Announcements

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Announcements Reminders
1. ES 123 Essay Due Friday November 24, 300
p.m. (BGS 10) 2. Lab final Tuesday December 5,
1030 - 1120 a.m. room NS-145 3. Course final
Saturday December 9, 200 - 400 p.m. NS-1
FORMAT Multiple choice (45 questions)
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Earth's magnetic field Introduction
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The magnetic compass

• The loadstone compass was discovered in China ca.
  1000 AD
• Used as the basis for marine navigation
• Columbus found that the compass needle changes
  direction at different locations
• The mystery of the compass was solved by William
  Gilbert in 1600

http//www.phy6.org/earthmag/demagint.htm
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William Gilbert and the Birth of Magnetism

• Personal Physician to Elizabeth I
• Published De Magnete in 1600, a 6-volume treatise
  documenting his experiments with magnetism
• Proposed that Earth behaves like a giant bar
  magnet
• Died in 1603 of bubonic plague

William Gilbert, 1544-1603
http//www.phy6.org/earthmag/demagint.htm
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Measuring Earths magnetic field
Three measurements Inclination angle measured
from horizontal Declination angle measured from
geographic north Intensity magnetic field
strength (units Teslas, nanoTeslas)
http//earth.leeds.ac.uk/dynamicearth/plates_move/
longterm/pmag2.htm
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Temporal Variations inEarths magnetic field
Secular Variations - Long-term (decades) changes
caused by fluid motion in Earth's outer core.
Diurnal Variations - Daily variations related
to interaction of the geomagnetic field with
solar wind. Magnetic Storms - Intense (1000 nT),
irregular and unpredictable variations associated
with solar flares.
http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter2/aurora_magnetosphere.html
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Secular Variation in Earth's magnetic field since
1600
http//swdcwww.kugi.kyoto-u.ac.jp/igrf/
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Migration of the magnetic north pole

• At the magnetic north pole, the inclination is
  90o (straight down)

http//gsc.nrcan.gc.ca/geomag/nmp/northpole_e.php
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The magnetosphere
Solar wind charged particles ejected from the
Sun Magnetosphere a zone of charged particles
trapped by Earths magnetic field Magnetotail
portion of a magnetosphere that is pushed away
from the sun by the solar wind
http//apollo.lsc.vsc.edu/classes/met130/notes/cha
pter2/aurora_magnetosphere.html
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The Van Allen radiation belts

• Two regions of charged particles trapped in
  magnetosphere
• Corkscrew-paths, bouncing between magnetic poles
• Outer belt is recharged by the solar wind
• Aurorae occur where outer belt intersects the
  lower atmosphere

http//www.physics.upenn.edu/thomsone/phys151/
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Characteristics of Earths magnetic field

• Nearly dipolar
• Approximately aligned with Earths rotation axis
• Changes slowly with time
• Spontaneously reverses every 200,000 years
• Is at least 3 Ga old

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What causes Earths magnetic field?
Self-exciting dynamo Elsasser, 1958

• If it were not being continually generated,
  Earths field would decay in 20,000 years
• Solution Self-exciting geodynamo
• Dynamo A machine for converting mechanical
  energy to electrical current flow (and magnetic
  field)

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What causes Earths magnetic field?
Inner core

• It has long been speculated that the geodynamo
  is caused by convection in the outer core
• Convection is driven by a) heat, and b)
  crystallization of the inner core

Core-mantle boundary
http//www.es.ucsc.edu/glatz/geodynamo.html
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Computer Model of the Geodynamo
Inner core

• In 1995, the first computer model was published
  (Glatzmaier-Roberts model) which successfully
  simulated the geodynamo, including spontaneous
  reversal
• Since then, several other groups have also
  succeeded

Core-mantle boundary
http//www.es.ucsc.edu/glatz/geodynamo.html
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Glatzmaier-Roberts model

• Snapshot of magnetic field lines produced by
  G-Rs computer program

http//www.es.ucsc.edu/glatz/geodynamo.html