Global geomagnetic field models for the past 3000 years: transient or permanent flux lobes?

Citation
Cg. Constable et al., Global geomagnetic field models for the past 3000 years: transient or permanent flux lobes?, PHI T ROY A, 358(1768), 2000, pp. 991-1008
Citations number
34
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Multidisciplinary
Journal title
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES
ISSN journal
1364-503X → ACNP
Volume
358
Issue
1768
Year of publication
2000
Pages
991 - 1008
Database
ISI
SICI code
1364-503X(20000315)358:1768<991:GGFMFT>2.0.ZU;2-0
Abstract
PSVMOD1.0 is a compilation of globally distributed palaeodirectional data f rom archaeomagnetic artefacts, lava flows, and lake sediments at 24 sites e valuated at 100 year intervals from 1000 BC to AD 1800. We estimate uncerta inty in these measures of declination and inclination by comparison with pr edictions from standard historical models in time-intervals of overlap, and use the 100-year samples and their associated uncertainties to construct a sequence of minimum structure global geomagnetic field models. Global pred ictions of radial magnetic field at the core mantle boundary (CMB), as well as inclination and declination anomalies at the Earth's surface, provide a n unprecedented view of geomagnetic secular variations over the past 3000 y ears, and demonstrate a consistent evolution of the field with time. Resolu tion of the models is poorest in the Southern Hemisphere, where only six of the 24 sites are located, several with incomplete temporal coverage. Low-f lux regions seen in the historical field near the North Pole are poorly res olved, but the Northern Hemisphere flux lobes are clearly visible in the mo dels. These lobes are not fixed in position and intensity, but they only ra rely venture into the Pacific hemisphere. The Pacific region is seen to hav e experienced significant secular variation: a strong negative inclination anomaly in the region, like that seen in 0-5 Ma models, persists from 1000 BC until AD 1000 and then gradually evolves into the smaller positive anoma ly seen today. On average bt tween 1000 BC and AD 1800, the non-axial-dipol e contribution to the radial magnetic field at the core-mantle boundary is largest in the north-central Pacific, and beneath Central Asia, with clear non-zonal contributions. At the Earth's surface, average inclination anomal ies are large and negative in the central Pacific, and most positive slight ly to the east of Central Africa. Inclination anomalies decrease with incre asing latitude. Average declinations are smallest in equatorial regions, ag ain with strong longitudinal variations, largest negative departures are ce ntred over Australia and Eastern Asia. Secular variation at the Earth's sur face is quantified by standard deviation of inclination and declination abo ut their average values, and at the CMB by standard deviation in radial mag netic field. All three show significant geographical variations, but appear incompatible with the idea that secular variation in the Pacific hemispher e is permanently attenuated by greatly enhanced conductivity in D " beneath the region.