Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Spin flip of Earth

  1. Apr 10, 2013 #1
    I was just reading in "Spin physics in semiconductors" by Dyaknonov section 1.2.4, page 4 in the footnote, how the earth's spin is expected to precess around its orbital angular momentum.

    Does anyone know if this has been considered as a possible cause for Geomagnetic reversal?

  2. jcsd
  3. Apr 10, 2013 #2
    I agree that such a correlation could have been plausible, but as far as I know there is none between the well known perbutations of the earth orbit and axis and the known geomagnetic reversals.

    Laskar and Vanyo did interesting research about that. You may also look at the modeling work of Gary Glatzmayer.

    Mind that the title of your thread sounds a bit deviant from science.
  4. Apr 18, 2013 #3
    I never heard of precession as an explanation. The precession of the earth's spin is periodic, with a cycle of 26,000 years. The earth's magnetic field flips randomly, but its average period is between 100,000 and 500,000 years. So the mismatch makes it extremely unlikely that one causes the other.

    Your reference claims that the flips of the earth's magnetic field occur on average every 450 KY, but vary a great deal.
    “A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field was the opposite. These periods are called chrons. The time spans of chrons are randomly distributed with most being between 0.1 and 1 million years[citation needed] with an average of 450,000 years.”

    This article claims that the precession of the earths spin takes place every 26 KY, like clockwork.
    “Axial precession is the movement of the rotational axis of an astronomical body, whereby the axis slowly traces out a cone. In the case of Earth, this type of precession is also known as the precession of the equinoxes, lunisolar precession, or precession of the equator. Earth goes through one such complete precessional cycle in a period of approximately 26,000 years or 1° every 72 years, during which the positions of stars will slowly change in both equatorial coordinates and ecliptic longitude.”

    The flips in the electron spin are usually caused by a radio frequency disturbance. So far as I know, there is no radio frequency disturbance that lasts 450 KY. There would have to be a driving field disturbance at the frequency of the flipping in order to make that analogy.

    According to dynamo theory, the average time may be related to the LCR resonance of the earth. LCR stands for inductance (L), capacitance (C) and resistance (C). Random motion of the liquid metal is driving the oscillation. However, the resonance forces the average period to be 450 KY.

    Aahh, but I should first introduce you to dynamo theory.

    The conventional explanation of what causes the flips is part of dynamo theory. Dynamo theory explains how magnetic fields are generated by the motion of electrically conducting fluids. Here is a summary of the theory as applied to the earth.

    The magnetic field of the earth is caused by an electric current generated by the motion of liquid metal in the center of the earth. The electric current is induced by the motion of the liquid metal. The motion of the liquid metal is caused by the combination of the earth’s spin and convection currents. Any magnetic field induces electric current into each dipole, which amplifies the magnetic field. A feed back loop ends up generating an electric current that circles the spin axis, resulting in a large magnetic dipole.

    The magnetic dipoles align in metastable arrangements. What makes the dipole metastable is the “inductance” of the liquid metal. However, there is nothing that in the long run prefers clockwise to anticlockwise. The motion of the liquid is basically random. So the magnetic dipole changes direction at random times, with an average spacing determined by the “inductance”, “capacitance”, and “resistivity” of the liquid metal.

    Dynamo theory and related experiments is very interesting. One general warning, though. Do not get it confused with ferromagnetism.

    The liquid at the core of the earth is liquid iron, liquid nickel and some impurities. Solid iron and solid nickel are ferromagnetic at temperatures below the Curie temperature. Therefore, the core would support a permanent magnetic dipole that never flipped if it were solid and below the Fermi temperature. So there are some nonscientists that have a belief that the earth is magnetic because it is made of iron.

    The core is liquid and way above the Fermi temperature. So the iron and nickel at the center of the earth is not ferromagnetic. The fact that the center of the earth is iron and nickel is a “coincidence” in terms of the magnetic field. Any electrical conducting fluid would generate a magnetic field if it was moved in the same way as in the earth. Most simulations of the "earth's magnetic field" so far have been performed with liquid sodium. Which is not ferromagnetic at all.

    Iron and nickel are electric conductors at the center of the earth. Metals don’t lose their conductivity when melted. Electric currents are induced in a moving liquid by magnetic fields. The “feed back loop” of inductance is what causes the magnetic field.

    The Wiki articles may be sufficient for your purposes.

    “In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales

    It was actually once believed that the dipole, which comprises much of the Earth's magnetic field and is misaligned along the rotation axis by 11.3 degrees, was caused by permanent magnetization of the materials in the earth. This means that dynamo theory was originally used to explain the Sun's magnetic field in its relationship with that of the Earth. However, this hypothesis, which was initially proposed by Joseph Larmor in 1919, has been modified due to extensive studies of magnetic secular variation, paleomagnetism (including polarity reversals), seismology, and the solar system's abundance of elements. Also, the application of the theories of Carl Friedrich Gauss to magnetic observations showed that Earth's magnetic field had an internal, rather than external, origin.”

    Pay wall abstract.
    “Various forms of the dynamo theory are presented in a graphic manner. Each of them depends on a flow pattern of presumably thermal convection in the earth's fluid core. The conducting fluid moves in magnetic fields generated by currents induced by the motion. Each of the flow patterns includes vortices, with helicity induced by the Coriolis force, that twist the magnetic fields in such a way as to regenerate an initial field. Some of them involve also differential rotation, with the parts of the core near the axis rotating more rapidly than the outer parts. Some forms of the theory are more successful than others in accounting qualitatively for the various observed aspects of the field, particularly the westward drifts and the occasional polarity reversals. The thermal energy source may be radioactivity of heat or crystallization at the inner-core surface.”
    Last edited: Apr 18, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook