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About 3,129 papers for “Secular variation”

SECULAR VARIATION

J. A. J. Matthews · Encyclopedia of Environmental Change · 2014

Cited by 0

Geomagnetic secular variation

Jeremy Bloxham, David Gubbins, Andrew Jackson · Philosophical Transactions of the Royal Society of London Series A Mathematical and Physical Sciences · 1989

Abstract We analyse over 175000 magnetic observations from an interval spanning 1695-1980 to produce a sequence of maps of the magnetic field at the core-mantle boundary; we find that even the earlier data enable us to determine reliable maps. We produce these maps at approximately 60-year intervals through the eighteenth and nineteenth centuries, and at 10-year intervals in the twentieth century. This span of maps is long enough to render straightforward the distinction between static and drifting features in the field: we observe that some features show no sign whatsoever of drift over the entire 285-year time interval, although others drift westwards. In particular, we observe that the secular variation is very low beneath the Pacific ocean, but beneath southern Africa and the South Atlantic ocean we observe rapid secular variation. We interpret the morphology of the static field in terms of a simple model of the dynamo, and conjecture that interactions between the core and the mantle are an important element of the process. As part of the static field we identify four main concentrations of flux, two in each hemisphere, at high latitudes: these features largely account for the Earth’s axial dipole moment. We find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models; much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots (a ‘core spot’) beneath southern Africa, early in this century. Nevertheless, we are able to construct maps that satisfy a set of necessary conditions for frozen-flux, and use these maps to construct maps of the core surface fluid flow, based on the steady flow hypothesis. Although we find no strong evidence against the steady flow hypothesis, we do find some grounds on which to doubt the validity of the flow maps.

Cited by 347

0 Permian Secular Variation

Zachary M. Menzo · 2013

This study uses 60 individual volcanic flows from the Early Permian in eastern Kazakhstan with the intention of analyzing to what extent secular variation of the geomagnetic field may be reduced during the Kiaman (Permo-Carboniferous) Reversed Polarity Superchron (KRPS). Such a study can be important because the confirmation of a relationship between secular variation and polarity reversal frequency during the Permian would allow for the use of paleosecular variation (PSV) data to infer the dynamo stability in the past, something that is currently not possible for times earlier than 160 Ma. The samples analyzed for the study were collected from a thick pile in the Chingiz Range of NE Kazakhstan with a U/Pb age of 283±2.4 Ma. The resulting data were compared to the data obtained by Dr. Bazhenov, who collected samples from the same volcanic pile for a parallel investigation. Nearly all of the paleomagnetic directions obtained in thermal demagnetization are seen to be SW and up, with a mean paleopole position at longitude (331.4E), and south-latitude (-37.9) confirming the expected mean Permian pole position. The resulting data also matched the anticipated high precision parameter (49.4), indicative of a stable geodynamo. This study therefore validates the correlation between secular variation and polarity reversal frequency.

Cited by 0

Secular Variation of Carloforte Latitude

A. Poma, S. Uras · International Astronomical Union Colloquium · 2000

Abstract The long series (1899.9–1979.0) of latitude observations made at Carloforte in the framework of the International Latitude Service is analyzed to determine a secular trend. The value of the linear rate obtained from the classical astrometric results agrees fairly well with the value derived from recent SLR and GPS observations carried out at the Cagliari Observatory.

Cited by 0Open access

Multipole Analysis and Secular Variation

D. E. Winch · Journal of geomagnetism and geoelectricity · 1968

The theory of multipoles is applied to the main field and secular variation field in order that the spherical harmonic coefficients of the secular variation field may be subdivided precisely into parts associated with the westward drifting, polewards drifting and nondrifting components of the main field.

Cited by 5Open access

Main field and secular variation

Joseph C. Cain · Reviews of Geophysics · 1995

The scope of this paper is the observational aspects of the Earth's main field and its secular change as known from historical observations. A few comments are also included in regard to paleomagnetic evidence, and a brief discussion is added on the main fields of other worlds of the Solar System. Such work as has been done in the past four years should eventually lead to a better understanding of the magnetic field of the Earth as well as planetary interiors. If the health of a discipline can be measured by the amount of controversy, this one must be extremely robust.

Cited by 9

The secular variation of latitudes

George C. Comstock · The Astronomical Journal · 1892

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Cited by 2

Secular variation of the geocenter

M. Greff‐Lefftz · Journal of Geophysical Research Atmospheres · 2000

The degree 1 deformations of Earth induced by a surface load are computed, in a reference frame related to the center of mass of the planet, using a theoretical approach (Love number formalism), at secular timescale, where Earth has a viscoelastic behavior. The temporal variation of the coordinates of the geocenter induced by the Pleistocenic deglaciation is investigated for two loading histories (three ice sheets with the same time dependence or ICE‐3G model) testing different viscosity profiles within the mantle. The present‐day secular variation of Earth's geocenter induced by postglacial rebound is found to be from 0.2 up to 0.5 mm/yr, and consequently, we suggest that observations from Space Geodesy of this rate could be used to constrain the viscosity contrast between upper and lower mantle.

Cited by 29Open access

Atmospheric Radiocarbon Variation and Secular Variation of Geomagnetic Moment

Kunihiko Kigoshi · The Quaternary Research (Daiyonki-Kenkyu) · 1977

A numerical computation of the secular variation of the production rate of atmospheric radiocarbon was carried using the secular variation of atmospheric radiocarbon concentration. In order to show the relation between atmospheric radiocarbon concentration and geomagnetism, the computed secular variation in radiocarbon production rate was converted to that of the geomagnetic moment, assuming a simple dependence of the production rate on the geomagnetic moment. The results of these computations of secular variation of the geomagnetic moment showed an essential discrepancy from the observed values in the period of de Vries's peak. This discrepancy shows that the simple dependence of the radiocarbon production rate on the geomagnetic moment does not hold in the computation of the secular variation of paleogeomagnetic moment from radiocarbon production rates. The results of this study indicate that the variation of the radiocarbon production rate depends not only on the variations in paleogeomagnetisms, but also upon the modulation of the magnetosphere by solar activities.

Cited by 0Open access

Secular variation in South African rainfall

University of the Witwatersrand, Johannesburg Institutional Repository on DSpace (University of the Witwatersrand, Johannesburg) · 2015

Annual rainfall totals over South Africa during
\nthe period 1910 - 1972, and where availability of data
\npermitted for the period 1880 - 197J, are examined for
\nevidence of secular variation. It is shown that no
\nevidence exists to support the view that the sub-continent
\nis undergoing progressive desiccation. With the aid of a
\nlinear model station records, grouped using principal
\ncomponent ana? vsis, are shown in some cases to possess
\ntrend on a micro-scale.
\nStrong evidence is provided to support the hypothesis
\nthat rainfall is oscillatory in nature. Oscillations in
\nthe ranges 16-20, 10-12, and 2 - 3 years are shown by
\ndifferent methods to be spatially dependent. A 3 - 4
\nyear oscillation is ubiquitous.
\nThe plausibility of using stochastic models to forecast
\nannual rainfall totals is investigated and tentative
\nforecasts made regarding the general pattern of rainfall
\nbehaviour over some regions of South Africa.

Cited by 0