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Richard Holme - One of the best experts on this subject based on the ideXlab platform.
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the geomagnetic jerk of 2003 5 characterisation with regional observatory Secular Variation data
Physics of the Earth and Planetary Interiors, 2018Co-Authors: Richard Holme, Yan Feng, Grace Cox, Yi JiangAbstract:Abstract The 2003.5 geomagnetic jerk was identified in geomagnetic records from satellite data, and a matching feature reported in Variations in length-of-day (ΔLOD), but detailed study has been hampered by lack of geomagnetic observatory data where it appears strongest. Here we examine Secular Variation (annual differences of monthly means) based on a new resource of 43 Chinese observatory records for 1998 until the present, focusing on 10 series of particularly high quality and consistency. To obtain a clean series, we calculate the covariance matrix of residuals between measurements and a state-of-the-art field model, CHAOS-6, and use eigenvalue analysis to remove noisy contributions from the uncorrected data. The magnitude of the most significant eigenvector correlates well with Dcx (corrected, extended Dst), suggesting the noise originates from unmodelled external magnetic field. Removal of this noise eliminates much coherent misfit around 2003—2005; nevertheless, the 2003.5 jerk is seen clearly in the first time derivative of the East component in Chinese data, and is also seen in the first time derivative of the vertical component in European data. Estimates of the jerk time are centred on 2003.5, but with some spatial Variation; this Variation can be eliminated if we allow a discontinuity in the Secular Variation as well as its temporal gradient. Both regions also provide evidence for a jerk around 2014, although less clearly than 2003.5. We create a new field model based on new data and CHAOS-6 to further examine the regional signals. The new model is close to CHAOS-6, but better fits Chinese data, although modelling also identifies some data features as unphysical.
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modeling the jovian magnetic field and its Secular Variation using all available magnetic field observations
Journal of Geophysical Research, 2016Co-Authors: Victoria Ridley, Richard HolmeAbstract:We present new models of Jupiter's internal magnetic field and Secular Variation from all available direct measurements from three decades of spacecraft observation. A regularized minimum norm approach allows the creation of smooth, numerically stable models displaying a high degree of structure. External field from the magnetodisk is modeled iteratively for each orbit. Jupiter's inner magnetosphere is highly stable with time, with no evidence for Variation with solar activity. We compare two spherical harmonic models, one assuming a field constant in time and a second allowing for linear time Variation. Including Secular Variation improves data fit with fewer additional parameters than increasing field complexity. Our favored solution indicates a ∼0.012% yr−1 increase in Jupiter's dipole magnetic moment from 1973 to 2003; this value is roughly one quarter of that for Earth. Inaccuracies in determination of the planetary reference frame cannot explain all the observed Secular Variation. Should more structure be allowed in the solutions, we find the northern hemispherical configuration resembles recent models based on satellite auroral footprint locations, and there is also evidence of a possible patch of reversed polar flux seen at the expected depth of the dynamo region, resembling that found at Earth and with implications for the Jovian interior. Finally, using our preferred model, we infer flow dynamics at the top of Jupiter's dynamo source. Though highly speculative, the results produce several gyres with some symmetry about the equator, similar to those seen at Earth's core-mantle boundary, suggesting motion on cylinders aligned with the rotation axis.
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Mapping geomagnetic Secular Variation at the core-mantle boundary
Geophysical Journal International, 2011Co-Authors: Richard Holme, Nils Olsen, F. L. BairstowAbstract:SUMMARY Data from recent satellite missions have vastly increased the resolution of models of the geomagnetic field, and itsfirst and second time derivatives – Secular Variation (SV) and Secular acceleration (SA). The spectra of both SV and SA are ‘blue’ at the core–mantle boundary, both well-fit by functions proportional to l(l + 1) where l is the spherical harmonic degree. The ratio of the two spectra defines a timescale for geomagnetic Variations of approximately 10 yrs for all resolvable harmonic degrees. The blue spectra should prevent meaningful maps of the SV being generated; nevertheless, the coherence of the maps up to harmonic degree 13 suggests that it is possible to obtain useful insight from their examination. Low SV is confirmed under the Pacific, but also revealed under the North Atlantic and Antarctica. These features are more readily explained in terms of dynamo control through thermal core–mantle coupling than by electromagnetic screening. Comparison with maps from measurements prior to the recent satellites, using the ‘Comprehensive Model’, suggests that models back to at least 1970 are sufficiently good to enable direct comparison of the SV.
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signal from noise in geomagnetic field modelling denoising data for Secular Variation studies
Geophysical Journal International, 2011Co-Authors: Ingo Wardinski, Richard HolmeAbstract:SUMMARY We analyse the residuals between a continuous geomagnetic field model of the internal field, C3FM, and observed Secular Variation. A large part of the residual Variations correlate closely with the DST-index, suggesting an origin from unmodelled external field Variations. Removal of this signal enhances the resolution of fine-scale detail in Secular Variation; this is useful in considering the phenomenology of geomagnetic jerks. The residual Variations between different observatories show even better correlation, suggesting the possibility of the construction of a proxy for the DST-index. Notable cross-correlation is also seen between the residuals and DST-index with a lag of about 55 months.
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a time dependent model of the earth s magnetic field and its Secular Variation for the period 1980 2000
Journal of Geophysical Research, 2006Co-Authors: Ingo Wardinski, Richard HolmeAbstract:[1] This study presents an investigation and description of the Secular Variation of the Earth's magnetic field between 1980 and 2000. A time-dependent model, C3FM (Continuous Covariant Constrained endpoints Field Model), of the main field and its Secular Variation between 1980 and 2000 is developed, with Gauss coefficients expanded in time on a basis of cubic B splines. This model is constrained to fit field models from high-quality vector measurements of Magsat in 1980 and Orsted in 2000 and to fit both magnetic observatory and repeat station Secular Variation estimates for the period in between. These Secular Variation estimates (first time derivatives) are derived from observatories monthly or annual means and repeat station data in order to reduce the contributions of crustal noise, annual, and semiannual Variation. On average, the model input consists Secular Variation estimates of the X, Y, and Z components at 130 locations per month. Treatment of covariance between the different components allows a higher temporal sensitivity of the model, due to the exclusion of some external field Variation. The model is computed up to degree and order 15. The model is a useful extension of the hitherto existing time-dependent description of the Secular Variation, GUFM which describes the Secular Variation until 1990. It reveals a short-term Secular Variation on subdecadal timescale and has a higher spatial resolution, than previously resolved. The model is also valuable to test the frozen flux hypothesis and to link features of the radial field at the core-mantle boundary to the geodynamo.
Nils Olsen - One of the best experts on this subject based on the ideXlab platform.
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Mapping geomagnetic Secular Variation at the core-mantle boundary
Geophysical Journal International, 2011Co-Authors: Richard Holme, Nils Olsen, F. L. BairstowAbstract:SUMMARY Data from recent satellite missions have vastly increased the resolution of models of the geomagnetic field, and itsfirst and second time derivatives – Secular Variation (SV) and Secular acceleration (SA). The spectra of both SV and SA are ‘blue’ at the core–mantle boundary, both well-fit by functions proportional to l(l + 1) where l is the spherical harmonic degree. The ratio of the two spectra defines a timescale for geomagnetic Variations of approximately 10 yrs for all resolvable harmonic degrees. The blue spectra should prevent meaningful maps of the SV being generated; nevertheless, the coherence of the maps up to harmonic degree 13 suggests that it is possible to obtain useful insight from their examination. Low SV is confirmed under the Pacific, but also revealed under the North Atlantic and Antarctica. These features are more readily explained in terms of dynamo control through thermal core–mantle coupling than by electromagnetic screening. Comparison with maps from measurements prior to the recent satellites, using the ‘Comprehensive Model’, suggests that models back to at least 1970 are sufficiently good to enable direct comparison of the SV.
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geomagnetic core field Secular Variation models
Space Science Reviews, 2010Co-Authors: Nicolas Gillet, Vincent Lesur, Nils OlsenAbstract:We analyse models describing time changes of the Earth’s core magnetic field (Secular Variation) covering the historical period (several centuries) and the more recent satellite era (previous decade), and we illustrate how both the information contained in the data and the a priori information (regularisation) affect the result of the ill-posed geomagnetic inverse problem. We show how data quality, frequency and selection procedures govern part of the temporal changes in the Secular Variation norms and spectra, which are sometimes difficult to dissociate from true changes of the core state. We highlight the difficulty of resolving the time variability of the high degree Secular Variation coefficients (i.e. the Secular acceleration), arising for instance from the challenge to properly separate sources of internal and of external origin. In addition, the regularisation process may also result in artificial changes in the model norms and spectra. Model users should keep in mind that such features can be mis-interpreted as the signature of physical mechanisms (e.g. diffusion). Finally, we present perspectives concerning core field modelling: imposing dynamical constraints (e.g. by means of data assimilation) reduces the non-uniqueness of the geomagnetic inverse problem.
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investigation of a Secular Variation impulse using satellite data the 2003 geomagnetic jerk
Earth and Planetary Science Letters, 2007Co-Authors: Nils Olsen, M. MandeaAbstract:Abstract Observatory monthly means provide an excellent opportunity to study the temporal changes of the magnetic field at a given location. Unfortunately, the uneven distribution of the present observatory network makes it difficult to determine the global field change pattern. Recently, we have developed an approach to extract satellite monthly means at a regular network of “virtual observatories” at 400 km altitude, based on CHAMP magnetic measurements. Using monthly means for 2001–2005 from those “virtual observatories” we investigate the space–time structure of the short-period Variation of the Earth's magnetic field by means of a Spherical Harmonic Expansion, followed by a separation into external (magnetospheric) and internal part. This allows, for the first time, to study the Secular Variation globally and directly from satellite magnetic data. Analyzing the time series of the magnetic field at the “virtual observatories” as well as those of the spherical harmonic expansion coefficients, we detect a Secular Variation impulse (an abrupt jump in the second time derivative of the magnetic field) in the CHAMP satellite data during the first months of the year 2003. The jerk occurred simultaneously in the northern and southern hemispheres in a rather limited area near 90° E, with maximum jerk strength at about ± 30° latitude, a region also characterized by a strong Secular acceleration (second time derivative of the magnetic field). We show that the 2003 geomagnetic jerk is not worldwide in occurrence and that there is an evidence for this event in the length-of-day Variation.
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core surface flow modelling from high resolution Secular Variation
Geophysical Journal International, 2006Co-Authors: Richard Holme, Nils OlsenAbstract:S U M M A R Y Data from the Orsted and CHAMP satellites have allowed modelling of the geomagnetic Secular Variation (SV ) with unprecedented accuracy. The spectrum of the SV is consistent with generation by advectively dominated processes. Based on the SV model, it is not possible to reject the frozen-flux hypothesis, but the spectrum of the SV implies that a conclusive test of frozen-flux is not possible. We parametrize the effects of diffusion as an expected misfit in the flow prediction due to departure from the frozen-flux hypothesis; at low spherical harmonic degrees, this contribution dominates the expected departure of the SV predictions from flow to the observed SV , while at high degrees the SV model uncertainty is dominant. We construct fine-scale core surface flows to model the SV . Flow non-uniqueness is a serious problem because the flows are sufficiently small scale to allow flow around non-uniqueness contours. Nevertheless, we find evidence to support previously suggested polar vortices. For this model of field and SV , predicted Variations in length of day from modelled core angular momentum vary over a large range, although there is evidence that this effect is reduced with longer time-series of magnetic data and better parametrization of the external magnetic field.
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a new approach to directly determine the Secular Variation from magnetic satellite observations
Geophysical Research Letters, 2006Co-Authors: Mioara Mandea, Nils OlsenAbstract:[1] Observatory monthly means provide an excellent opportunity to study the temporal changes of the geomagnetic field at a given location. Unfortunately, determination of the global pattern of changes using observatory data is hampered by their uneven distribution. Satellite data provide excellent global coverage, but the spacecraft movement makes direct comparisons of satellite and observatory data difficult. To investigate short-period Secular Variation in observatory and satellite data, we developed an approach to extract satellite monthly means for “virtual observatories” at 400 km altitude, using CHAMP magnetic measurements. Comparison of these virtual observatory monthly means with the corresponding ground values shows a remarkably well-correlated signal at time-scales of months to years, which is beyond the temporal resolution limit of recent global models. Here, we describe this newly developed approach, its validation, and discuss how it can be used to understand short-period changes of the recent geomagnetic field.
Philippe Lanos - One of the best experts on this subject based on the ideXlab platform.
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geomagnetic field Variations in western europe from 1500 bc to 200 ad part ii new intensity Secular Variation curve
Physics of the Earth and Planetary Interiors, 2013Co-Authors: Gwenaël Hervé, Annick Chauvin, Philippe LanosAbstract:Abstract In order to extend the Secular Variation curve (SVC) of archaeointensity in Western Europe to the first millennium BC, we studied 24 kilns and hearths in place, two displaced hearths and six sets of pottery sherds from French archaeological sites. Archaeological artefacts, radiocarbon and dendrochronology dated the acquisition of the thermoremanent magnetization (TRM) carried by the studied objects. Rock magnetism experiments suggest that the main carrier of the magnetization is a Ti-poor titanomagnetite. Archaeointensity was determined by the Thellier–Thellier classical protocol with pTRM-checks. A strict criteria set was applied to select only the most reliable results with linear NRM–TRM diagrams (55% of total specimens). This study demonstrates that pottery sherds with two TRMs give reliable archaeointensities in the low-temperature interval, if the NRM–TRM diagram is adequately adjusted. Eighteen new mean archaeointensities (14 corrected from the anisotropy of TRM and 16 from cooling rate) were computed. The comparison with previously published Western Europe paleointensities show a strong dispersion between data primarily due to their variable quality. Western Europe data were weighted following the archaeointensity protocol, the number of specimens per site and the type of studied materials, in order to better highlight the Secular Variation of archaeointensity during the first millennium BC. The SVC, built with sliding windows of 160 years shifted every 50 years, presents (at Paris) a maximum of 90 μT around 800 BC and a minimum of 60 μT around 250 BC. These archaeointensity maximum and minimum correspond to cusps of the geomagnetic field direction in Western Europe. This new curve is consistent with Mesopotamian and Eastern Europe data. The archaeointensity Secular Variation in Western Europe predicted by global geomagnetic models CALS3k.4, ARCH3k.1 and ARCH3k_cst.1 is smoother than our SVC. We used our directional dataset ( Herve et al., 2013 ) to build a new Western Europe VGPs and VDMs mean curves. Comparison with the predictions given by the global models points out a possible persistent non-dipole fields effect over Europe between 1000 BC and 600–500 BC. Finally, we note that the strong Variations of intensity of the geomagnetic field (with a mean decrease rate per century close to 6 μT) will be useful for archaeomagnetic dating purposes.
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Geomagnetic field Variations in Western Europe from 1500BC to 200AD. Part I: Directional Secular Variation curve
Physics of the Earth and Planetary Interiors, 2013Co-Authors: Gwenaël Hervé, Annick Chauvin, Philippe LanosAbstract:To improve the geomagnetic field Secular Variation curve (SVC) of Western Europe during protohistoric times, archaeomagnetic directions of 39 archaeological kilns or hearths from France were investigated. The dating of each archaeological structure was established with archaeological or chronometric methods. Thirty-seven of these structures are dated from the first millennium BC, one from the end of the second millennium BC and the last one from the fourth millennium BC. Thermomagnetic curves, unblocking temperatures and coercivities suggest that the main carrier of the remanent magnetization is a Ti-poor titanomagnetite. Archaeodirections were obtained by alternating field and thermal demagnetizations on almost 900 specimens. The anisotropy tensor of thermoremanent magnetization was determined for 35 structures and 22 mean archaeodirections were corrected for anisotropy. The new archaeodirections are very consistent with previously published data. A new directional SVC was built using bivariate statistics with selected Western Europe data located within 1000 km of Paris. Selection criteria include the number of samples, the dating reliability and the accuracy of the mean archaeodirection. Resulting Secular Variation between 1500BC and 0AD mainly shows large changes in declination, while inclinations are bracketed between ∼65° and ∼75°. The declinations show a strong maximum with values ∼30-35° around 800-750BC, followed by a sharp decrease to values around 0° at 500BC and close to −7° around 250BC. The main features of the Secular Variation from 1500BC to 0AD appear to be a dominant westward drift and two major changes around 800 and 250BC. Compared to the global and regional geomagnetic models, the new reference data are better fitted by ARCH3k_cst.1 and SCHA.DIF.3k than by ARCH3k.1 and CALS3k.4 models. The strong Variation of the archaeodirection between 1000 and 500BC makes archaeomagnetism very useful for dating purposes.
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Archaeomagnetic Secular Variation in the UK during the past 4000 years and its application to archaeomagnetic dating
Physics of the Earth and Planetary Interiors, 2007Co-Authors: I. Zananiri, Philippe Lanos, C.m. Batt, D.h. Tarling, P. LinfordAbstract:This paper examines the limitations and deficiencies of the current British archaeomagnetic calibration curve and applies several mathematical approaches in an attempt to produce an improved Secular Variation curve for the UK for use in archaeomagnetic dating. The dataset compiled is the most complete available in the UK, incorporating published results, PhD theses and unpublished laboratory reports. It comprises 620 archaeomagnetic (directional) data and 238 direct observations of the geomagnetic field, and includes all relevant information available about the site, the archaeomagnetic direction and the archaeological age. A thorough examination of the data was performed to assess their quality and reliability. Various techniques were employed in order to use the data to construct a Secular Variation (SV) record: moving window with averaging and median, as well as Bayesian statistical modelling. The SV reference curve obtained for the past 4000 years is very similar to that from France, most differences occurring during the early medieval period (or Dark Ages). Two examples of dating of archaeological structures, medieval and pre-Roman, are presented based on the new SV curve for the UK and the implications for archaeomagnetic dating are discussed.
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archaeomagnetism in italy a compilation of data including new results and a preliminary italian Secular Variation curve
Geophysical Journal International, 2006Co-Authors: Evdokia Tema, Ian Hedley, Philippe LanosAbstract:Published Italian archaeomagnetic data are combined with new data from the Geneve and Torino laboratories. A total of 74 directional data is presented with age estimates falling between 1300 BC to 1600 AD, including results from volcanic deposits of unquestionable age. The data set has been analysed using the Bayesian stochastic approach for curve building to produce a preliminary Italian Secular Variation (SV) curve. Comparison with the French SV curve shows a general agreement but some significant differences are also observed. The new Italian SV curve can be used for archaeomagnetic dating of Italian artefacts, even though caution must be paid for the period 9th12th century AD and times older than 8th century BC, when only few data are available and error envelopes are large.
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first archaeomagnetic Secular Variation curve for the iberian peninsula comparison with other data from western europe and with global geomagnetic field models
Geochemistry Geophysics Geosystems, 2006Co-Authors: Miriam Gomezpaccard, Annick Chauvin, Philippe Lanos, G Mcintosh, M L Osete, G Catanzariti, V C Ruizmartinez, J I NunezAbstract:[1] A first Secular Variation (SV) curve for the Iberian Peninsula was computed by hierarchical Bayesian method using a total of 134 archaeomagnetic directions with ages ranging from −775 to 1959 A.D. A general agreement is observed between the Iberian curve and the French and German SV curves, although some interesting differences were found, such as the occurrence of lower inclinations between the 11th and 14th centuries in the Iberian curve. The analysis of these three reference curves indicates that SV in western Europe is characterized by three major directional changes at −125, 200, and 1350 A.D. It is suggested that these cusps are regional features of the geomagnetic field. The Iberian curve has been compared with the predictions of the Jackson, CALSK7K.2, and Hongre global models. Despite large differences recognized between these models, even for the dipolar terms, they predict reasonably well the Iberian archaeomagnetic SV.
Steve P Lund - One of the best experts on this subject based on the ideXlab platform.
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paleomagnetic Secular Variation records from holocene sediments of lake victoria 0 5 s 33 3 e
The Holocene, 2021Co-Authors: Steve P Lund, Ellen Platzman, Thomas C JohnsonAbstract:We have conducted a paleomagnetic study of Holocene sediments from Lake Victoria in order to develop a high-resolution record of paleomagnetic Secular Variation (PSV). This study has recovered PSV ...
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late quaternary paleomagnetic Secular Variation recorded in deep sea sediments from the demerara rise equatorial west atlantic ocean
Physics of the Earth and Planetary Interiors, 2017Co-Authors: Steve P Lund, Delia W Oppo, William T CurryAbstract:Abstract We have carried out a paleomagnetic/rock magnetic study of two gravity cores and two multicores from the Demerara Rise (∼8°N), adjacent to NE South America. The magnetic measurements indicate that there is a stable natural remanent magnetization (NRM) carried primarily by detrital magnetite/titanomagnetite that preserves the local pattern of paleomagnetic Secular Variation (PSV). The two gravity cores have consistent patterns of directional variability. The rock magnetic intensities in both gravity cores vary by less than a factor of three. Relative paleointensity estimates have been derived by normalizing the NRM to Chi, ARM, and SIRM. Both gravity cores show the same pattern of relative paleointensity variability. 27 calibrated radiocarbon dates from our studied gravity cores and one additional piston core (Huang et al., 2014) have been used to build chronologies for the two gravity cores. Core 25GGC has bulk sedimentation rates varying from 18 to 22 cm/ky and contains a PSV record for the last 19 ka; core 9GGC has bulk sedimentation rates of 9–17 cm/ky and contains a PSV record for the last 28 ka. There are no other published, good-quality, well-dated full-vector PSV records within 4000 km of the sites, a region which constitutes almost 20% of the Earth’s surface area. Our relative paleointensity records are consistent with other global records under the assumption of field intensity being largely a global-scale process. We have compared our directional PSV data statistically with eight other good-quality, well-dated low-latitude PSV records. Our statistical analysis shows that our Demerara Rise directional PSV records are consistent with those other studies and that the Late Quaternary Equatorial field variability is significantly lower than much longer-duration (780 ka to 5 Ma) variability.
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a full vector paleomagnetic Secular Variation record psv from pyramid lake nevada from 47 17 ka evidence for the successive mono lake and laschamp excursions
Earth and Planetary Science Letters, 2017Co-Authors: Steve P Lund, Joseph C Liddicoat, Larry Benson, Robert M Negrini, Scott MensingAbstract:Abstract We have carried out a paleomagnetic study of late-Pleistocene Pyramid Lake core PLC08-1 (1680 cm). Our goals were to develop a full-vector paleomagnetic Secular Variation (PSV) record for the core, establish a paleomagnetic chronostratigraphy for the lake based on correlation of the PSV record to other dated PSV records in the region, compare that chronostratigraphy with previously developed radiocarbon and ash chronologies, and search for evidence of the Mono Lake Excursion and Laschamp Excursion. We have recovered a full-vector PSV record (inclination, declination, relative paleointensity) for the interval 47 ka to 17 ka. Twenty radiocarbon dates and four dated ashes provided a chronostratigraphic framework for this record. We have also used the link between our PSV and other dated PSV records to develop an independent PSV chronostratigraphy for the core. The PSV chronostratigraphy is not significantly different from that estimated by the radiocarbon and ash chronologies. We note the existence of two intervals of anomalous paleomagnetic directions. The younger interval, centered at 34.1 ± 0.4 ka , has the characteristic vector component features of the Mono Lake Excursion. The older interval, centered at 40.9 ± 0.5 ka , has the characteristic paleomagnetic signature of the Laschamp Excursion. This is the first time both intervals of excursional behavior have been found in the same sediment record from the western USA. Our new PSV record also corroborates previous estimates of the Mono Lake Excursion directional field behavior ( Liddicoat and Coe, 1979 ) and age ( Benson et al., 2003 ).
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full vector paleomagnetic Secular Variation records from latest quaternary sediments of lake malawi 10 0 s 34 3 e
Quaternary Science Reviews, 2016Co-Authors: Steve P Lund, Ellen Platzman, Thomas C JohnsonAbstract:Abstract We have conducted a paleomagnetic study of Late Quaternary sediments from Lake Malawi, East Africa, in order to develop a high-resolution record of paleomagnetic Secular Variation (PSV). This study has recovered PSV records from two cores (3P, 6P) in northern Lake Malawi (10.0°S, 34.3°E). The PSV appears to be recorded in fine-grained detrital magnetite/titanomagnetite grains. Detailed af demagnetization of the natural remanence (NRM) shows that a distinctive characteristic remanence (ChRM) is demagnetized from ∼20 to 80 mT, which decreases simply toward the origin. The resulting directional PSV records for 3P and 6P are easily correlatable with 29 distinct inclination features and 29 declination features. The statistical character of the PSV in both cores is consistent with Holocene PSV noted at other Holocene equatorial sites. Radiocarbon dating of the cores is based on 18 independent radiocarbon dates and four dated stratigraphic horizons that can be correlated into each core. The final directional PSV time series cover the last 24,000 years with an average sediment accumulation rate of ∼30 cm/kyr. We have also developed a relative paleointensity estimate for these PSV records based on normalizing the NRM (after 20 mT af demagnetization) by the SIRM (after 20 mT af demagnetization). Changing sedimentation patterns complicate any attempt to develop a single paleointensity record for the entire core lengths. We have developed a relative paleointensity record for the last 6000 years that has 14 correlatable features including 5 notable peaks in intensity. Three of these peaks are synchronous with paleointensity highs farther north in SE Europe/SW Asia/Egypt but two of the peaks are at times of low paleointensity farther north. We interpret this to indicate that Lake Malawi (10°S) is at least partly under the influence of a different flux-regeneration region of the outer-core dynamo. A relative paleointensity record was also developed for ∼11,000–24,000 YBP; the general pattern appears to be consistent with other published records, but our confidence in the correlations is more limited.
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character of holocene paleomagnetic Secular Variation in the tangent cylinder evidence from the chukchi sea
Physics of the Earth and Planetary Interiors, 2016Co-Authors: Steve P Lund, Lloyd D Keigwin, Dennis A DarbyAbstract:Abstract We have carried out a paleomagnetic study on three deep-sea cores from the Chukchi Sea (72°N) in order to characterize the Holocene paleomagnetic Secular Variation (PSV) in this high-latitude region. The Chukchi Sea lies within the geomagnetic-field tangent cylinder and PSV variability in this region might be expected to have a different pattern than PSV at sites located outside the tangent cylinder at lower latitudes. We have recovered correlatable directional PSV records and relative paleointensity records from all three cores. 15 radiocarbon dates were used to develop a chronostratigraphy for the PSV records. These records constitute the highest-resolution full-vector PSV records ever recovered from such high latitudes. We have compared our results with other previous studies from the region and find that our overall PSV is consistent with these other studies, although there are sometimes age differences up to 1000 years between correlatable PSV features. Our statistical PSV characteristics indicate that field variability (VGP angular dispersion) is lower than in regions just south of the Chukchi Sea and outside the tangent cylinder, but our records are probably not long enough to completely characterize PSV. However, our results are consistent with the only other published VGP angular dispersion results from inside the tangent cylinder (Antarctica, 79°S).
Hagay Amit - One of the best experts on this subject based on the ideXlab platform.
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Geomagnetic core field models and Secular Variation forecasts for the 13th International Geomagnetic Reference Field (IGRF-13)
Earth Planets and Space, 2020Co-Authors: I. Wardinski, Hagay Amit, Diana Saturnino, A. Chambodut, Benoit Langlais, M. Mandea, E. ThebaultAbstract:Observations of the geomagnetic field taken at Earth's surface and at satellite altitude are combined to construct continuous models of the geomagnetic field and its Secular Variation from 1957 to 2020. From these parent models, we derive candidate main field models for the epochs 2015 and 2020 to the 13th generation of the International Geomagnetic Reference Field (IGRF). The Secular Variation candidate model for the period 2020-2025 is derived from a forecast of the Secular Variation in 2022.5, which results from a multi-variate singular spectrum analysis of the Secular Variation from 1957 to 2020.
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on equatorially symmetric and antisymmetric geomagnetic Secular Variation timescales
Physics of the Earth and Planetary Interiors, 2017Co-Authors: Hagay Amit, Maelie Coutelier, Ulrich R ChristensenAbstract:Abstract It has been suggested that the Secular Variation (SV) timescales of the geomagnetic field vary as 1 / l (where l is the spherical harmonic degree), except for the dipole. Here we propose that the same scaling law applies for SV timescales defined for different symmetry classes of the geomagnetic field and SV. We decompose the field and its SV into symmetric and antisymmetric parts and show in geomagnetic field models and numerical dynamo simulations that the corresponding SV timescales also vary as 1 / l , again except for the dipole. The time-average antisymmetric/symmetric SV timescales are larger/smaller than the total, respectively. The difference in SV timescales between these two symmetry classes is probably due to different degrees of alignment of the core flow with different magnetic field structures at the core-mantle boundary. The symmetric dipole SV timescale in the recent geomagnetic field and in long-term time-averages from numerical dynamos is below the extrapolated 1 / l curve, whereas before ∼ 1965 the geomagnetic dipole tilt was rather steady and the symmetric dipole SV timescale exceeded the extrapolated 1 / l curve. We hypothesize that the period of nearly steady geomagnetic dipole tilt between 1810–1965 was anomalous for the geodynamo. Overall, the deviation of the dipole SV timescales from the 1 / l curves may indicate that magnetic diffusion contributes to the dipole SV more than it does for higher degrees.
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can downwelling at the top of the earth s core be detected in the geomagnetic Secular Variation
Physics of the Earth and Planetary Interiors, 2014Co-Authors: Hagay AmitAbstract:Abstract It has been argued based on recent seismic and mineral physics studies that the top of Earth’s liquid outer core is stably stratified. Here I analyze persistent geomagnetic Secular Variation features on the core–mantle boundary to examine whether a kinematic signature of core fluid upwelling/downwelling can be detected. I focus on regions of intense high-latitude geomagnetic flux patches that may be maintained by fluid downwelling. In order to identify persistent patterns, the radial field and its Secular Variation are stacked in the flux patch moving reference frame. These stacked images are compared with forward solutions to the radial induction equation based on idealized field-flow models. Clear advective Secular Variation signature below North America indicates that these intense flux patches may exhibit significant mobility. Stretching signature in the form of persistent positive Secular Variation correlated with the intense flux patch below the Southern Indian Ocean may be considered as regional scale geomagnetic evidence for whole core convection, although pure toroidal flow cannot be outruled.
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accounting for magnetic diffusion in core flow inversions from geomagnetic Secular Variation
Geophysical Journal International, 2008Co-Authors: Hagay Amit, Ulrich R ChristensenAbstract:SUMMARY We use numerical dynamos to investigate the possible role of magnetic diffusion at the top of the core. We find that the contribution of radial magnetic diffusion to the Secular Variation is correlated with that of tangential magnetic diffusion for a wide range of control parameters. The correlation between the two diffusive terms is interpreted in terms of the Variation in the strength of poloidal flow along a columnar flow tube. The amplitude ratio of the two diffusive terms is used to estimate the probable contribution of radial magnetic diffusion to the Secular Variation at Earth-like conditions. We then apply a model where radial magnetic diffusion is proportional to tangential diffusion to core flow inversions of geomagnetic Secular Variation data. We find that including magnetic diffusion does not change dramatically the global flow but some significant local Variations appear. In the non frozen-flux core flow models (termed ‘diffusive’), the hemispherical dichotomy between the active Atlantic and quiet Pacific is weaker, a cyclonic vortex below North America emerges and the vortex below Asia is stronger. Our results have several important geophysical implications. First, our diffusive flow models contain some flow activity at low latitudes in the Pacific, suggesting a local balance between magnetic field advection and diffusion in that region. Second, the cyclone below North America in our diffusive flows reconciles the difference between mantle-driven thermal wind predictions and frozen-flux core flow models, and is consistent with the prominent intense magnetic flux patch below North America in geomagnetic field models. Finally, we hypothesize that magnetic diffusion near the core surface plays a larger role in the geomagnetic Secular Variation than usually assumed.
