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Core surface flow models from decadal and subdecadal secular variation of the main geomagnetic field
Get PDFThe 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
Fluidbewegung an der Kern-Mantel-Grenze abgeleitet aus der Säkularvariation des Geomagnetischen Hauptfeldes
No full textTitle 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
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Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Core surface flow models from decadal and subdecadal secular variation of the main geomagnetic field
No full textCore surface flow models from decadal and subdecadal secular variation of the main geomagnetic field
No full textDispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
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