Guilielmi Gilberti Colcestrensis, medici londinensis, De magnete

Gilbert, W. S.

doi:10.5479/sil.113709.39088016899940

Cited by 49 publications

(61 citation statements)

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“…geomagnetism | paleomagnetism | Thellier paleointensity experiment | multidomain correction | time-averaged dipole field I n 1600, William Gilbert (1) articulated that Earth's magnetic field was well approximated by a bar magnet centered along its rotation axis. This model has become elaborated for the timeaveraged field as the geocentric axial dipole (GAD) hypothesis, which essentially all paleogeographic reconstructions rely on.…”

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confidence: 99%

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

Wang

Kent

Rochette

2015

Proc. Natl. Acad. Sci. U.S.A.

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The geomagnetic field is predominantly dipolar today, and highfidelity paleomagnetic mean directions from all over the globe strongly support the geocentric axial dipole (GAD) hypothesis for the past few million years. However, the bulk of paleointensity data fails to coincide with the axial dipole prediction of a factor-of-2 equator-to-pole increase in mean field strength, leaving the core dynamo process an enigma. Here, we obtain a multidomain-corrected Pliocene-Pleistocene average paleointensity of 21.6 ± 11.0 μT recorded by 27 lava flows from the Galapagos Archipelago near the Equator. Our new result in conjunction with a published comprehensive study of single-domain-behaved paleointensities from Antarctica (33.4 ± 13.9 μT) that also correspond to GAD directions suggests that the overall average paleomagnetic field over the past few million years has indeed been dominantly dipolar in intensity yet only ∼60% of the present-day field strength, with a long-term average virtual axial dipole magnetic moment of the Earth of only 4.9 ± 2.4 × 10 22 A·m 2 .geomagnetism | paleomagnetism | Thellier paleointensity experiment | multidomain correction | time-averaged dipole field I n 1600, William Gilbert (1) articulated that Earth's magnetic field was well approximated by a bar magnet centered along its rotation axis. This model has become elaborated for the timeaveraged field as the geocentric axial dipole (GAD) hypothesis, which essentially all paleogeographic reconstructions rely on. Paleomagnetic field directions for the past 5 My for both normal and reverse polarity chrons from around the globe show that the mean inclinations closely correspond to the GAD model (2), which predicts a simple relationship between mean inclination, I, and site latitude, Lat: tan(I) = 2 × tan(Lat). The GAD hypothesis also predicts that the mean field intensity should vary with latitude, as [1 + 3 × cos 2 (90°− Lat)] 1/2 , whereby the intensity at the poles is twice that at the Equator. The modern field [i.e., International Geomagnetic Reference Field (IGRF) (3)] is dominated by an axial dipole with steep field directions at the poles about twice the intensity (∼60 μT) compared with that (∼30 μT) of near-horizontal directions in the equatorial belt and corresponds to a virtual axial dipole moment (VADM) of ∼8 × 10 22 A·m 2 . However, the expected latitudinal dependence has yet to be shown in the current global paleointensity database PINT2014.01 (4) even for the most recent past 5 My. Analyses of these data using various selection criteria tend to show a puzzling near uniformity of mean intensities with latitude and largely because of the dominance of data from 15°t o 30°latitude with ages of 0.03-0.5 Ma, which average close to the present field intensity, would suggest the time-averaged field for normal and reversed polarity chrons over the past 5 My was also near the present VADM of ∼8 × 10 22 A·m 2 ( Fig. 1 and SI Appendix, SI Text and Fig. S1).The factor-of-2 difference between the poles and Equator provides the maximum signal to res...

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confidence: 99%

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

Wang

Kent

Rochette

2015

Proc. Natl. Acad. Sci. U.S.A.

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“…A landmark in the study of Earth's magnetism was the publication of "De Magnete" by William Gilbert in 1600 [Gilbert, 1600]. Considered by many to mark the beginning of modern science, the book was certainly the beginning of the scientifi c study of terrestrial magnetism.…”

Section: By William B Cade IIImentioning

confidence: 99%