Title: Computer Simulation of Geodynamo: Dipole Field Generation and Reversals

Aturho: Akira Kageyama, Jinghong Li, and Tetsuya Sato

Keyworkds: Geodynamo, MHD


Abstract:
It is well known that the earth's magnetic field is nearly a dipole and its 
direction ---the magnetic north and south--- has reversed a lot of times in 
the earth's history. (The last reversal of the geomagnetic field took place 
about 0.8 million years ago.) It is one of the most challenging subjects in 
geophysics to understand the generation and reversal mechanisms of the earth's 
dipole field. 
The generation process of the earth's magnetic field is sometimes called the 
geodynamo, since it is believed to be generated by a magneto-hydrodynamic (MHD
) dynamo process. The MHD dynamo is an energy conversion process in 
electrically conducting fluids (such as plasmas or liquid metals) from the 
fluidsﾕ convection energy into the magnetic energy. In the geodynamoﾕs case, 
the dynamo medium is the liquid iron in the earthﾕs deep interior. 
The earth is, roughly speaking, divided into three spherical regions; the 
inner core (radius r < 1300km), the outer core (1300km < r <3500km), and the 
mantle ( 3500km < r < 6400km ). The outer core is made of iron in the liquid 
state. (The inner core is solid iron.) 
We performed computer simulations of the MHD dynamo in a rotating spherical 
shell vessel, which is a model of the earthﾕs outer core. We followed the 
thermal convection motion of an electrically conducting fluid in the spherical 
shell by numerically solving the time development of the MHD equations. 
It is known that a thermal convection motion in a rapidly rotating spherical 
shell is organized as columnar convection cells (Busse columns) as a result of 
strong constraint of Coriolis force. We found that helical flow structures in 
these columnar convection cells naturally incorporates the so-called alpha-
effect, and as a result, a dipole field is naturally generated by the MHD 
dynamo process [1]. We have demonstrated a physical mechanism of how the 
dipole field is generated.
We have also found in our subsequent simulations that the generated dipole 
moment spontaneously reverses its polarity [2,3,4]. Our simulations indicate 
that the generated magnetic energy alternate between a high-energy state and a 
low-energy state. An interesting feature found in our simulation is that the 
dipole polarity can reverse only at high-energy state where north-south 
symmetry of the convection pattern is broken and the columnar convection cells 
become vulnerable.

[1] A. Kageyama, and T. Sato, Phys. Rev. E, 55, (1997)
[2] M. M. Ochi, A. Kageyama, and T. Sato, Phys. Plasmas, 6, 777 (1999)
[3] A. Kageyama, M. M. Ochi, and T. Sato, Phys. Rev. Lett. 82, 5409 (1999)
[4] J. Li, A. Kageyama, and T., Science, 295, 2877 (2002).