16 research outputs found
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 âŒ5% 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 30â50 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 todayâs field, with an intensity structure similar to todayâs 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 Earthâs magnetic fiel
Fluidbewegung an der Kern-Mantel-Grenze abgeleitet aus der SĂ€kularvariation 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 ĂRSTED 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 SĂ€kularvariation im Zeitraum von 1980 und 2000
gelegt worden. Dabei wurden die beiden Endpunkte des Zeitintervals so gewÀhlt,
dass sie mit den Satellitenmissionen MAGSAT 1980 und CHAMP, ĂRSTED in 2000
zusammenfallen. Diese Satellitenmissionen erlaubten es, rÀumlich
hochaufgelöste Modelle des Erdmagnetfeldes zu erstellen.
Im ersten Teil dieser Studie befasse ich mich mit der SĂ€kularvariation,
beobachtet ĂŒber 70 Jahre an drei verschiedenen Stationen: Eskdalemuir
(Schottland), Hermanus (SĂŒdafrika) und Kakioka (Japan). Hier versuche ich die
SĂ€kularvariation mittels eines einfachen deterministischen Modells zu
beschreiben, d. h. eine Beschreibung der SĂ€kularvariation als (gewichtete)
Ăberlagerung von PeriodizitĂ€ten hervorgerufen durch Prozesse ausserhalb der
ErdatmosphÀre, z. B. 11-Jahres-Zyklus der Sonne, Sonnenrotation, etc. Dabei
vermag dieser einfache Ansatz die SĂ€kularvariation der einzelnen Stationen
wiederzugeben, aber jedes Modell ist eben nur fĂŒr eine bestimmte Station
gĂŒltig, also ist somit die Aussagekraft dieser Modelle lokal beschrĂ€nkt. Mehr
noch; die Schlussfolgerung, dass die SĂ€kularvariation und insbesondere, dass
Geomagnetische Jerks durch externe Prozesse verursacht werden, ist nicht
zutreffend.
Im zweiten Teil meiner Arbeit entwerfe ich ein Modell der SĂ€kularvariation
zwischen 1980 und 2000 basierend auf zeitlich variablen Gauss-Koeffizienten.
Hier werden die bereits angesprochenen Hauptfeldmodelle der
Satellitenmissionen verwendet, um das Modell der SĂ€kularvariation in den
Endpunkten festzulegen.
Anhand dieses Modells ist es mir möglich Eigenschaften, wie z. B. rÀumliche
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 fĂŒr den Zeitraum 1590 - 1990 von Jackson et al.
(2000) dar.
Last but not least, erlaubt das Modell RĂŒckschlĂŒsse auf die rĂ€umliche
Konfiguration des Geodynamos; es weist auf die Existenz von Konvektionsrollen
hin.
Im dritten Teil meiner Arbeit entwickle ich mögliche Modelle fĂŒr die
Fluidbewegung an der Kern-Mantel-Grenze zwischen 1980 - 2000 unter Verwendung
meines Modells fĂŒr das Hauptfeld und seiner SĂ€kularvariation. Die allgemeine
Vorstellung ist, dass die SĂ€kularvariation 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 abschlieĂenden Bemerkungen ein.
Aber die Fluidbewegung beschreibt nicht nur die SĂ€kularvariation sondern auch
eine davon unabhÀngige Observable, nÀmlich die Variation der TageslÀnge
Interannual Fluctuations of the Core Angular Momentum Inferred from Geomagnetic Field Models
International audienc
Modelling Internal and External Geomagnetic Fields Using Satellite Data
International audienc
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 â 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 TimeâVarying 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 timeâvarying 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 quadrupoleâtoâdipole ratio of 0.27 for the steady magnetic field. The timeâvarying 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 timeâvarying 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
International audienc