scholarly journals Repeat station data compared to a global geomagnetic field model

2013 ◽  
Vol 55 (6) ◽  
Author(s):  
Monika Korte ◽  
Vincent Lesur
Keyword(s):  
Magnetic Field ◽  
Secular Variation ◽  
Global Model ◽  
Time Interval ◽  
Night Time ◽  
Core Field ◽  
Repeat Station ◽  
Station Data ◽  
Long Term Trends

<p>Geomagnetic repeat station surveys with local variometers for improved data reductions have been carried out in Germany for about ten years. For nearly the same time interval the satellites Ørsted and CHAMP have provided a good magnetic field data coverage of the whole globe. Recent global field models based on these satellite data together with geomagnetic observatory data provide an improved description of the core field and secular variation. We use the latest version of the GFZ Reference Internal Magnetic Model to compare the magnetic field evolution predicted by that model between 2001 and 2010 to the independent repeat station data collected over the same time interval in Germany. Estimates of crustal bias at the repeat station locations are obtained as averages of the residuals, and the scatter or trend around each average provides information about influences in the data from field sources not (fully) described by the global model. We find that external magnetic field signal in the order of several nT, including long-term trends, remains both in processed annual mean and quiet night time repeat station data. We conclude that the geomagnetic core field secular variation in this area is described to high accuracy (better than 1 nT/yr) by the global model. Weak long-term trends in the residuals between repeat station data and the model might indicate induced lithospheric anomalies, but more data are necessary for a robust analysis of such signals characterized by very unfavorable signal-to-noise ratio.</p>

scholarly journals Development of an improved geomagnetic reference field of Antarctica

1999 ◽  
Vol 42 (2) ◽  
Author(s):  
A. De Santis ◽  
M. Chiappini ◽  
J. M. Torta ◽  
R. R. B. von Frese
Keyword(s):  
Magnetic Field ◽  
Secular Variation ◽  
Reference Model ◽  
Reference Field ◽  
Spherical Cap ◽  
Core Field ◽  
The Core ◽  
Spherical Cap Harmonic Analysis ◽  
The Antarctic ◽  
Magnetic Observatories

The properties of the Earth's core magnetic field and its secular variation are poorly known for the Antarctic. The increasing availability of magnetic observations from airborne and satellite surveys, as well as the existence of several magnetic observatories and repeat stations in this region, offer the promise of greatly improving our understanding of the Antarctic core field. We investigate the possible development of a Laplacian reference model of the core field from these observations using spherical cap harmonic analysis. Possible uses and advantages of this approach relative to the implementations of the standard global reference field are also considered.

Modelling the core magnetic field of the Earth

1982 ◽  
Vol 306 (1492) ◽  
pp. 179-191 ◽  
Keyword(s):  
Magnetic Field ◽  
Spherical Harmonics ◽  
Secular Variation ◽  
Long Wavelength ◽  
Core Field ◽  
The Core ◽  
The Earth ◽  
High Degree ◽  
Current Systems ◽  
The Magnetic Field

The magnetic field generated in the core of the Earth is often represented by spherical harmonics of the magnetic potential. It has been found from looking at the equations of spherical harmonics, and from studying the values of the spherical harmonic coefficients derived from data from Magsat, that this is an unsatisfactory way of representing the core field. Harmonics of high degree are characterized by generally shorter wavelength expressions on the surface of the Earth, but also contain very long wavelength features as well. Thus if it is thought that the higher degree harmonics are produced by magnetizations within the crust of the Earth, these magnetizations have to be capable of producing very long wavelength signals. Since it is impossible to produce very long wavelength signals of sufficient amplitude by using crustal magnetizations of reasonable intensity, the separation of core and crustal sources by using spherical harmonics is not ideal. We suggest that a better way is to use radial off-centre dipoles located within the core of the Earth. These have several advantages. Firstly, they can be thought of as modelling real physical current systems within the core of the Earth. Secondly, it can be shown that off-centred dipoles, if located deep within the core, are more effective at removing long wavelength signals of potential or field than can be achieved by using spherical harmonics. The disadvantage is that it is much more difficult to compute the positions and strengths of the off-centred dipole fields, and much less easy to manipulate their effects (such as upward and downward continuation). But we believe, along with Cox and Alldredge & Hurwitz, that the understanding that we might obtain of the Earth’s magnetic field by using physically reasonable models rather than mathematically convenient models is very important. We discuss some of the radial dipole models that have been proposed for the nondipole portion of the Earth’s field to arrive at a model that agrees with observations of secular variation and excursions.

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

1981 ◽  
Vol 374 (1756) ◽  
pp. 15-33 ◽  
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.

scholarly journals Long-term variations of the mesospheric wind field at mid-latitudes

2007 ◽  
Vol 25 (8) ◽  
pp. 1779-1790 ◽  
Author(s):  
D. Keuer ◽  
P. Hoffmann ◽  
W. Singer ◽  
J. Bremer
Keyword(s):  
Solar Activity ◽  
Wind Field ◽  
Meridional Wind ◽  
Time Interval ◽  
Wind Component ◽  
Model Calculations ◽  
Measuring Techniques ◽  
Long Term Trends ◽  
Solar Influence

Abstract. Continuous MF radar observations at the station Juliusruh (54.6° N; 13.4° E) have been analysed for the time interval between 1990 and 2005, to obtain information about solar activity-induced variations, as well as long-term trends in the mesospheric wind field. Using monthly median values of the zonal and the meridional prevailing wind components, as well as of the amplitude of the semidiurnal tide, regression analyses have been carried out with a dependence on solar activity and time. The solar activity causes a significant amplification of the zonal winds during summer (increasing easterly winds) and winter (increasing westerly winds). The meridional wind component is positively correlated with the solar activity during summer but during winter the correlation is very small and non significant. Also, the solar influence upon the amplitude of the semidiurnal tidal component is relatively small (in dependence on height partly positive and partly negative) and mostly non-significant. The derived trends in the zonal wind component during summer are below an altitude of about 83 km negative and above this height positive. During the winter months the trends are nearly opposite compared with the trends in summer (transition height near 86 km). The trends in the meridional wind components are below about 85 km positive in summer (significant) and near zero (nonsignificant) in winter; above this height during both seasons negative trends have been detected. The trends in the semidiurnal tidal amplitude are at all heights positive, but only partly significant. The detected trends and solar cycle dependencies are compared with other experimental results and model calculations. There is no full agreement between the different results, probably caused by different measuring techniques and evaluation methods used. Also, different heights and observation periods investigated may contribute to the detected differences.

scholarly journals Reconstruction of secular variation in seawater sulfate concentrations

2014 ◽  
Vol 11 (9) ◽  
pp. 13187-13250 ◽  
Author(s):  
T. J. Algeo ◽  
G. M. Luo ◽  
H. Y. Song ◽  
T. W. Lyons ◽  
D. E. Canfield
Keyword(s):  
Secular Variation ◽  
Ocean Model ◽  
Rate Of Change ◽  
Time Interval ◽  
Microbial Sulfate Reduction ◽  
Seawater Sulfate ◽  
History Of ◽  
Quantitative Analyses ◽  
Late Neoproterozoic

Abstract. Long-term secular variation in seawater sulfate concentrations ([SO42–]SW) is of interest owing to its relationship to the oxygenation history of Earth's surface environment, but quantitative approaches to analysis of this variation remain underdeveloped. In this study, we develop two complementary approaches for assessment of the [SO42–] of ancient seawater and test their application to reconstructions of [SO42–]SW variation since the late Neoproterozoic Eon (< 650 Ma). The first approach is based on two measurable parameters of paleomarine systems: (1) the S-isotope fractionation associated with microbial sulfate reduction (MSR), as proxied by Δ34SCAS-PY, and (2) the maximum rate of change in seawater sulfate, as proxied by ∂ δ34SCAS / ∂ t (max). This "rate method" yields an estimate of the maximum possible [SO42–]SW for the time interval of interest, although the calculated value differs depending on whether an oxic or an anoxic ocean model is inferred. The second approach is also based on Δ34SCAS-PY but evaluates this parameter against an empirical MSR trend rather than a formation-specific ∂ δ34SCAS / ∂ t (max) value. The MSR trend represents the relationship between fractionation of cogenetic sulfate and sulfide (i.e., Δ34Ssulfate-sulfide) and ambient dissolved sulfate concentrations in 81 modern aqueous systems. This "MSR-trend method" is thought to yield a robust estimate of mean seawater [SO42–] for the time interval of interest. An analysis of seawater sulfate concentrations since 650 Ma suggests that [SO42–]SW was low during the late Neoproterozoic (< 5 mM), rose sharply across the Ediacaran/Cambrian boundary (to ~ –10 mM), and rose again during the Permian to levels (~ 10–30 mM) that have varied only slightly since 250 Ma. However, Phanerozoic seawater sulfate concentrations may have been drawn down to much lower levels (~ 1–4 mM) during short (&amp;lesssim; 2 Myr) intervals of the Cambrian, Early Triassic, Early Jurassic, and possibly other intervals as a consequence of widespread ocean anoxia, intense MSR, and pyrite burial. The procedures developed in this study offer potential for future high-resolution quantitative analyses of paleoseawater sulfate concentrations.

scholarly journals The Mag.num core field model as a parent for IGRF-13, and the recent evolution of the South Atlantic Anomaly

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
M. Rother ◽  
M. Korte ◽  
A. Morschhauser ◽  
F. Vervelidou ◽  
J. Matzka ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Secular Variation ◽  
Field Model ◽  
South Atlantic ◽  
Model Parameters ◽  
The South ◽  
South Atlantic Anomaly ◽  
Core Field ◽  
The Core ◽  
Observatory Data

AbstractWe present the GFZ candidate field models for the $$13{\mathrm{th}}$$ 13 th  Generation International Geomagnetic Reference Field (IGRF-13). These candidates were derived from the geomagnetic core field model, which is constrained by Swarm satellite and ground observatory data from November 2013 to August 2019. Data were selected from magnetically quiet periods, and the model parameters have been obtained using an iteratively reweighted inversion scheme approximating a robust modified Huber norm as a measure of misfit. The root mean square misfit of the model to Swarm and observatory data is in the order of 3–5 nT for mid and low latitudes, with a maximum of 44 nT for the satellite east component data at high latitudes. The time-varying core field is described by order 6 splines and spherical harmonic coefficients up to degree and order 20. We note that the temporal variation of the core field component of the model is strongly damped and shows a smooth secular variation that suits well for the IGRF, where secular variation is represented as constant over 5-year intervals. Further, the external field is parameterised by a slowly varying part and a more rapidly varying part controlled by magnetic activity and interplanetary magnetic field proxies. Additionally, the Euler angles of the magnetic field sensor orientation are co-estimated. A widely discussed feature of the geomagnetic field is the South Atlantic Anomaly, a zone of weak and decreasing field strength stretching from southern Africa over to South America. The IGRF and indicate that the anomaly has developed a second, less pronounced eastern minimum at Earth’s surface since 2007. We observe that while the strong western minimum continues to drift westwards, the less pronounced eastern minimum currently drifts eastward at Earth’s surface. This does not seem to be linked to any eastward motion at the core–mantle boundary, but rather to intensity changes of westward drifting flux patches contributing to the observed surface field. Also, we report a sudden change in the secular variation measured at two South Atlantic observatories around 2015.0, which occurred shortly after the well-known jerk of 2014.0.

scholarly journals Low-frequency lower E-region wind and reflection height measurements as sensor for climate variability

2008 ◽  
Vol 6 ◽  
pp. 331-335 ◽  
Author(s):  
C. Jacobi
Keyword(s):  
Oblique Incidence ◽  
Lower Thermosphere ◽  
Low Frequency ◽  
Time Interval ◽  
Radio Waves ◽  
Reflection Height ◽  
E Region ◽  
Long Term Trends ◽  
Mlt Region

Abstract. Measurements of reflection heights of low-frequency (LF) radio waves at oblique incidence and estimates of mesosphere/lower thermosphere (MLT) region horizontal winds applying the D1 spaced receiver method on LF field strength registrations are analyzed with respect to possible long-term trends and interdecadal variability in the time interval from ~1980 to date. While no clear signal of mesospheric height trend is registered during the last two decades, significant trends of MLT horizontal winds are found. These trends are non-linear, in particular a change of trends around 1990 is found, which is probably connected with changes in tropospheric and stratospheric conditions at that time.

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