Geo Dynamo

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    Gary A Glatzmaier

    The Geodynamoaleomagnetic records indicate that the geomagnetic field has existed for at least three billion years.

    However, based on the size and electrical conductivity of the Earth's core, the field, if it were notontinually being generated, would decay away in only about 20,000 years since the temperature of th

    ore is too high to sustain permanent magnetism. In addition, paleomagnetic records show that the dip

    olarity of the geomagnetic field has reversed many times in the past, the mean time between reversals

    eing roughly 200,000 years with individual reversal events taking only a couple thousand years.

    These observations argue for a mechanism within the Earth's interior that continually generates the

    eomagnetic field. It has long been speculated that this mechanism is a convective dynamo operating

    he Earth's fluid outer core, which surrounds its solid inner core, both being mainly composed of iron.

    The solid inner core is roughly the size of the moon but at the temperature of the surface of the sun. T

    onvection in the fluid outer core is thought to be driven by both thermal and compositional buoyancyources at the inner core boundary that are produced as the Earth slowly cools and iron in the iron-rich

    luid alloy solidifies onto the inner core giving off latent heat and the light constituent of the alloy. Th

    uoyancy forces cause fluid to rise and the Coriolis forces, due to the Earth's rotation, cause the fluid

    lows to be helical. Presumably this fluid motion twists and shears magnetic field, generating new

    magnetic field to replace that which diffuses away.

    However, until now, no detailed dynamically self-consistent model existed that demonstrated this coul

    ctually work or explained why the geomagnetic field has the intensity it does, has a strongly dipole-

    ominated structure with a dipole axis nearly aligned with the Earth's rotation axis, has non-dipolar fie

    tructure that varies on the time scale of ten to one hundred years and why the field occasionallyndergoes dipole reversals. In order to test the convective dynamo hypothesis and attempt to answer

    hese longstanding questions, the first self-consistent numerical model, the Glatzmaier-Roberts model,

    was developed that simulates convection and magnetic field generation in a fluid outer core surroundin

    olid inner core (Figure 1) with the dimensions, rotation rate, heat flow and (as much as possible) the

    material properties of the Earth's core [1-5]. The magnetohydrodynamic equations that describe this

    roblem are solved using a spectral method (spherical harmonic and Chebyshev polynomial expansion

    hat treats all linear terms implicitly and nonlinear terms explicitly [4]. These equations are solved ove

    nd over, advancing the time dependent solution 15 days at a time.
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    Fig.1 A snapshot of the region (yellow) where the fluid flow is the greatest. The core-mantle boundary

    he blue mesh; the inner core boundary is the red mesh. Large zonal flows (eastward near the inner c

    nd westward near the mantle) exist on an imaginary "tangent cylinder" due to the effects of large

    otation, small fluid viscosity, and the presence of the solid inner core within spherical shell of the ouluid core. (click on image to download, 0.15 Mb)

    The resulting three-dimensional numerical simulation of the geodynamo, run on parallel supercompute

    t the Pittsburgh Supercomputing Center and the Los Alamos National Laboratory, now spans more th

    00,000 years. The simulated magnetic field has an intensity and a dipole dominated structure that is v

    imilar to the Earth's (Figure 2) and a westward drift of the non-dipolar structures of the field at the

    urface that is essentially the same as the 0.2 degrees/year measured on the Earth. Our solution illustra

    ow the influence of the Earth's rotation on convection in the fluid outer core is responsible for this

    magnetic field structure and time dependence [1].

    Fig.2 A snapshot of the 3D magnetic field structure simulated with the Glatzmaier-Roberts geodynamo

    model. Magnetic field lines are blue where the field is directed inward and yellow where directed

    utward. The rotation axis of the model Earth is vertical and through the center. A transition occurs a

    he core-mantle boundary from the intense, complicated field structure in the fluid core, where the fie

    s generated, to the smooth, potential field structure outside the core. The field lines are drawn out to
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    wo Earth radii. Magnetic field is rapped around the "tangent cylinder" due to the shear of the zonal

    luid flow (Fig. 1). (click on image to download, 0.15 Mb)

    n addition, about 36,000 years into the simulation the magnetic field underwent a reversal of its dipol

    moment (Figure 3), over a period of a little more than a thousand years. The intensity of the magnetic

    ipole moment decreased by about a factor of ten during the reversal and recovered immediately after,

    imilar to what is seen in the Earth's paleomagnetic reversal record. Our solution shows how convecti

    n the fluid outer core is continually trying to reverse the field but that the solid inner core inhibits

    magnetic reversals because the field in the inner core can only change on the much longer time scale oiffusion [2]. Only once in many attempts is a reversal successful, which is probably the reason why t

    mes between reversals of the Earth's field are long and randomly distributed.

    Fig.3 Like in Fig. 2, but 500 years

    before the middle of a magnetic

    dipole reversal,

    at the middle of the reversal, and

    500 years after the middle of the

    reversal. (click on images to

    download, 0.15 Mb each)

    After the first magnetic reversal, we continued our simulation on two different branches: one by

    ontinuing to prescribe a uniform heat flux out of the core at the core-mantle boundary and the other b

    rescribing a heterogeneous heat flux there that is similar to the Earth's present pattern. The former haot reversed again; the latter underwent two more reversals, roughly 100,000 years apart. This

    emonstrates the influence the thermal structure in the lower mantle has on the style of convection and

    magnetic field generation in the fluid core below.

    One part of this numerical solution is the rotation rate of the solid inner core relative to the surface,

    which evolves according to the torque applied on the inner core by the generated magnetic field. Our

    olution shows how the field couples the inner core to the eastward flowing fluid above it (Figure 4a),

    eeping it in co-rotation [5]. This mechanism is analogous to a synchronous electric motor for which t

    ield, carried eastward by the fluid, acts like the rotating field in the stator and the inner core acts like

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    Fig.4 (a) A snapshot of the simulated magnetic field structure within the core, with lines blue where

    utside the solid inner core and yellow where inside. Again, the rotation axis is vertical. (click on ima

    o download, 0.24 Mb) (b) A schematic image illustrating the super-rotation of the inner core relativehe Earth's surface.

    The inner core in our simulation initially rotated between 2 and 3 degrees longitude per year faster tha

    he solid mantle and surface [1, 5]. This prediction in 1995 [1] for the Earth motivated two seismologi

    rom Columbia University in early 1996 to search for evidence of this super-rotation in 30 years of

    eismic data. They found evidence that supports our prediction and published it in July 1996 [6], (Figu

    b). More recent simulations of ours that now include a simple parameterization for the gravitational

    oupling that may exist between the mantle and the inner core have a much smaller inner core rotation

    mplitude; however, this rotation is still predominantly eastward relative to the model Earth's surface.

    For more about the Geodynamo, visit these links:

    NOVA's Magnetic Storm

    Geodynamo Computations at the Pittsburgh Supercomputing Center


    1] G.A. Glatzmaier and P.H. Roberts, "A three-dimensional convective dynamo solution with rotating

    nd finitely conducting inner core and mantle," Phys. Earth Planet. Inter., 91, 63-75 (1995).

    2] G.A. Glatzmaier and P.H. Roberts, "A three-dimensional self-consistent computer simulation of a

    eomagnetic field reversal," Nature, 377, 203-209 (1995).

    3] G.A. Glatzmaier and P.H. Roberts, "On the magnetic sounding of planetary interiors," Phys. Earth

    lanet. Inter., 98, 207-220 (1996).
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    4] G.A. Glatzmaier and P.H. Roberts, "An anelastic evolutionary geodynamo simulation driven by

    ompositional and thermal convection," Physica D, 97, 81-94 (1996).

    5] G.A. Glatzmaier and P.H. Roberts, "Rotation and magnetism of Earth's inner core," Science, 274,

    887-1891 (1996).

    6] X. Song and P. Richards, "Seismological evidence for differential rotation of the Earth's inner core

    Nature, 382, 221-224 (1996).