Public message 3284 SCIENCE Area 19:52 Wednesday 4Sep91 To: MIKE DAVEY Re: EARTH'S MAGNETI

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Public message 3284 SCIENCE Area 19:52 Wednesday 4-Sep-91 From: HENRY SHAW To: MIKE DAVEY Re: EARTH'S MAGNETIC FIELD 2 ...continued from previous It has been known since shortly after Gauss first measured the strength of the field in ~1830 that the magnetic dipole moment of the earth is decreasing. This decrease has averaged about 0.05%/yr since 1830. There are, however, significant fluctuations about this average and in about 1960, the rate of decrease increased significantly (see Fraser-Smith, A. C., 1987, Centered and eccentric geomagnetic dipoles and their poles, 1600-1985. Reviews of Geophysics, v25, pp.1-16.). The magnetic field of the earth is *not* a simple dipole (the field associated with, say, an "ideal" bar magnet). Though the dipole field is the dominant one today at the surface, the whole field is a bumpy, lumpy sort of thing. Mathematically, one can describe the total field by a sum of functions known as spherical harmonics. [Conceptually, this is similar to describing the motions of a vibrating guitar string; there is the fundamen- tal frequency and there are also higher frequency "harmonic" vibrations.] As the dipole field of the earth decays, higher-order "bumps" in the field grow, which implies that the total energy stored in the field has remained approximately constant (Verosub, K., and Cox, A., 1971, Changes in the total magnetic energy external to the earth's core, Jour. Geomagnetism and Geoelectricity, v23, pp. 235-42). The field we see at the surface of the earth is probably the result of a the sum over the fields produced by a number of convective cells in the outer core. Each of which produces a poloidal field. As the size, shape, and location of each of these cells changes over time, the net field we see at the surface undergoes corre- sponding changes. There is ample evidence that the present time-variation of the field (the decreasing dipole moment) has not always been the case. Over short timescales (thousands of years), one can determine the strength of the paleo-field by measuring the strength of the remanent magnetism of baked clay pots and bricks and comparing that to the intensity produced in the same samples by heating and cooling them in magnetic fields of varying strength in the laboratory. Because of the "bumpiness" of the field over the surface of the earth at any given time, one cannot infer the strength of the dipole from these types of measurements made at a single site; one needs data from around the world. Such data have been obtained on artifacts dating back more than 12,000 years, and the resulting curve, consisting of 1167 individual determinations for the variations in the intensity of the dipole field shows that the dipole moment has decreased for the past 2000 years, extending the historical record. The decay over the past 1000 years has been particularly fast. From 6000 to 3000 years before present, however, there was a significant INCREASE in the strength of the dipole (McElhinny, M., and Senanayake, W. E., 1982, Variations in the geomagnetic dipole 1: the past 50,000 years. J. Geomag. Geoelect. v34, pp.39-51.). continued next message... --- TBBS v2.1/NM * Origin: Diablo Valley PCUG-BBS, Walnut Creek, CA 510/943-6238 (1:161/55)

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