Ecology Letters (2011) 14: 1170–1181
Interactions among species drive the ecological and evolutionary processes in ecological communities. These interactions are effectively key components of ...biodiversity. Studies that use a network approach to study the structure and dynamics of communities of interacting species have revealed many patterns and associated processes. Historically these studies were restricted to trophic interactions, although network approaches are now used to study a wide range of interactions, including for example the reproductive mutualisms. However, each interaction type remains studied largely in isolation from others. Merging the various interaction types within a single integrative framework is necessary if we want to further our understanding of the ecological and evolutionary dynamics of communities. Dividing the networks up is a methodological convenience as in the field the networks occur together in space and time and will be linked by shared species. Herein, we outline a conceptual framework for studying networks composed of more than one type of interaction, highlighting key questions and research areas that would benefit from their study.
Ten archeointensity results have been obtained from brick and ceramic fragments collected in France and precisely dated to between the tenth and eighteenth centuries. Intensity experiments were ...performed using the Triaxe protocol taking into account cooling rate and thermoremanent magnetization anisotropy effects. Together with our previous results from France and Belgium, we computed a geomagnetic field intensity variation curve for Western Europe covering the past 1100 years. This curve is characterized by a general decreasing trend at the millennial timescale punctuated by three short intensity peaks, during the twelfth century, around 1350–1400 AD and ∼1600 AD. A similar evolution but with smoother variations due to data scatter is also observed in Western Europe and to a lesser extent in Eastern Europe when all available archeointensity data fulfilling quality criteria are used. Comparison of our archeointensity variation curve with the climatic record derived from fluctuations in length of the Swiss glaciers shows a good temporal concordance between all geomagnetic field intensity maxima detected in Western Europe over the past millennium and colder episodes. A comparison is further discussed between these intensity maxima and episodes of low rates of 14C production. A common pattern of variations between both records is recognized between the middle of the tenth and of the beginning of eighteenth centuries. If significant, such coincidences suggest a dual geomagnetic and solar origin for the century‐scale climate and radionuclide production variations during at least the past millennium.
Key Points
New French archeointensity results dated to the past 1100 years were obtained
Three century‐scale intensity peaks in Western Europe over the past 1100 years
Geomagnetic field may partly modulate climate and 14C production rates
We investigated how the noise in satellite magnetic data affects magnetic lithospheric field models derived from these data in the special case where this noise is correlated along satellite orbit ...tracks. For this we describe the satellite data noise as a perturbation magnetic field scaled independently for each orbit, where the scaling factor is a random variable, normally distributed with zero mean. Under this assumption, we have been able to derive a model for errors in lithospheric models generated by the correlated satellite data noise. Unless the perturbation field is known, estimating the noise in the lithospheric field model is a non-linear inverse problem. We therefore proposed an iterative post-processing technique to estimate both the lithospheric field model and its associated noise model. The technique has been successfully applied to derive a lithospheric field model from CHAMP satellite data up to spherical harmonic degree 120. The model is in agreement with other existing models. The technique can, in principle, be extended to all sorts of potential field data with "along-track" correlated errors.
The forthcoming Swarm satellite mission is a constellation of three satellites dedicated to the study of the geomagnetic field. The orbital characteristics of the mission, which includes a pair of ...satellites flying side by side, has prompted new efforts in data processing and modeling. A consortium of several research institutions has been selected by the European Space Agency (ESA) to provide a number of Level-2 data products which will be made available to the scientific community. Within this framework, specific tools have been tailor-made to better recover the lithospheric magnetic field contribution. These tools take advantage of gradient properties measured by the lower pair of Swarm satellites and rely on a regional modeling scheme designed to better detect signatures of small spatial scales. We report on a processing chain specifically designed for the Swarm mission. Using an End-to-End simulation, we show that the tools developed are operational. The chain generates a model that meets the primary scientific objectives of the Swarm mission. We also discuss refinements that could also be implemented during the Swarm operational phase to further improve lithospheric field models and reach unprecedented spatial resolution.
The eleventh generation of the International Geomagnetic Reference Field (IGRF) was agreed in December 2009 by a task force appointed by the International Association of Geomagnetism and Aeronomy ...(IAGA) Division V Working Group V-MOD. New spherical harmonic main field models for epochs 2005.0 (DGRF-2005) and 2010.0 (IGRF-2010), and predictive linear secular variation for the interval 2010.0–2015.0 (SV-2010-2015) were derived from weighted averages of candidate models submitted by teams led by DTU Space, Denmark (team A); NOAA/NGDC, U.S.A. (team B); BGS, U.K. (team C); IZMIRAN, Russia (team D); EOST, France (team E); IPGP, France (team F); GFZ, Germany (team G) and NASA-GSFC, U.S.A. (team H). Here, we report the evaluations of candidate models carried out by the IGRF-11 task force during October/November 2009 and describe the weightings used to derive the new IGRF-11 model. The evaluations include calculations of root mean square vector field differences between the candidates, comparisons of the power spectra, and degree correlations between the candidates and a mean model. Coefficient by coefficient analysis including determination of weighting factors used in a robust estimation of mean coefficients is also reported. Maps of differences in the vertical field intensity at Earth’s surface between the candidates and weighted mean models are presented. Candidates with anomalous aspects are identified and efforts made to pinpoint both troublesome coefficients and geographical regions where large variations between candidates originate. A retrospective analysis of IGRF-10 main field candidates for epoch 2005.0 and predictive secular variation candidates for 2005.0–2010.0 using the new IGRF-11 models as a reference is also reported. The high quality and consistency of main field models derived using vector satellite data is demonstrated; based on internal consistency DGRF-2005 has a formal root mean square vector field error over Earth’s surface of 1.0 nT. Difficulties nevertheless remain in accurately forecasting field evolution only five years into the future.
A long standing question in geomagnetism is whether the time variation of the induced crustal field is a detectable quantity and, if so, at which spatial wavelengths. We tackle this problem with the ...help of a forward modeling approach using a vertically integrated susceptibility (VIS) grid of the Earth's crust. For spherical harmonic degrees 15–90, we estimate the root mean square of the crustal magnetic field secular variation to amount 0.06–0.12 nT/yr at the terrestrial surface between epochs 1960–2002.5. The geographical distribution of the signal shows absolute values reaching 0.65–1.30 nT/yr over South America. Unfortunately, most of the world magnetic observatories currently lie on quasi‐stationary features where the crustal field signal variations are expected to be very low. However, this long sought signal could be detected over well chosen regions, provided that satellite, observatory, and repeat station measurements are available over several decades.
The Magnetic Field of the Earth’s Lithosphere Thébault, Erwan; Purucker, Michael; Whaler, Kathryn A. ...
Space science reviews,
08/2010, Letnik:
155, Številka:
1-4
Journal Article, Conference Proceeding
Recenzirano
The lithospheric contribution to the Earth’s magnetic field is concealed in magnetic field data that have now been measured over several decades from ground to satellite altitudes. The lithospheric ...field results from the superposition of induced and remanent magnetisations. It therefore brings an essential constraint on the magnetic properties of rocks of the Earth’s sub-surface that would otherwise be difficult to characterize. Measuring, extracting, interpreting and even defining the magnetic field of the Earth’s lithosphere is however challenging. In this paper, we review the difficulties encountered. We briefly summarize the various contributions to the Earth’s magnetic field that hamper the correct identification of the lithospheric component. Such difficulties could be partially alleviated with the joint analysis of multi-level magnetic field observations, even though one cannot avoid making compromises in building models and maps of the magnetic field of the Earth’s lithosphere at various altitudes. Keeping in mind these compromises is crucial when lithospheric field models are interpreted and correlated with other geophysical information. We illustrate this discussion with recent advances and results that were exploited to infer statistical properties of the Earth’s lithosphere. The lessons learned in measuring and processing Earth’s magnetic field data may prove fruitful in planetary exploration, where magnetism is one of the few remotely accessible internal properties.
CHAMP satellite data and ground-based magnetic observations are used and combined to map the lithospheric magnetic field over China by means of the revised spherical cap harmonic analysis （R-SHCA） ...modeling technique. The magnetic field is de- scribed to a spatial resolution of 150 km at the mean Earth＇s radius, which represents a good compromise between the resolu- tions afforded by surface and satellite data. We compare the magnetic anomalies modeled at the regional scale with composite regions containing large-scale of tectonic structures. These regions, including the Tarim Basin and the Tibetan Plateau, are correlated with regional magnetic anomalies at satellite altitude but contain a significant number of small-scale and complex magnetic structures at the mean Earth＇s radius. These magnetic anomalies are globally consistent with the known geological features in China but also offer a way to delineate the contours of the geological blocks and to discuss the connection between magnetic anomalies and the heat flow distribution in this region.
We have recently proposed the revised spherical cap harmonic analysis (R‐SCHA) modeling technique. The new mathematical functions represent faithfully the spatial variations of potential fields in a ...restricted area. In this paper, we tackle the inverse problem and outline the efficiency of the new basis functions with respect to real magnetic data. Processing simultaneously repeat stations, observatory, aeromagnetic, and CHAMP satellite data provides our first vector lithospheric field model over France, which extends from surface to 500 km of altitude. The magnetic field is represented with a minimum horizontal spatial representation of 40 km at the mean Earth radius. The magnetic lithospheric map consistency is confirmed with a comparison to known geological features. The model variation with altitude also suggests that the major French magnetic anomaly, the Paris basin anomaly, is produced by a deep‐rooted geological structure. These results demonstrate the superiority of regional modeling over global modeling for delineating small‐scale details in the lithospheric field. In view of forthcoming satellite missions, like Swarm, the revised spherical cap harmonic analysis method will help to accurately represent the lithospheric field for more detailed geological interpretations.
We derive a lithospheric magnetic field model up to equivalent spherical harmonic degree 1000 over southern Africa. We rely on a joint inversion of satellite, near-surface, and ground magnetic field ...data. The input data set consists of magnetic field vector measurements from the CHAMP satellite, across-track magnetic field differences from the Swarm mission, the World Digital Magnetic Anomaly Map, and magnetic field measurements from repeat stations and three local INTERMAGNET observatories. For the inversion scheme, we use the revised spherical cap harmonic analysis (R-SCHA), a regional analysis technique able to deal with magnetic field measurements obtained at different altitudes. The model is carefully assessed and displayed at different altitudes and its spectral content is compared to high-resolution global lithospheric field models. By comparing the shape of its spectrum to a statistical power spectrum of Earth's lithospheric magnetic field, we infer the mean magnetic thickness and the mean magnetization over southern Africa.
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