the earth’s magnetic field
DESCRIPTION
The Earth’s Magnetic Field. Source: NASA http://www.nasm.si.edu/research/ceps/etp/earth/img/dipole_earth.gif. Source: NASA http://www.nasm.si.edu/research/ceps/etp/earth/img/magneto.gif. spher. harmonic analyses. magnetic reversals, secular variation. Earth’s Magnetic Field. - PowerPoint PPT PresentationTRANSCRIPT
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The Earth’s Magnetic Field
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Source: NASAhttp://www.nasm.si.edu/research/ceps/etp/earth/img/dipole_earth.gif
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Source: NASAhttp://www.nasm.si.edu/research/ceps/etp/earth/img/magneto.gif
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spher. harmonic analyses
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magnetic reversals, secular variation
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Earth’s Magnetic Field
• Mostly a dipole field (modern and paleomagnetic evidence)
• Can change direction (polarity)
• Secular variation (small-scale changes in direction and magnitude)
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Magnetic Units:
• Magnetic field – Tesla
field mostly described in micro-Tesla
or
gamma (γ) 1γ = 10-9 T
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http://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm
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http://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm
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http://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm
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Mathematical description:
• Spherical harmonic analyses
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Short-term changes:
• Careful measurements since 1600
• Navigational purposes
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http://gsc.nrcan.gc.ca/geomag/nmp/early_nmp_e.php
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Paleomagnetism – history of the earth’s magnetic field• Earth is dipole field
– Relationship between Inclination and latitude
tan I = 2 tan λ
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Identification of magnetic minerals in rocks
Rocks are assemblages of diamagnetic,
paramagnetic, and ferrimagnetic minerals.
Concentration of ferrimagnetic minerals is
usually very small (< 1 %)Microphotograph of a thin section of norite
dolomite
limestone
sandstone shale
Sedimentary rocks
Volcanic rocks
granitegabbro
basalt1
10-1
10-2
10-3
10-4
10-5Mag
neti
c su
scep
tibi
lity
, κ (
SI
unit
s)
Median values and ranges of the magnetic susceptibility in common rock types
Modified from Lowrie, 1997
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Magnetic Polarity
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Duration of a Polarity Chron
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Generation of the Magnetic Field
• No big fat bar magnet
• Dynamic processes inside the outer core
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Early (Simplistic) Model
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Ultimate driver of magnetic field ?
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Magnetic Field of Other Planets
• Mercury – yes, but weak• Venus – no, or very weak• Mars – no clear evidence for field, but
might have had field in the past• Jupiter – yes, very strong (20,000 times
earth)• Saturn – yes 500 times earth• Uranus – yes, strong tilt with respect to
rotational axis
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Source: NASAhttp://photojournal.jpl.nasa.gov/jpeg/PIA02008.jpg
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