Global Field Models Research Articles

Archaeomagnetic research in the Andean highlands

Eleven archaeomagnetic samples were collected from several sites near Porco, Bolivia as part of the Proyecto Arqueológico Porco-Potosí. These samples were collected in order to 1) explore the availability of appropriate sample materials in the Bolivian Andes, and 2) assess the feasibility of pursuing reference curve research for this region. In addition, this research explored the archaeomagnetic dating application of reference curves constructed from the global geomagnetic field model gufm1 ( Jackson et al., 2000), which covers the period from AD 1590–1990. This model is particularly applicable to our project, since many of the project sites are late prehispanic (pre-AD 1532) and early historic (ca. AD 1532–1825) in age. Of the 11 samples, three appeared to contain an insufficient amount of ferromagnetic minerals to provide reliable results, while eight samples produced acceptable-to-good results. The data from the eight good samples were compared to a reference curve for the period of AD 1600–2000 that was calculated for this project from the global field model, gufm1. The archaeomagnetic dates obtained for two of the samples agreed with archaeological expectations, while those for four other samples marginally or greatly postdated the archaeological age expectations. Two additional samples produced significantly different directions and could not be dated against the reference curve. It is likely that the age discrepancies for four of the samples stem from inaccuracies in the archaeomagnetic data, the archaeological age expectations, the calculated reference curve, or some combination of these factors.

On the persistence of geomagnetic flux lobes in global Holocene field models

To investigate the longevity and robustness of high-latitude flux patches in the geomagnetic field at the core–mantle boundary, we present time-dependent models of the geomagnetic field for the past 7000 years. Our models use the same data set as previously used for time-dependent archaeomagnetic field modelling, but constrained with additional priori models from time averages of field models covering the last 150, 400 and 3000 years. We find that the data are consistent with flux patches existing in both north and south hemispheres for the past 7000 years, and that the northern hemisphere patches at least have highly dynamic behaviour. Simple averaging of the historical field may not be appropriate to obtain a characteristic time-averaged model of the field for comparison with other geophysical observables. Our results should inform geodynamo studies of thermal core–mantle coupling, and of possible long-term structure in the geomagnetic field.

Establishing a first archaeointensity record for the SW Pacific

The dearth of archaeomagnetic intensity data from the southern hemisphere is a limiting factor in evaluating models of global geomagnetic field evolution during the Holocene. Here we present high quality microwave archaeointensity data obtained from 34 ceramic fragments (21 archaeological contexts) from the Duke of York Islands, Fiji and Vanuatu, SW Pacific. Complementary Thellier-type experiments, corrected for anisotropy give good agreement with the microwave results. The majority of the new data prior to 250 AD exhibit significantly lower intensity than predicted by current global field models (CALS3k.3 and ARCH3k) for the region, with an apparent intensity minimum at 250 BC reaching as low as 50% of the present-day field strength. Between 400 AD and 1500 AD, the data are broadly consistent with the global field models but with a 20% higher field between 1200 and 1400 AD. These new results therefore imply that the geomagnetic field has a greater range of variability than predicted and that further data from the region are essential to better constrain the global field models. The results also demonstrate that establishing an archaeomagnetic reference curve for dating SW Pacific ceramic artefacts is feasible, which has potentially significant implications for Pacific archaeological research.

Candidates for multiple impact craters?: Popigai and Chicxulub as seen by the global high resolution gravitational field model EGM2008

Abstract. In 2008 the new Earth Gravitational Model (EGM2008) was released. It contains a complete set of spherical harmonic coefficients of the Earth's gravitational potential (Stokes parameters) to degree 2190 and order 2159 and selected orders to degree 2190, that can be used for evaluation of various potential quantities with both the unprecedented accuracy and high spatial resolution. Two such quantities, the gravity anomaly and second-order radial derivative of the disturbing potential, were computed over selected areas with known impact craters. The displays of these derivatives for two such sites clearly show not only the strong circular-like features known to be associated with them but also other symmetrical structures which appear to make them multiple impact sites. At Popigai, Siberia, the series of circular features fall in a line from the "primary crater" in the southeast (SE) direction. At Chicxulub, Yucatán, there appears to be one more crater close to the "primary" in the northeast (NE) direction, as well as possibly others in the vicinity of the main crater (SW). Gravity information alone is not, however, proof of impact craters but it is useful in identifying candidate sites for further study, for examination by geologists and geophysicists. In the case of Chicxulub, a very recent single seismic profile suggests that a more likely explanation for the observed circular like gravity signal from EGM2008 NE of the "primary" is a pre-impact basin.

On the use of global potential field models for regional interpretation of the West and Central African Rift System

The use in regional interpretations of the Earth Gravity Model (EGM08) and Earth magnetic model (EMAG2) is evaluated by comparison to ground gravity and aeromagnetic data in the central sector of the West and Central African Rift System (WCARS). The comparison includes upward continuation, spectral analysis and pseudogravity calculation and statistical evaluation. A correlation between EMAG2 (which contains roughly 25 km resolution aeromagnetic data in the region) and near-surface aeromagnetic data over WCARS is only true for the very low wavelength part but a strong similarity between EGM08 and ground gravity data can be confirmed. Interpretation of the EGM08 data allows identifying and confirming the position of major structural trends, and provides new information on the crustal architecture. The lineaments limiting different grabens forming the Logone Birni basin and in the southern Chad basin are identified. The results presented here show the use of EGM08 data for regional interpretations over Cameroon and adjacent countries, which can overcome the absence and sparseness of data in developing countries and remote areas.

Error distribution in regional inversion of potential field data

An increasing number of regional inversions of gravity and magnetic field data have recently been computed, some based on satellite and ground data, some on satellite data only. In each case it is important to quantify the errors resulting from the confined data region or the satellite parameters. In this study we investigate the error distribution of regional inversions for scalar gravity and vector geomagnetic data from satellite data only. We include the effects of satellite altitude, size of the data region, signal-to-noise ratio and spherical harmonic degree of an a priori global field model. We evaluate our results on the basis of a regional inversion of the Earth's magnetic field.

Geomagnetic observations on Tristan da Cunha, South Atlantic Ocean

Few geomagnetic ground observations exist of the Earth's strongest core field anomaly, the South Atlantic Anom- aly (SAA). The geomagnetic repeat station on the island Tristan da Cunha, located half-way between South Africa and South America at 37° 05' S, 12° 18' W, is therefore of crucial importance. We have conducted several sets of repeat station measurements during magnetically quiet conditions (Kp 2o or less) in 2004. The procedures are de- scribed and the results are compared to those from earlier campaigns and to the predictions of various global field models. Features of the local crustal bias field and the solar quiet daily variation are discussed. We also evaluate the benefit of continuous magnetic field recordings from Tristan da Cunha, and argue that such a data set is a very valuable addition to geomagnetic satellite data. Recently, funds were set up to establish and operate a magnetome- ter station on Tristan da Cunha during the Swarm magnetic satellite mission (2011-2014).

Comparison of eight years magnetic field data from Cluster with Tsyganenko models in the inner magnetosphere

Abstract. Eight years of magnetic field data, taken while the four Cluster spacecraft pass through, or adjacent to, the equatorial ring current, have been surveyed to investigate the effects on the Earth's magnetic field of the externally driven current systems connecting the ionosphere, cusp and ring current regions. This study extends previous work to cover a greater range of orbit location and external conditions. We compare the modeled magnetic field from different global field models (Tsyganenko, 1989, 1996, and 2001, hereafter T89, T96 and T01) with data from the four Cluster spacecraft. Comparing with the different models allow us not only to characterize each model's performance, but also provides insight into the physical sources of observed signals. The data generally deviate much less from the expected model field during the years close to the solar minimum, implying that the models perform better during weaker geomagnetic activity. There are particular deviations from the models associated with the ring current (well-defined smooth trends) and region 2 field aligned currents (FACs) or low-altitude cusp FACs (sharp bipolar signatures). During the ring current crossings (through perigee, at 4–5 RE), the T96 model always overestimates the ring current, while the T01 and T89 models sometimes underestimate it. The sharp bipolar signatures are not always sampled, implying a localized extent, but only the T96 and T01 models include forms for the region 2 FACs and T01 appears to model these better. Overall, all deviations from T01 are much smaller than for the other models, indicating that this model achieves the best fit to the data. The 4 Cluster spacecraft observe nearly the same signatures at small separations (during the early years of the mission) but do sample different signatures at the large separations (during the later years). Using the four-spacecraft technique, we infer that the region 2 FACs, with a transverse thickness of ~0.17–0.54 RE, and cusp FACs, with a thickness of ~0.06–0.12 RE, are very stable in size and location.

What ancient scalar satellite data can tell us about the 1969 geomagnetic jerk?

Abstract The linearity of the secular variation of the Earth’s magnetic field is irregularly broken by sudden changes in its trend, known as geomagnetic jerks. Detecting these events in the temporal variations of the magnetic field components continues to be an exciting topic, mainly recently when their investigation rely not more only on observatory data, but as well as on measurements provided by satellites. We have also been interested by these magnetic events, and one of the central aims of our work has been to determine whether the satellite scalar data, obtained in the sixties and early seventies are able to reveal the existence of such a geomagnetic jerk, previously detected around 1969, in observatory annual or monthly means. For this purpose, we have used available OGO2, OGO4 and OGO6 satellite intensity data covering the period 1965–1971. Another motivation of re-processing and analyzing these old satellite datasets has been to better estimate their role in the global core field modeling, especially in describing the secular variation. Our results indicate that these ancient magnetic satellite datasets contain valuable information to characterize the secular variation over the time-span they are available, and allow to detect the geomagnetic jerk around 1969.

Geomagnetic field for 0–3 ka: 1. New data sets for global modeling

Paleomagnetic and archeomagnetic records are used in both regional and global studies of Earth's magnetic field. We present a description and assessment of five newly compiled data sets, also used in the companion paper by Korte et al. (2009) to produce a series of time‐varying spherical harmonic models of the geomagnetic field for the last 3000 years. Data are drawn from our compilation of lake sediment records and from the online database, GEOMAGIA50v2. The five selections are available from the EarthRef Digital Archive at http://earthref.org/cgi‐bin/erda.cgi?n=944. Data are grouped according to the source of material, and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows (the ARCH3k_dat data set) and for sediments (SED3k_dat). The overall number of data is 55% greater than in previous compilations. Constrained data sets were selected using different criteria for each group. Winnowing of archeological data was based on uncertainties supplied by the original data providers. The lake sediment data assessment relied on preassigned age uncertainties and one or more of the following: comparisons with archeomagnetic data from the same region, regional consistency among several lakes, and consistency with global archeomagnetic models. We discuss relative merits of a larger unconstrained data set or a smaller (possibly) more reliable one. The constrained data sets eliminate a priori up to 35% of the available data in each case and rely on potentially subjective assessments of data quality. Given the limited data available our analyses indicate that iterative rejection of a small number (1–1.5%) of outlying data during global field modeling is a preferable approach. Specific regional comparisons among the models and data support the conclusion that Korte et al.'s outlier‐free CALS3k.3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0–3 ka field; the ARCH3k.1 model provides a better fit to the denser European archeomagnetic data and may be better in that region.

Geomagnetic field hemispheric asymmetry and archeomagnetic jerks

We investigate the origin of the so-called archeomagnetic jerks detected in the French archeomagnetic record over the past three millennia. Although only very large-scale global archeomagnetic field models are currently available, we show that the occurrence of archeomagnetic jerks is intimately linked to what we define as “most eccentric” events, i.e., periods of time when a simple description of the geomagnetic field in terms of an eccentric dipole reveals the center of this eccentric dipole to strongly move away from the Earth's center. From the behavior of the much better known historical field, we interpret the evolution of the center of the eccentric dipole as reflecting the production and gathering of flux patches at the core–mantle boundary within preferential hemispheres. Archeomagnetic jerks would thus correspond to episodes of maximum geomagnetic field hemispheric asymmetry. Such “most eccentric” events could also provide an explanation for some of the properties previously reported in the long-term paleomagnetic field.

TOWARDS A NEW APPROACH TO ARCHAEOMAGNETIC DATING IN EUROPE USING GEOMAGNETIC FIELD MODELLING*

We introduce a new approach to producing secular variation curves for archaeomagnetic dating, using global geomagnetic field modelling techniques. Using published palaeosecular variation curves for five European locations as input, and a previous global model for regularization, we produce a model for the period ad 0–1900, which reproduces the input curves within their 95% confidence intervals, and successfully fits data from other regions in Europe. Our global model performs as well as a previous regional model, but additionally ensures consistency with physical constraints. We show that the curves currently used for dating are not mutually consistent, and therefore that in the future this new approach will provide more reliable curves for archaeomagnetic dating.

Efficient and Rapid Estimation of the Accuracy of GRACE Global Gravitational Field Using the Semi‐Analytical Method

AbstractIn this paper, firstly, the new combined error model of cumulative geoid height influenced by three error sources including the inter‐satellite range‐rate of K‐band ranging system, orbital position of GPS receiver and non‐conservative force of accelerometer from GRACE satellites is established using the semi‐analytical method. Secondly, the dependability of error models is demonstrated based on matching relationship among the accuracy indexes of key payloads. Finally, the accuracy of global gravitational field up to degree and order 120 is effectively and rapidly estimated from GRACE Level 1B measured observation errors of the year 2006 publicized by the Jet Propulsion Laboratory (JPL) in the USA, and the cumulative geoid height error is 18.368 cm at degree 120, which preferably accords with the Earth's gravitational field model EIGEN‐GRACE02S provided by the Geo‐ForschungsZentrum Potsdam (GFZ) in Germany. This work can provide theoretical foundation and calculational guarantee for the efficient and rapid estimation of the accuracy of high‐degree global gravitational field model in the future international satellite gravity measurement mission (e.g. GRACE Follow‐On, degree 360).

GeoForschungsZentrum Anomaly Magnetic Map (GAMMA): A candidate model for the World Digital Magnetic Anomaly Map

The World Digital Magnetic Anomaly Map (WDMAM) is an ongoing effort toward the mapping of worldwide available aeromagnetic data. It is led by a task force of the International Association for Geomagnetism and Aeronomy (IAGA) and aims at distributing a global map in printed and digital forms. In this paper, we describe in detail our candidate model which has to be evaluated by the IAGA task force together with five other candidate maps. After discussing the quality of the available data, we show a simple but effective method applied to successfully process, reduce, and merge together individual compilations. The near‐surface data are corrected using global field models and further refined with two‐dimensional polynomial corrections. After the upward continuation to 5 km altitude, data are resampled to a 3 minute grid and merged together. We then calculate a spherical harmonic model up to degree 199 and analyze the magnetic spectrum of the global map. This helps us to confirm that wavelengths larger than 400 km are spurious at a global scale in aeromagnetic compilations. Therefore we substitute them using a satellite‐based lithospheric field model (MF5) to degree 100. Finally, our proposed candidate map for WDMAM is presented.

An improved geomagnetic data selection algorithm for global geomagnetic field modelling

A spherical harmonic degrees 60, global internal field model is described (called BGS/G/L/0706). This model includes a degree 15 core and piecewise-linear secular variation model and is derived from quiet-time Ørsted and Champ satellite data sampled between 2001.0 and 2005.0. For the satellite data selection, a wide range of geomagnetic index and other data selection filters have been used to best isolate suitably quiet magnetospheric and ionospheric conditions. Only a relatively simple, degree one spherical harmonic, external field model is then required. It is found that a new ‘Vector Magnetic Disturbance’ index (VMD), the existing longitude sector A indices, the auroral zone index IE, and the polar cap index PC are better than Kp and Dst at rejecting rapidly varying external field signals at low, middle, auroral and polar latitudes. The model quality is further enhanced by filling spatial and temporal gaps in the quiet data selection with a second selection containing slightly more disturbed data. It is shown that VMD provides a better parametrization than Dst of the large-scale, rapidly changing, external field. The lithospheric field model between degrees 16 and 50 is robust and displays good coherence with other recently published models for this epoch. BGS/G/L/0706 also shows crustal anomalies consistent with other studies, although agreement is poorer in the southern polar cap. Intermodel coherency reduces above about degree 40, most likely due to incompletely filtered signals from polar ionospheric currents and auroral field aligned currents. The absence of the PC index for the southern hemisphere for 2003 onwards is a particular concern.

Global lithospheric magnetic field modelling by successive regional analysis

Abstract Present lithospheric field models, like the MF4 and CM4, are produced by least squares estimation using spherical harmonic basis functions with global support. Accounting for the different properties of magnetic data at low and high latitudes, a method that can take regional differences into account is proposed. Using four years of CHAMP satellite data, a detailed lithospheric magnetic field snapshot is obtained at 400 km altitude over the entire sphere by stitching together a dense coverage of regional models. The individual forward models computed on a quasi-regular grid over the Earth are then transformed to spherical harmonics by direct integration. Despite the stitching procedure, the long wavelength lithospheric features are correctly reproduced and small scale features are well resolved. Without regularization, the resulting model is stable to spherical harmonic degree 56. In addition to accounting for regionally varying noise levels, the proposed technique is also well suited to deal with incomplete data coverage issues when combining satellite with near surface data. The method could therefore make an important contribution to one of the main goals of the Swarm mission: to close the spectral gap between satellite and near-surface magnetic surveys.

Introducing localized constraints in global geomagnetic field modelling

Abstract A set of functions is defined that can be used for modelling the internal part of the geomagnetic field. These functions are represented in term of spherical harmonics of a given maximum degree L and are centred at specific latitudes and longitudes. The number of functions needed and the positions of their centres are such that any potential field of maximum spherical harmonic degree L can be modelled. Formulae are obtained to transform between the potential field representation using these functions and a classic spherical harmonic representation. The shape of these functions can be optimized to make them reasonably localized, and from there it is shown how a localized constraint can be applied to an internal geomagnetic field model. The technique is demonstrated by means of models built from a few months of the Swarm mission synthetic data set.

The effect of gravity and pressure in the electrodynamics of the low‐latitude ionosphere

The source of the plasma drift structure in the low‐latitude ionosphere during magnetically quiet times is generally understood. Nearly all the quiet time electric field structure has been attributed to divergences in the neutral wind current dynamo. However, there are other current drivers active in the low latitude ionosphere: a gravity‐driven current (g × B drift of O+) and a gradient‐pressure current (∇P × B drift of e and O+). The gravity‐driven current is important in the development of the Rayleigh‐Taylor instability and equatorial spread F (ESF) but is ignored in global dynamo electric field models. The gradient pressure and gravity terms in the ionosphere momentum equation normally oppose each other. This paper examines the magnitude of the large‐scale electric fields generated by divergences in these lesser currents in a coupled ionosphere‐electrodynamics model. The low‐latitude electric fields generated by the neutral wind dynamo are altered only slightly by these additional terms. The resulting vertical electric fields (zonal plasma drifts) do not change significantly but the vertical plasma drifts are affected by ∼10 to 15 m/s in the hours before sunrise and after sunset. These low‐latitude plasma drift differences are due entirely to the inclusion of the gravity‐driven current. The gravity current term produces a downward drift after sunset, which reduces the evening prereversal enhancement. Additionally, the positive vertical drift before sunrise is sometimes large enough to cause a predawn vertical enhancement. The gradient pressure terms can be ignored for large and medium scale ionosphere features.

Testing global storm‐time electric field models using particle spectra on multiple spacecraft

During a magnetic storm on 17–18 February 1998, several spacecraft sampled the inner magnetosphere over a wide range of L shells simultaneously and provided a unique opportunity to obtain particle spectra as a function of L shells, latitude, and local time. We present phase space modeling in a dipole magnetic field and with model electric fields during the recovery phase of that storm to explain the ion spectrograms obtained on three spacecraft (POLAR, EQUATOR‐S, and FAST). The particle signatures studied are from the late recovery phase of the storm, but the particles are affected by the electric fields along their trajectories also during main phase. Our goal is to test and possibly improve global electric field models, which are crucial to the evolution of the storm‐time particle distributions. We backtrace ion distributions from the satellite locations and keep track of charge exchange losses. We use the Volland‐Stern, Weimer 96, Weimer 2000, and modifications of Weimer models to best fit instantaneous potential measurements made by the electric field instrument on POLAR and the ion drift meter instrument on the Defense Meteorological Satellite Program fleet of satellites. We incorporate corotation and a simple, global axisymmetric inductive electric field due to the ring current changes. Significant differences with ion spectral observations do exist and cannot be accounted for simply by modification of existing models. Explaining those differences requires addition of local inductive electric fields or nightside injections.

Survey tracks current position of South Magnetic Pole

The search for the Earth's magnetic dip poles—the principal points on the surface where the geomagnetic field is vertical—was driven initially by a combination of adventurism and the need for magnetic compass correction information for navigation. The position and drift of the magnetic poles remain topics of popular interest today and are of scientific value; e.g., for testing the accuracy of global field models in polar regions. The South Magnetic Pole, first reached in January 1909 by Edgeworth David, Douglas Mawson, and Alistair Mackay during an epic man‐hauled sledge journey onto the interior of the Antarctic Plateau, drifted out to sea in about 1960.