Core surface flow models from decadal and subdecadal secular variation of the main geomagnetic field

The main objective of this thesis is an investigation and description of the secular variation of the Earth's magnetic field between 1980 and 2000. In particular, in the first part of this study the phenomenon of geomagnetic jerks are investigated by means of a deterministic model, which basically gives a description of the secular variation as a parameterization of typical periodicities of the external field. I argue that the conclusion drawn from this approach is not valid for explaining jerks as caused by external events and not valid to describe the global secular variation. In the second part a time--dependent model of the secular variation between 1980 and 2000 is developed. The endpoints of the time interval were chosen, because of the availability of high quality field models from satellite data for these epochs. The Gauss coefficients are expanded in time as function of cubic B--splines. This model is forced to fit field models from high quality vector measurements from MAGSAT in 1980 and OERSTED in 2000. The methodology is new. The model is a valuable extension of the hitherto existing time--dependent description of the secular variation, the GUFM which was valid until 1990. Unlike GUFM the model is based on observatory monthly means, and the knot spacing of the cubic B--Splines tighter than GUFM. Therefore it reveals a short term secular variation on subdecadal time scale, which was not as yet resolved. The model is also valuable to test the frozen flux hypothesis and to link some of the morphology of the radial field at the core--mantle boundary to the geodynamo. The third part of this thesis deals with the inversion of the time--dependent field and secular variation model for different kinds of core surface flow ...thesi

Earth's magnetic field is probably not reversing

The geomagnetic field has been decaying at a rate of ∼5% per century from at least 1840, with indirect observations suggesting a decay since 1600 or even earlier. This has led to the assertion that the geomagnetic field may be undergoing a reversal or an excursion. We have derived a model of the geomagnetic field spanning 30–50 ka, constructed to study the behavior of the two most recent excursions: the Laschamp and Mono Lake, centered at 41 and 34 ka, respectively. Here, we show that neither excursion demonstrates field evolution similar to current changes in the geomagnetic field. At earlier times, centered at 49 and 46 ka, the field is comparable to today’s field, with an intensity structure similar to today’s South Atlantic Anomaly (SAA); however, neither of these SAA-like fields develop into an excursion or reversal. This suggests that the current weakened field will also recover without an extreme event such as an excursion or reversal. The SAA-like field structure at 46 ka appears to be coeval with published increases in geomagnetically modulated beryllium and chlorine nuclide production, despite the global dipole field not weakening significantly in our model during this time. This agreement suggests a greater complexity in the relationship between cosmogenic nuclide production and the geomagnetic field than is commonly assumed

International Geomagnetic Reference Field: the 12th generation

The 12th generation of the International Geomagnetic Reference Field (IGRF) was adopted in December 2014 by the Working Group V-MOD appointed by the International Association of Geomagnetism and Aeronomy (IAGA). It updates the previous IGRF generation with a definitive main field model for epoch 2010.0, a main field model for epoch 2015.0, and a linear annual predictive secular variation model for 2015.0-2020.0. Here, we present the equations defining the IGRF model, provide the spherical harmonic coefficients, and provide maps of the magnetic declination, inclination, and total intensity for epoch 2015.0 and their predicted rates of change for 2015.0-2020.0. We also update the magnetic pole positions and discuss briefly the latest changes and possible future trends of the Earth’s magnetic fiel

Fluidbewegung an der Kern-Mantel-Grenze abgeleitet aus der Säkularvariation des Geomagnetischen Hauptfeldes

Title page Contents, Abstract, Acknowledgements 1 Earth's magnetic field 1 1.1 Observation of the geomagnetic field 1 1.2 Origin and temporal variation 2 1.3 Outline of this thesis 6 2 A deterministic model of the temporal variation of the geomagnetic field 9 2.1 Analysis of the external field variation 10 2.2 Modelling of disturbance fields 13 2.3 Arguments against the external field variations cause of jerks 19 2.4 Conclusions 28 3 Time--dependent modelling of the geomagnetic field 29 3.1 Data 29 3.2 Data processing 33 3.3 Some fundamental concepts of Inverse Problems 34 3.4 Method 36 3.5 Results 49 4 Qualitative analysis of the core--surface fluid motions 65 4.1 Inversion for core--surface flows 65 4.2 Maps of core--surface flows 77 4.3 Discussion and Conclusion 94 5 Conclusion and Prospects 99 Bibliography 101 Appendices A-C 111The main objective of this thesis is an investigation and description of the secular variation of the Earth's magnetic field between 1980 and 2000\. In particular, in the first part of this study the phenomenon of geomagnetic jerks are investigated by means of a deterministic model, which basically gives a description of the secular variation as a parameterization of typical periodicities of the external field. I argue that the conclusion drawn from this approach is not valid for explaining jerks as caused by external events and not valid to describe the global secular variation. In the second part a time-dependent model of the secular variation between 1980 and 2000 is developed. The endpoints of the time interval were chosen, because of the availability of high quality field models from satellite data for these epochs. The Gauss coefficients are expanded in time as function of cubic B-splines. This model is forced to fit field models from high quality vector measurements from MAGSAT in 1980 and ØRSTED in 2000. The methodology is new. The model is a valuable extension of the hitherto existing time-dependent description of the secular variation, the GUFM which was valid until 1990. Unlike GUFM the model is based on observatory monthly means, and the knot spacing of the cubic B-Splines tighter than GUFM. Therefore it reveals a short term secular variation on subdecadal time scale, which was not as yet resolved. The model is also valuable to test the frozen flux hypothesis and to link some of the morphology of the radial field at the core-mantle boundary to the geodynamo. The third part of this thesis deals with the inversion of the time-dependent field and secular variation model for different kinds of core surface flow. These flows allow prediction of the decadal change of the length of the day, an observable which is independent to geomagnetic data. The prediction of some of the flows have the right tendency, but differ in slope from the observed change of the length of the day.In dieser Studie ist ein besonderes Augenmerk auf die Beschreibung des Erdmagnetfeldes und seiner Säkularvariation im Zeitraum von 1980 und 2000 gelegt worden. Dabei wurden die beiden Endpunkte des Zeitintervals so gewählt, dass sie mit den Satellitenmissionen MAGSAT 1980 und CHAMP, ØRSTED in 2000 zusammenfallen. Diese Satellitenmissionen erlaubten es, räumlich hochaufgelöste Modelle des Erdmagnetfeldes zu erstellen. Im ersten Teil dieser Studie befasse ich mich mit der Säkularvariation, beobachtet über 70 Jahre an drei verschiedenen Stationen: Eskdalemuir (Schottland), Hermanus (Südafrika) und Kakioka (Japan). Hier versuche ich die Säkularvariation mittels eines einfachen deterministischen Modells zu beschreiben, d. h. eine Beschreibung der Säkularvariation als (gewichtete) Überlagerung von Periodizitäten hervorgerufen durch Prozesse ausserhalb der Erdatmosphäre, z. B. 11-Jahres-Zyklus der Sonne, Sonnenrotation, etc. Dabei vermag dieser einfache Ansatz die Säkularvariation der einzelnen Stationen wiederzugeben, aber jedes Modell ist eben nur für eine bestimmte Station gültig, also ist somit die Aussagekraft dieser Modelle lokal beschränkt. Mehr noch; die Schlussfolgerung, dass die Säkularvariation und insbesondere, dass Geomagnetische Jerks durch externe Prozesse verursacht werden, ist nicht zutreffend. Im zweiten Teil meiner Arbeit entwerfe ich ein Modell der Säkularvariation zwischen 1980 und 2000 basierend auf zeitlich variablen Gauss-Koeffizienten. Hier werden die bereits angesprochenen Hauptfeldmodelle der Satellitenmissionen verwendet, um das Modell der Säkularvariation in den Endpunkten festzulegen. Anhand dieses Modells ist es mir möglich Eigenschaften, wie z. B. räumliche Ausbreitung, der drei bekannten Geomagnetischen Jerks dieses Zeitraumes (1983, 1991 und 1999) zu untersuchen. Weiterhin ermöglicht das Modell den Nachweis von magnetischer Diffusion an der Kern-Mantel-Grenze und stellt eine Weiterentwicklung des Modells für den Zeitraum 1590 - 1990 von Jackson et al. (2000) dar. Last but not least, erlaubt das Modell Rückschlüsse auf die räumliche Konfiguration des Geodynamos; es weist auf die Existenz von Konvektionsrollen hin. Im dritten Teil meiner Arbeit entwickle ich mögliche Modelle für die Fluidbewegung an der Kern-Mantel-Grenze zwischen 1980 - 2000 unter Verwendung meines Modells für das Hauptfeld und seiner Säkularvariation. Die allgemeine Vorstellung ist, dass die Säkularvariation an der Kern-Mantel-Grenze durch Advektion und Diffusion erzeugt wird. Das wird genau durch die Induktionsgleichung beschrieben. Wichtig hierbei ist die Rolle der Diffusion, ich gehe darauf in den abschließenden Bemerkungen ein. Aber die Fluidbewegung beschreibt nicht nur die Säkularvariation sondern auch eine davon unabhängige Observable, nämlich die Variation der Tageslänge

Interannual Fluctuations of the Core Angular Momentum Inferred from Geomagnetic Field Models

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Modelling Internal and External Geomagnetic Fields Using Satellite Data

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Regional outer core kinematics from the time dependence of intense geomagnetic flux patches

International audienceObservations of the geomagnetic field by surface observatories and dedicated satellite missions such as the Swarm constellation provide constraints on the dynamics in Earth's outer core. In particular, global core flow models estimated by inversion of the radial magnetic induction equation provide an image of the circulation of the electrically conductive fluid at the top of the core. However, in these models the poloidal flow is much less robust than the toroidal core flow. Here, we infer regional outer core kinematics from the temporal variability of high-latitude intense geomagnetic flux patches. We develop an algorithm to fit anisotropic 2D-Gaussians to the shape of those flux patches in order to infer their area, amplitude and level of anisotropy. The temporal variabilities of these properties are used to quantify contraction, expansion, amplification, weakening and horizontal shear. Comparisons with idealized kinematic scenarios based on synthetic field and flow models allow to infer regional outer core kinematics. We found that some geomagnetic flux patches exhibit expansion and weakening corresponding to fluid upwellings, whereas other patches exhibit contraction and intensification corresponding to downwellings. In both cases the patches' area and amplitude relations follow hyperbolic curves. Our results show that the geomagnetic flux patches are affected by upwelling more often than by downwelling during the historical period. Equatorially symmetric poloidal flow prior to ≈ 1910 is inferred for the western intense patches. Kinematic scenarios where the field and flow structures centers coincide failed to reproduce the geomagnetic flux patches behavior. We recover the flux concentration efficiency of intense geomagnetic flux patches with an upwelling that resides two times its radius size away from the center of the flux patch. We also found a significant level of anisotropy over long periods for the historical geomagnetic flux patches. Anisotropic magnetic flux patches that are elongated in the direction of the shear flow may explain the east-west oriented present-day field at high latitudes of the southern Hemisphere. Overall, stretching effects at the top of the core can be deduced from our analysis of regional SV and allow further inferences on the poloidal part of the core flow

Correlated Time‐Varying Magnetic Fields and the Core Size of Mercury

International audienceMercury is characterized by a very peculiar magnetic field, as it was revealed by the MESSENGER mission. Its internal component is highly axisymmetric, dominated by the dipole, and very weak. This in turns leads to a very dynamic magnetosphere. It is known that there exist relationships between the internally generated field and the external field, although their dynamics are complex. In this study we derive steady and time‐varying spherical harmonic models of Mercury's magnetic field using MESSENGER measurements and interpret these models both in terms of correlated features and of the internal structure of Mercury. The influence of the hemispheric data distribution of MESSENGER is evaluated to grant the robustness of our models. We find a quadrupole‐to‐dipole ratio of 0.27 for the steady magnetic field. The time‐varying models reveal periodic and highly correlated temporal variations of internal and external origins. This argues for externally inducing and internally induced sources. The main period is 88 days, the orbital period of Mercury around the Sun. There is no measurable time lag between variations of external and internal magnetic fields, which place an upper limit of 1 S/m for the mantle conductivity. Finally, the compared amplitudes of external and internal time‐varying field lead to an independent (from gravity studies) estimate of the conductive core radius, at 2,060 ± 22 km. These analyses will be further completed with the upcoming BepiColombo mission and its magnetic field experiment, but the presented results already lift the veil on some of the magnetic oddities at Mercury

Robust Characteristics of the Laschamp and Mono Lake Geomagnetic Excursions: Results From Global Field Models

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