The Origin of Planetary Magnetic Fields
e O g o a eta y ag et c e ds
Hao Cao
Department of Earth and Space Sciences, University of California, Los Angeles
Prof. Christopher T. Russell: A Celebration of a Life in Science
May‐8th‐2013
Not Every Planet Has a Magnetic Field y
g
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Mercury
Venus
Earth
Mars
J it
Jupiter
S t
Saturn
U
Uranus
N t
Neptune
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Here are the Questions
Here are the Questions
• Why some planets have magnetic fields?
Why some planets have magnetic fields?
• How strong are the magnetic fields inside planets?
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g
p
• What determines the strength of the magnetic field?
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g
• The magnetic puzzles of Mercury
…
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Having Magnetic Fields is Non‐trivial
Having Magnetic Fields is Non
trivial
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Not Every Flow Can Generate Magnetic Fields
Not Every Flow Can Generate Magnetic Fields
[P k 1970]
[Parker 1970]
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Realization of Dynamos Inside Planets
Realization of Dynamos Inside Planets
• Large region of conducting fluids must g
g
g
be present
‐ liquid iron core for terrestrial planets ‐ metallic hydrogen for gas giants
metallic hydrogen for gas giants
‐ ionic ocean for ice giants • Enough energy must be available to drive fast and complex flows thermal‐chemical
chemical convection
convection
‐ thermal
‐ mechanically driven (e.g. impact, precession, libration)
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Metallic Hydrogen
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How Strong are the Magnetic Fields inside Planets?
g
g
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MHD Waves in the Earth’ss Core
MHD Waves in the Earth
Core
Torsional Oscillation (Alfven wave)
(
)
[Braginsky 1970]
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MHD Waves in the Earth’ss Core
MHD Waves in the Earth
Core
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MHD Waves in the Earth’ss Core
MHD Waves in the Earth
Core
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What Determines the Magnetic Field Strength?
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g
• Force balance: flow will keep amplifying the magnetic field until Lorentz force becomes strong enough to act back on the l
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b
h
b k
h
flow
• Energy balance: Ohmic dissipation becomes comparable to available power
il bl
[Christenseen 2010]
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The Magnetic Field Strength of Mercury and Saturn
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g
y
[Christensen 2010]
[Christensen 2010]
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The Magnetic Geometry of Mercury
The Magnetic Geometry of Mercury
[Anderson + 2011]
[Johnson + 2012]
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Here is the Essence of Our Solution
Here is the Essence of Our Solution
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Here is One Realization of Our Solution
Here is One Realization of Our Solution
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Summary & Discussion
Summary & Discussion
• The general principal of dynamo process is EM induction
• For planets to have dynamos, conducting fluids and enough energy are needed
• Except for Jupiter, magnetic fields of planets have only Except for Jupiter, magnetic fields of planets have only
been measured in‐situ
• The magnetic field strength inside the dynamo region can only be probed indirectly
only be probed indirectly
• The strong north‐south asymmetry observed for Mercury’s magnetic field can be reconciled with symmetric thermal boundary condition
boundary condition • The essence is that flows with similar heat transfer efficiency can differ in their ability to generate magnetic fields
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“Measure what is measurable, and make measurable what is not so.”
‐ Galileo Galilei
Galileo Galilei
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