Planetary dynamos: Dipole-multipole transition and dipole reversals Ulrich Christensen Max-Planck-Institute for Solar System Research Katlenburg-Lindau, Germany in collaboration with Hagay Amit, Julien Aubert, Erik King, Carsten Kutzner, Kumiko Hori, Gauthier Hulot, Ajay Manglik, Peter Olson, Martin Schrinner, Andreas Tilgner, Johannes Wicht
20
Embed
Planetary dynamos: Dipole-multipole transition and dipole reversals · Planetary dynamos: Dipole-multipole transition and dipole reversals Ulrich Christensen Max-Planck-Institute
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Planetary dynamos: Dipole-multipole transition and dipole reversals
Ulrich ChristensenMax-Planck-Institute for Solar System Research
Katlenburg-Lindau, Germany
in collaboration withHagay Amit, Julien Aubert, Erik King, Carsten Kutzner, Kumiko
Hori, Gauthier Hulot, Ajay Manglik, Peter Olson, Martin Schrinner, Andreas Tilgner, Johannes Wicht
The geodynamo
Fe,Ni +10% light element (Si, S, O)
Fe,Ni + 2 - 4% light element
• Thermal /compositional convection in liquid outer iron core
• Electromagnetic induction by motion through the existing magnetic field sets up electrical current
• Currents produce magnetic field as needed for induction
⇒ Selfsustained dynamo
Dipole reversals
A few reversals per million years
Stochastic (vs. periodic at Sun) Mill. years
Low magnetic field strength during reversal (but non-zero)
Field during reversal dominated by multipole components
Field morphology
Uranus and Neptune have multipolar fields
Earth
Br
Pic
ture
33
Neptune
Br
Earth, Mercury, Jupiter and Saturn have dipole-dominated magnetic fields
Outline of geodynamo models
Direct numerical simulation of fundamental equations of magnetohydrodynamics
Some parameters not Earth-like
Solve equations of thermal / compositional con-vection and magnetic induction in a rotating and electrically conducting spherical shell
Tangentcylinder
Fluid outer core
Solid inner core
Governing equationsBBT
r
rRa*uEPueuu
t
u
oz
È
××∇++∇=∇+×+∇⋅+∂∂
)(2)( 2
. Inertia Coriolis Viscosity Buoyancy Lorentz
T
Pr
ETu
t
T 2∇=∇⋅+∂∂
Advection Diffusion
B
Pm
EuBBu
t
B
2∇+∇⋅=∇⋅+∂∂
Advection Induction Diffusion
00 =⋅∇=⋅∇ Bu
Magnetic field morphology Ra/Rac= 114 E=10-5 Pm=0.8 Ra/Rac= 161 E=10-5 Pm=0.5