMB by

a spherical Harmonic Analysis


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Models of the magnetic field at the CMB at selected epoch

from 1751 to 1980 (Bloxham and Gubbins,1985):

i. static flux bundles (permanent regions of intense flux observed under Arctic

Canada, Siberia and Antartica, the central Pasific Ocean and The Persian

ii. Static zero-flux patches (permanent regions of very low flux observed at the

North Pole, under Easter Island, in the northen Pacific Ocean, and in many

model near the south pole

iii. Rapidly drifting flux spots (observed in the southern hemisphere from around

90o, drifting westward towards South America with changes in intensity

iv. Localized field oscilations (such as that under Indonesia)


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Contours plots of the radial field at the CMB.

Contours interval is 100 T. Solid contours

represent flukx into the core broken contours flukx

out of the core. The bold contpurs represent zero

radial field (Bloxham and Gubbins,1985)

IV. The Earth Magnetic Field

At The Core Mantle Boundary


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Map of The Radial Component ofThe Magnetic Field for CMB 1980. Contour interval is 100 T;Solid

contours represent flux into the core, broken contour flux out of the core;

bold contours represent zero radial field. The two main pairs of lobes (1,3)

and (2,4) are indicated, as are the patches of low radial field (5 and 6) near

the poles (after Gubbins and Bloxham, 1987)


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Convection Rolls In A Rapidly Rotating Sphere


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Correlation of Magnetic Stratigraphy

in Seven Cores From Antartic. Greek letters denote

faunal zone. Inset: Source of cores (after, Opdyke

et.al 1966)

IV. REVERSALS OF THE EARTHS

MAGNETIC FIELD


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Timescale For Geomagnetic Reversals.Each short

horizontal lines shows the gae as determined by

potassium-argon dating and the magnetic polarity

(normal or reversed) of one of volcanic cooling unit.

Normal polarity intervals are shown by the solid portions

of the field normal column and reversed polarity

intervals by the solid portions of the field reversed

column. The duration of events is based in part on

paleomagnetic data from sediment and magnetic profiles

(After Cox, 1969)


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Inclination of Short-Period Events in a Deep-Sea Core From The Southern

Ocean. Polarity log at right, clear is reversed (After Watkins,1968)


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Polarity Bias of The Geomagnetic Field

During The Phanerozoic.Overlapping 50 Ma averages of polarity ratios as

observed in paleomagnetic result are shown together with the limits of thestandard error (After Irving and Pullaiah, 1976)


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Estimated Mean Reversal Rate

From Present Back To 165 MA

(After Mcrill and Mc Fadden, 1990)


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Geomagnetic Virtual Dipole Moment, Paleosecular Variation and Reversal

Chronology Since The Triassic. Numbers by the dots are number of

chronological units. On the polarity timescale, normal periods are black

and reversed periods white(After Prevort et.al. 1990)


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CARBON STORAGE IN PEATLANDS

Peatland C stores (t C/ha)

Soil Biomass C Absorption

Global 1181-1537 ND 0.2-0.5

Tropics 1700-1600 500 0.3-0.9

Temperate 1300 120 0.17-0.29

Source : Parish (2002)


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PEATLANDS AS A SOURCE OF CO2


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Earths Magnetic Field : Source

self-exciting dynamo in the Earths molten outercore

fluid motions (convection cells) inducemagnetic fields(magnetohydrodynamics)

molten Fe and Ni, excellentconductors

energy source

radioactive decay gravitational energy

(sinking of heavy particles)


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P l i


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History of the geomagnetic field

magnetic remanence

a fossil magnetization recorded during formation and deposition

of natural materials

thermoremanence magnetization - acquisition of geomagneticfield direction at time of cooling below Curie temperature

depositional remanence magnetization - alignment of magneticminerals/grains with the geomagnetic field

records dipole and non-dipole field orientations

basis of magnetostratigraphy : reversals, excursions, and secularvariation

Paleomagnetism


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Paleomagnetism

Dipole approximation

best fit to Earths actual field

geocentric axial dipole (GAD)

tan I = 2 tan

D = 0o everywhere

as much as 20% differencebetween dipole approximationand actual field

non-dipole field (regional,short term variability)

Earths magnetic field is a vector

three elements at any point in timeand space

declination

inclination

intensity


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Secular Variation difference between GAD

approximation and actualfield (non-dipole field)

temporal variations on

timescales of 102-106years

useful for dating andcorrelation purposes

all three elements

(decliantion, inclination,intensity) are reproducibleon a regional scale of3000-5000 km.

Northeastern U.S Stacked

Inclination

50 807060

0

2000

4000

6000

8000

10000

12000

Magnetostratigraphy

the earth magnetic’s field

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