Geomagnetic spikes on the core-mantle boundary

📝 Abstract

Extreme variations of Earth’s magnetic field occurred in the Levant region around 1000 BC, when the field intensity rapidly rose and fell by a factor of 2. No coherent link currently exists between this intensity spike and the global field produced by the core geodynamo. Here we show that the Levantine spike must span > 60 degrees longitude at Earth’s surface if it originates from the core-mantle boundary (CMB). Several low intensity data are incompatible with this geometric bound, though age uncertainties suggest these data could have sampled the field before the spike emerged. Models that best satisfy energetic and geometric constraints produce CMB spikes 8-22 degrees wide, peaking at O(100) mT. We suggest that the Levantine spike reflects an intense CMB flux patch that grew in place before migrating northwest, contributing to growth of the dipole field. Estimates of Ohmic heating suggest that diffusive processes likely govern the ultimate decay of geomagnetic spikes.

💡 Analysis

Extreme variations of Earth’s magnetic field occurred in the Levant region around 1000 BC, when the field intensity rapidly rose and fell by a factor of 2. No coherent link currently exists between this intensity spike and the global field produced by the core geodynamo. Here we show that the Levantine spike must span > 60 degrees longitude at Earth’s surface if it originates from the core-mantle boundary (CMB). Several low intensity data are incompatible with this geometric bound, though age uncertainties suggest these data could have sampled the field before the spike emerged. Models that best satisfy energetic and geometric constraints produce CMB spikes 8-22 degrees wide, peaking at O(100) mT. We suggest that the Levantine spike reflects an intense CMB flux patch that grew in place before migrating northwest, contributing to growth of the dipole field. Estimates of Ohmic heating suggest that diffusive processes likely govern the ultimate decay of geomagnetic spikes.

📄 Content

ARTICLE Received 10 Feb 2016 | Accepted 11 Apr 2017 | Published 30 May 2017 Geomagnetic spikes on the core-mantle boundary Christopher Davies1,2 & Catherine Constable2 Extreme variations of Earth’s magnetic field occurred in the Levant region around 1000 BC, when the field intensity rapidly rose and fell by a factor of 2. No coherent link currently exists between this intensity spike and the global field produced by the core geodynamo. Here we show that the Levantine spike must span 460 longitude at Earth’s surface if it originates from the core–mantle boundary (CMB). Several low intensity data are incompatible with this geometric bound, though age uncertainties suggest these data could have sampled the field before the spike emerged. Models that best satisfy energetic and geometric constraints produce CMB spikes 8–22 wide, peaking at O(100) mT. We suggest that the Levantine spike reflects an intense CMB flux patch that grew in place before migrating northwest, contributing to growth of the dipole field. Estimates of Ohmic heating suggest that diffusive processes likely govern the ultimate decay of geomagnetic spikes. DOI: 10.1038/ncomms15593 OPEN 1 School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK. 2 Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093–0225, USA. Correspondence and requests for materials should be addressed to C.D. (email: c.davies@leeds.ac.uk). NATURE COMMUNICATIONS | 8:15593 | DOI: 10.1038/ncomms15593 | www.nature.com/naturecommunications 1 A key challenge to understanding the geodynamo process is to characterize and explain the most extreme spatial and temporal variations of Earth’s magnetic field. Dipole dominance, high- and low-latitude intense flux patches and weak variations in the Pacific hemisphere1,2 are robust characteristics of the field over the past few hundred years. Similar features have been reproduced in geodynamo simulations3 and have been used as criteria for assessing whether simulations produce Earth-like behaviour4–6. Whether these kinds of spatial variations can reflect long-term behaviour of the geomagnetic field requires detailed observations preceding the historical period. The highest geomagnetic field intensities on record have been recovered from archeomagnetic artefacts from the Levantine region dated at around 1000 BC. At this time the global field was unusually strong, with an increasing7–9 axial dipole moment (ADM) of B95–100 ZAm2. Yet the intensities recorded in Jordan10 and Israel11 around 980 BC correspond to local virtual ADMs (VADMs) of approximately 200 ZAm2. The detection of high VADMs in Turkey to the North12, in Georgia13 to the East, and highs 150–300 years earlier in China to the North-East14 lends support for a somewhat broader regional extent for this extreme feature, and an increasing number of high quality intensity data from distinct archaeological sites in Syria15 provide additional temporal context for Middle Eastern intensity variations over the past 9,000 years. However, the morphology and spatial extent of the Levantine spike are presently unknown. Recent work suggests the existence of a second geomagnetic spike in North America that may be coeval with the Levantine spike16. Here we focus on the Levant region before discussing the issue of multiple spikes. The Levantine geomagnetic spike is shown in Fig. 1, which presents six sets of spatially binned paleointensity data from sites with ages younger than 5000 BC and northern hemisphere locations from 15 to 60 E as downloaded from the online Geomagia.v3 database17 (http://geomagia.gfz-potsdam.de/ ) on 4 December, 2015. We use the VADM representation for the data to minimize geographical effects due to axial dipole variation and show all data with age and intensity uncertainties as assigned (or not) by the original authors. The spike is clearly visible in the 10–40 N, 30–45 E geographic bin in Fig. 1e, but not elsewhere. Previous attempts to relate the rate of field changes inferred from the spike to fluid flow at the top of the core require velocities that are much faster than those corresponding to present secular variation and flow morphologies that are very different to those obtained from frozen flux inversions of geomagnetic data or from the present generation of geodynamo simulations18,19 The 2,138 archeomagnetic intensity data shown in Fig. 1 form a subset of a much larger globally distributed collection of observations (480,000 over the interval 8000 BC to 1660 AD), that is dominated by directions and relative paleointensities from sediments and has been used to produce recent Holocene field models. A selection of maps of radial magnetic field, Br, at the core–mantle boundary (CMB) are predicted from snapshots of the CALS10k.2 model9 and shown in Fig. 2. From 2000 BC, flux at high northern latitudes moves equator-ward, slightly weakening the dipole. By 1500 BC a sma

This content is AI-processed based on ArXiv data.