A comparison of geomagnetic secular variation as recorded by historical, archaeomagnetic and palaeomagnetic measurements

Palaeomagnetic methods can extend the documentary record of changes in the Earth’s magnetic field far into the past. Tolerable agreement is found between various methods, demonstrating the geophysical value of palaeomagnetic experiments. Combining results from the different approaches of investigating secular change can lead to a better perspective and to superior models of geomagnetic field behaviour. Lake sediments have recently been found to hold remarkably detailed signatures of past field changes. A mathematical approach to formulating an empirical description of global geomagnetic field behaviour is proposed and applied to palaeomagnetic data spanning the last 10 ka.

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Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

Earth Planets and Space ◽

10.1186/s40623-020-01279-y ◽

2020 ◽

Vol 72 (1) ◽

Author(s):

Sabrina Sanchez ◽

Johannes Wicht ◽

Julien Bärenzung

Keyword(s):

Magnetic Field ◽

Secular Variation ◽

Geomagnetic Field ◽

Wave Number ◽

Main Magnetic Field ◽

Candidate Model ◽

Uncertainty Estimates ◽

Geomagnetic Field Models ◽

Dipole Configuration

Abstract The IGRF offers an important incentive for testing algorithms predicting the Earth’s magnetic field changes, known as secular variation (SV), in a 5-year range. Here, we present a SV candidate model for the 13th IGRF that stems from a sequential ensemble data assimilation approach (EnKF). The ensemble consists of a number of parallel-running 3D-dynamo simulations. The assimilated data are geomagnetic field snapshots covering the years 1840 to 2000 from the COV-OBS.x1 model and for 2001 to 2020 from the Kalmag model. A spectral covariance localization method, considering the couplings between spherical harmonics of the same equatorial symmetry and same azimuthal wave number, allows decreasing the ensemble size to about a 100 while maintaining the stability of the assimilation. The quality of 5-year predictions is tested for the past two decades. These tests show that the assimilation scheme is able to reconstruct the overall SV evolution. They also suggest that a better 5-year forecast is obtained keeping the SV constant compared to the dynamically evolving SV. However, the quality of the dynamical forecast steadily improves over the full assimilation window (180 years). We therefore propose the instantaneous SV estimate for 2020 from our assimilation as a candidate model for the IGRF-13. The ensemble approach provides uncertainty estimates, which closely match the residual differences with respect to the IGRF-13. Longer term predictions for the evolution of the main magnetic field features over a 50-year range are also presented. We observe the further decrease of the axial dipole at a mean rate of 8 nT/year as well as a deepening and broadening of the South Atlantic Anomaly. The magnetic dip poles are seen to approach an eccentric dipole configuration.

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Instability of thermoremanence and the problem of estimating the ancient geomagnetic field strength from non-single-domain recorders

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1507986112 ◽

2015 ◽

Vol 112 (36) ◽

pp. 11187-11192 ◽

Cited By ~ 6

Author(s):

Ron Shaar ◽

Lisa Tauxe

Keyword(s):

Magnetic Field ◽

Field Strength ◽

Geomagnetic Field ◽

Single Domain ◽

Standard Theory ◽

Systematic Bias ◽

The Past ◽

Deep Interior ◽

Relationship Of ◽

Geomagnetic Field Strength

Data on the past intensity of Earth’s magnetic field (paleointensity) are essential for understanding Earth’s deep interior, climatic modeling, and geochronology applications, among other items. Here we demonstrate the possibility that much of available paleointensity data could be biased by instability of thermoremanent magnetization (TRM) associated with non-single-domain (SD) particles. Paleointensity data are derived from experiments in which an ancient TRM, acquired in an unknown field, is replaced by a laboratory-controlled TRM. This procedure is built on the assumption that the process of ancient TRM acquisition is entirely reproducible in the laboratory. Here we show experimental results violating this assumption in a manner not expected from standard theory. We show that the demagnetization−remagnetization relationship of non-SD specimens that were kept in a controlled field for only 2 y show a small but systematic bias relative to sister specimens that were given a fresh TRM. This effect, likely caused by irreversible changes in micromagnetic structures, leads to a bias in paleointensity estimates.

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High-resolution paleomagnetic secular variation and relative paleointensity records from the western Canadian Arctic: implication for Holocene stratigraphy and geomagnetic field behaviourThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.GEOTOP Contribution 2008-0024.

Canadian Journal of Earth Sciences ◽

10.1139/e08-039 ◽

2008 ◽

Vol 45 (11) ◽

pp. 1265-1281 ◽

Cited By ~ 31

Author(s):

Francesco Barletta ◽

Guillaume St-Onge ◽

James E.T. Channell ◽

André Rochon ◽

Leonid Polyak ◽

...

Keyword(s):

Dipole Moment ◽

High Resolution ◽

Secular Variation ◽

Geomagnetic Field ◽

Special Issue ◽

Polar Climate ◽

Climate Stability ◽

Relative Paleointensity ◽

Holocene Stratigraphy ◽

Field Behaviour

Two piston cores recovered from the Chukchi and the Beaufort seas document Arctic Holocene geomagnetic field behaviour and highlight the potential of secular variation and relative paleointensity as a regional chronostratigraphic tool. Several centennial- to millennial-scale Holocene declination and inclination features can be correlated in both cores, with other high-resolution western North American lacustrine and volcanic paleomagnetic records and with records of changes in Earth’s dipole moment, supporting the geomagnetic origin of these features and implying that they are associated with changes in Earth’s dipole moment.

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Observations of the Earth's magnetic field made in Edinburgh from 1670 to the present day

Transactions of the Royal Society of Edinburgh Earth Sciences ◽

10.1017/s0263593300002029 ◽

1994 ◽

Vol 85 (4) ◽

pp. 239-252 ◽

Cited By ~ 23

Author(s):

D. R. Barraclough

Keyword(s):

Magnetic Field ◽

Mathematical Models ◽

Secular Variation ◽

Geomagnetic Field ◽

Upper Atmosphere ◽

The Difference ◽

Time Changes ◽

Relative Measurements ◽

Magnetic North ◽

Made In

AbstractMagnetic observations made at the same site give valuable information about the time changes (the secular variation) of the geomagnetic field. This paper gives details of all known measurements of the geomagnetic field in and around Edinburgh since the earliest observation of magnetic declination (the difference between true and magnetic north) by George Sinclair in 1670. Early observations of the strength of the field were only relative measurements. Approximate conversion factors are derived to enable these data to be expressed in modern absolute units (nanoteslas). Observed values of declination, inclination and the horizontal intensity of the geomagnetic field are plotted and compared with values computed from mathematical models of the field covering the interval 1690 to 1990, inclusive. The earlier observations were not corrected for the effects of the rapidly varying magnetic fields caused by electric currents in the upper atmosphere. The consequences of this are estimated.

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Some archaeomagnetic results from Greece

The Annual of the British School at Athens ◽

10.1017/s0068245400013733 ◽

1963 ◽

Vol 58 ◽

pp. 8-13 ◽

Cited By ~ 11

Author(s):

J. C. Belshé ◽

K. Cook ◽

R. M. Cook

Keyword(s):

Magnetic Field ◽

The Other ◽

Secular Change ◽

Magnetic Oxides ◽

Archaeological Remains ◽

Main Field ◽

Thermoremanent Magnetization ◽

The Past ◽

The Earth ◽

The Magnetic Field

Many clays and stones contain particles of magnetic oxides of iron. These particles, if heated above their Curie points, which range up to 670° C., lose whatever magnetism they have; and when they cool back through their Curie points, they acquire a new ‘thermoremanent’ magnetization under the influence of the surrounding magnetic field, which generally is the magnetic field of the earth. That field is changing continuously, both in direction and intensity, and the course of its secular change is not yet understood; the change is compound, one factor being the main field, which may be fairly stationary over long periods, and the other being the numerous minor regional fields, which move and alter relatively quickly and largely determine the local variations in the magnetic field. So it is dangerous to extrapolate values for local variations either for more than a century or two in time or for more than five to ten degrees in space. At present the best hope for discovering past changes in the earth's field is from the thermoremanent magnetization of burnt clays and stones, where the date of the burning is reasonably closely fixed from other evidence. Such knowledge is obviously of interest to geophysicists, but for periods and places where the past course of the earth's field has been ascertained, archaeomagnetism—that is the study of the thermoremanent magnetization of archaeological remains—can help archaeologists too. It should be evident on reflection that if an archaeomagnetic specimen is to be useful certain requirements are necessary. First, the locality where it was magnetized must be known. Secondly, for the study of direction, the sample's orientation at the time when it was magnetized must be recorded, so that the inclination [or dip] and declination [or compass bearing] of its own thermoremanent magnetism can be related to the horizontal and to true North respectively.

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On the determination of the size of the Earth’s core from observations of the geomagnetic secular variation

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1981.0009 ◽

1981 ◽

Vol 374 (1756) ◽

pp. 15-33 ◽

Cited By ~ 30

Keyword(s):

Magnetic Field ◽

Secular Variation ◽

Magnetic Data ◽

Time Interval ◽

Perfect Conductor ◽

Geomagnetic Secular Variation ◽

The Core ◽

First Case ◽

Fractional Error ◽

The Magnetic Field

When the magnetic field of a planet is due to self-exciting hydromagnetic dynamo action in an electrically conducting fluid core surrounded by a poorly-conducting ‘mantle', a recently proposed method (Hide 1978,1979) can in principle be used to find the radius r c of the core from determinations of secular changes in the magnetic field B in the accessible region above the surface of the planet, mean radius r s , with a fractional error in r c of the order of, but somewhat larger than, the reciprocal of the magnetic Reynolds number of the core. It will be possible in due course to apply the method to Jupiter and other planets if and when magnetic measurements of sufficient accuracy and detail become available, and a preliminary analysis of Jovian data (Hide & Malin 1979) has already given encouraging results. The ‘magnetic radius’ ̄r̄ c of the Earth’s molten iron core has been calculated by using one of the best secular variation models available (which is based on magnetic data for the period 1955-75), and compared with the ‘seismological’ value of the mean core radius, r c = 3486 ± 5 km. Physically plausible values of r̄ c are obtained when terms beyond the centred dipole ( n = 1) and quadrupole ( n = 2) in the series expansion in spherical harmonics of degree n = 1,..., ^ n ,..., n * are included in the analysis (where 2 ≼ ^ n ≼ n *≼ ∞). Typical values of the fractional error ( r̄ c - r c ) / r c amount to between 0.10 and 0.15. Somewhat surprisingly, this error apparently depends significantly on the value of the small time interval considered; the error of 2% found in the first case considered, for which ^ n — n * = 8 and for the time interval 1965-75, is untypically low. These results provide observational support for theoretical models of the geomagnetic secular variation that treat the core as an almost perfect conductor to a first approximation except within a boundary layer of typical thickness much less than 1 km at the core-mantle interface.

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Multipole Analysis. II. Geomagnetic Secular Variation

Australian Journal of Physics ◽

10.1071/ph690481 ◽

1969 ◽

Vol 22 (4) ◽

pp. 481 ◽

Cited By ~ 4

Author(s):

RW James

Keyword(s):

Magnetic Field ◽

Secular Variation ◽

Time Trends ◽

Earth’S Magnetic Field ◽

Geomagnetic Secular Variation ◽

Rates Of Change ◽

Multipole Analysis ◽

Earth's Magnetic Field

The method of multipole analysis described in Part I is applied to the Earth's magnetic field for various epochs between 1845 and 1965, allowing the geomagnetic secular variation to be illustrated by time trends in the multipole parameters. The rates of change of the multipole parameters are used to separate the secular variation into non.drifting, meridional drifting, and longitudinal drifting components, which are discussed in detail for the epoch 1965.

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Geomagnetic secular variation forecast using the NASA GEMS ensemble Kalman filter: A candidate SV model for IGRF-13

10.21203/rs.3.rs-45638/v2 ◽

2020 ◽

Author(s):

Andrew Tangborn ◽

Weijia Kuang ◽

Terence Sabaka ◽

Ce Ye

Keyword(s):

Kalman Filter ◽

Secular Variation ◽

Ensemble Kalman Filter ◽

Geomagnetic Field ◽

Model Bias ◽

Geomagnetic Secular Variation ◽

Forecast Period ◽

Time Period ◽

Correction Scheme ◽

Geomagnetic Field Models

Abstract We have produced a 5-year mean secular variation (SV) of the geomagnetic field for the period 2020-2025. We use the NASA Geomagnetic Ensemble Modeling System (GEMS), which consists of the NASA Goddard geodynamo model and ensemble Kalman filter (EnKF) with 400 ensemble members. Geomagnetic field models are used as observations for the assimilation, including gufm1 (1590-1960), CM4 (1961-2000) and CM6 (2001-2019). The forecast involves a bias correction scheme that assumes that the model bias changes on timescales much longer than the forecast period, so that they can be removed by successive forecast series. The algorithm was validated on the time period 2010-2015 by comparing with CM6 before being applied to the 2020-2025 time period. This forecast has been submitted as a candidate predictive model of IGRF-13 for the period 2020-2025.

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