Early geodynamo work

Regarding the February cover article on the quest for a laboratory geodynamo (PHYSICS TODAY, February 2006, page 13), it is important, as Isaac Newton said, to "stand on the shoulders of giants" as we advance our understanding. Unfortunately, the present generation often fails to do so. Consider, for example, the Bullard–Rikitake dynamo theory, which explains not only the axial field but also its periodic spontaneous reversal, as observed in ocean-bottom cores. Here is the background:

In the early 1950s, Edward Bullard and a student of his named Rikitake built a geodynamo at the University of Newcastle upon Tyne in the UK. It consisted of two counter-rotating iron cylinders about two meters in diameter, connected electrically by an equatorial layer of mercury. It generated an axial magnetic field that spontaneously reversed its direction every 20 minutes, as Earth's field is known to do every 10 000 years or so.

The actual geodynamo has yet another peculiar and unexplained property: It is substantially off-center by about 10% of Earth's diameter. Earth's field is about 0.6 gauss in Siberia, and about 0.1 gauss in the diametrically opposite region in the southern Atlantic Ocean, as I pointed out in a paper presented in the 1950s at a symposium at Newcastle. This asymmetry is considerably harder to explain than the field generation itself or its periodic reversals.

I mention this for readers who may be interested in joining this fascinating field of experimental geophysics.

The different research groups cited in Bertram Schwarzschild's story about experiments using a laboratory analogue of the geodynamo may have been unaware of my old paper published back in 1963.¹ The diagram of the apparatus in the PHYSICS TODAY story and the corresponding diagram in my paper show almost identical designs. Apart from the Helmholtz coils needed for a dynamo seed field, both figures show two propellers driving the liquid metal in opposite directions. My paper was stimulated by the pioneering work of Walter M. Elsasser.

Reference

1. 1.F. Winterberg, Phys. Rev. 131, 29 (1963) [SPIN].

[Editor's note: Daniel Lathrop, one of the researchers consulted for the original story, was invited to respond to Friedwardt Winterberg's comments.]

Lathrop comments: After the first two successful liquid metal dynamos,¹ there has been considerable recent activity in experiments seeking dynamo action in less constrained flows.² Much initial motivation for these experiments had been the work of Martin L. Dudley and Ronald W. James³ from 1989. It is clear now, in hindsight, that Winterberg's 1963 paper⁴ predates these experimental attempts and much of the earlier motivating theory. His paper gives a detailed analysis of different experimental possibilities for probing dynamo action using liquid metals.

Plainly, it has been an oversight of the community to not have recognized Winterberg's contribution before now.

References

1. 1.A. Gailitis et al., Phys. Rev. Lett. 86, 3024 (2001) [MEDLINE]; R. Stieglitz, U. Muller, Phys. Fluids 13, 561 (2001) [SPIN].
2. 2.P. Odier, J.-F. Pinton, S. Fauve, Phys. Rev. E 58, 7397 (1998) [SPIN]; N. L. Peffley, A. Cawthorne, D. Lathrop, Phys. Rev. E 61, 5287 (2000) [MEDLINE]; E. J. Spence et al., Phys. Rev. Lett. 96, 055002 (2006) [MEDLINE].
3. 3.M. L. Dudley, R. W. James, Proc. R. Soc. A 425, 407 (1989) [INSPEC].
4. 4.F. Winterberg, Phys. Rev. 131, 29 (1963) [SPIN].

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