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Does Zonal Swishing Play a Part in Earth’s Magnetic Field Reversals?

Zonal swishing in the Earth's outer core (Credit: Akira Kageyama, Kobe University)


Why does the Earth’s magnetic field ‘flip’ every million years or so? Whatever the reason, or reasons, the way the liquid iron of the Earth’s outer core flows – its currents, its structure, its long-term cycles – is important, either as cause, effect, or a bit of both.

The main component of the Earth’s field – which defines the magnetic poles – is a dipole generated by the convection of molten nickel-iron in the outer core (the inner core is solid, so its role is secondary; remember that the Earth’s core is well above the Curie temperature, so the iron is not ferromagnetic).

But what about the fine structure? Does the outer core have the equivalent of the Earth’s atmosphere’s jet streams, for example? Recent research by a team of geophysicists in Japan sheds some light on these questions, and so hints at what causes magnetic pole flips.

About the image: This image shows how an imaginary particle suspended in the liquid iron outer core of the Earth tends to flow in zones even when conditions in the geodynamo are varied. The colors represent the vorticity or “amount of rotation” that this particle experiences, where red signifies positive (east-west) flow and blue signifies negative (west-east) flow. Left to right shows how the flow responds to increasing Rayleigh numbers, which is associated with flow driven by buoyancy. Top to bottom shows how flow responds to increasing angular velocities of the whole geodynamo system.

The jet stream winds that circle the globe and those in the atmospheres of the gas giants (Jupiter, Saturn, etc) are examples of zonal flows. “A common feature of these zonal flows is that they are spontaneously generated in turbulent systems. Because the Earth’s outer core is believed to be in a turbulent state, it is possible that there is zonal flow in the liquid iron of the outer core,” Akira Kageyama at Kobe University and colleagues say, in their recent Nature paper. The team found a secondary flow pattern when they modeled the geodynamo – which generates the Earth’s magnetic field – to build a more detailed picture of convection in the Earth’s outer core, a secondary flow pattern consisting of inner sheet-like radial plumes, surrounded by westward cylindrical zonal flow.

This work was carried out using the Earth Simulator supercomputer, based in Japan, which offered sufficient spatial resolution to determine these secondary effects. Kageyama and his team also confirmed, using a numerical model, that this dual-convection structure can co-exist with the dominant convection that generates the north and south poles; this is a critical consistency check on their models, “We numerically confirm that the dual-convection structure with such a zonal flow is stable under a strong, self-generated dipole magnetic field,” they write.

This kind of zonal flow in the outer core has not been seen in geodynamo models before, due largely to lack of sufficient resolution in earlier models. What role these zonal flows play in the reversal of the Earth’s magnetic field is one area of research that Kageyama and his team’s results that will now be able to be pursued.

Sources: Physics World, based on a paper in the 11 February, 2010 issue of Nature. Earth Simulator homepage

Comments on this entry are closed.

  • Torbjorn Larsson OM February 20, 2010, 2:00 PM

    Totally off topic, “swishing” is one of my fav english words.

    Not that the presence or absence of a magnetic field seems to make much difference to us – all our small atmospheric planets (Venus, Earth, Mars, Titan) inherently looses ions at very closely the same rate despite having different types of fields (I have this on good authority), and the solar wind produced atmospheric loss is AFAIU not even an order of magnitude greater (but here I’m on shakier grounds) – but presumably it will make a difference for habitable zones around other systems.

    The more we know, the merrier it is.

  • DrFlimmer February 21, 2010, 3:11 AM

    At least the magnetic field shields us from cosmic rays, either by the sun or extra-solar sources. And since there are quite a lot high-energetically ionizing particles involved, the magnetic field is good shield for us. It’s no wonder that no airplane is supposed to fly over the poles….

  • Lawrence B. Crowell February 21, 2010, 5:42 AM

    As I remember the important part of this physics is the boundary layer between the inner and outer core. It is there that the convective flow sets up currents which run the magnetic dynamo.

    If the earth’s magnetic field were to turn off permanently the atmosphere of Earth would be over time blown into space by solar wind. Mars has been buffetted by this and is left with a residual atmosphere. Venus has no appreciable magnetic field but has a much bigger atmosphere, so it might take longer to bleed that off.

    LC

  • DrFlimmer February 21, 2010, 7:12 AM

    @ LBC

    Venus has a very thick atmosphere with very heavy molecules that cannot be blown away so easily, I think. Also, it’s a bit larger than Mars, which makes it easier for Venus to keep its atmosphere.

    And Mercury has a magnetic field, but not a significant atmosphere, afaik.

  • Aqua February 21, 2010, 9:20 AM

    “(the inner core is solid, so its role is secondary; remember that the Earth’s core is well above the Curie temperature, so the iron is not ferromagnetic).”

    …well above the Curie temperature…..meaning that the NiFe core materials become paramagnetic or randomly aligned atoms… BUT is that still true under increased pressures and in the presence of a low gauss high volume magnetic field? OR would that material actually become superconducting as it condenses closer toward degenerate matter?

  • Torbjorn Larsson OM February 21, 2010, 11:04 AM

    At least the magnetic field shields us from cosmic rays,

    No, that’s dominantly atmospheric stopping power. Or hydrospheric, if you ask the fishes. :-D

    If the earth’s magnetic field were to turn off permanently the atmosphere of Earth would be over time blown into space by solar wind.

    That is the question, isn’t it?

    For ion escape as such it doesn’t matter. A weaker and smaller induced field means higher escape rate over a smaller surface vs a stronger and larger dynamo field with lower escape rate over a larger surface.

    Presumably this first order balancing effect means that Earth field doesn’t protect us against solar wind in any effective sense either. The escape rates should be the same. (Taking into account that the solar wind intensity should depend on distance to the sun, I presume.)

    And again, for systems where the solar wind means much more so secondary effects are important, it may be very different.

  • Torbjorn Larsson OM February 21, 2010, 11:13 AM

    Oh btw, regarding cosmic rays. Isn’t it that the sun magnetic field makes a difference?

    IIRC the recent lull in the solar wind output has changed the cosmic ray density 2-3 times or so. (Vague memory.) Presumably increasing it, as the wind blows the magnetic bubble up, doesn’t it?

  • Jean Tate February 22, 2010, 6:06 AM

    The inner core is important, and so is the outer core-mantle boundary; their radii provide the all-but-rigid boundaries within which the inner core’s flows are constrained. In other words, they set the boundary conditions, as Lawrence B. Crowell said.

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