Analysis of Geomagnetic Variability by Empirical Orthogonal Functions

2020 ◽  
Author(s):  
Chi-Hua Chung ◽  
Benjamin Fong Chao
Keyword(s):  
Geomagnetic Field ◽  
Empirical Orthogonal Functions ◽  
Spatial Structures ◽  
Orthogonal Functions ◽  
Core Mantle Boundary ◽  
The Core ◽  
Orthogonal Function ◽  
Secular Variations ◽  
Complex Eof ◽  
Westward Drift

<p>We examine the secular variations of global geomagnetic field on long temporal scales using the IGRF model given in Gauss coefficients for 1900 - 2020. We apply the Empirical Orthogonal Function (EOF) analysis to the geomagnetic field truncated at degree 6 and downward continue it to the core-mantle boundary (CMB) under the assumption of an insulating mantle. The first three EOF modes show the periods around 120, 75 and 60 years with corresponding spatial structures. These oscillational modes potentially support the manifestation of magnetic, Archimedes and Coriolis (MAC) waves in the stably stratified layer near CMB (Buffett, 2016). We also model and decompose the geomagnetic field to standing and drifting components according to trajectories of the Gauss coefficients similarly to Yukutake (2015). We then use the Complex EOF (CEOF) analysis on the drifting field. The results indicate the presence of the westward drift phenomenon but only weakly given the fact that the westward drift has only completed a fraction of a cycle during this time.</p>

scholarly journals Persistent westward drift of the geomagnetic field at the core–mantle boundary linked to recurrent high-latitude weak/reverse flux patches

2020 ◽  
Vol 222 (2) ◽  
pp. 1423-1432
Author(s):  
Andreas Nilsson ◽  
Neil Suttie ◽  
Monika Korte ◽  
Richard Holme ◽  
Mimi Hill
Keyword(s):  
Northern Hemisphere ◽  
High Latitude ◽  
Geomagnetic Field ◽  
Outer Core ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
The Past ◽  
Convection Rolls ◽  
Westward Drift

SUMMARY Observations of changes in the geomagnetic field provide unique information about processes in the outer core where the field is generated. Recent geomagnetic field reconstructions based on palaeomagnetic data show persistent westward drift at high northern latitudes at the core–mantle boundary (CMB) over the past 4000 yr, as well as intermittent occurrence of high-latitude weak or reverse flux patches. To further investigate these features, we analysed time-longitude plots of a processed version of the geomagnetic field model pfm9k.1a, filtered to remove quasi-stationary features of the field. Our results suggest that westward drift at both high northern and southern latitudes of the CMB have been a persistent feature of the field over the past 9000 yr. In the Northern Hemisphere we detect two distinct signals with drift rates of 0.09° and 0.25° yr−1 and dominant zonal wavenumbers of m = 2 and 1, respectively. Comparisons with other geomagnetic field models support these observations but also highlight the importance of sedimentary data that provide crucial information on high-latitude geomagnetic field variations. The two distinct drift signals detected in the Northern Hemisphere can largely be decomposed into two westward propagating waveforms. We show that constructive interference between these two waveforms accurately predicts both the location and timing of previously observed high-latitude weak/reverse flux patches over the past 3–4 millennia. In addition, we also show that the 1125-yr periodicity signal inferred from the waveform interference correlates positively with variations in the dipole tilt over the same time period. The two identified drift signals may partially be explained by the westward motion of high-latitude convection rolls. However, the dispersion relation might also imply that part of the drift signal could be caused by magnetic Rossby waves riding on the mean background flow.

scholarly journals A coupled core-mantle evolution: review and future prospects

2020 ◽  
Vol 7 (1) ◽  
Author(s):  
Takashi Nakagawa
Keyword(s):  
Inner Core ◽  
Future Prospects ◽  
Core Mantle Boundary ◽  
Mantle Evolution ◽  
The Core ◽  
Secular Variations ◽  
Onset Timing ◽  
Polarity Reversals ◽  
Geomagnetic Polarity

Abstract In this review, I provide the current status and future prospects for the coupled core-mantle evolution and specifically summarize the constraints arising from geomagnetism and paleomagnetism on the long-term secular variations of the geomagnetic field. The heat flow across the core-mantle boundary (CMB) is essential for determining the best-fit scenario that explains the observational data of geomagnetic secular variations (e.g., onset timing of the inner core growth, geomagnetic polarity reversals, and westward drift) and should include the various origins of the heterogeneous structures in the deep mantle that have affected the heat transfer across the core-mantle boundary for billions of years. The coupled core-mantle evolution model can potentially explain the onset timing of the inner core and its influence on the long-term geomagnetic secular variations, but it is still controversial among modeling approaches on the core energetics because the paleomagnetic data contains various uncertainties. Additionally, with the coupled core-mantle evolution model in geodynamo simulations, the frequency of the geomagnetic polarity reversals can be explained with the time variations of the heat flow across the CMB. Additionally, the effects of the stable region in the outermost outer core to the magnetic evolution are also crucial but there would be still uncertain for their feasibility. However, despite this progress in understanding the observational data for geomagnetic secular variations, there are several unresolved issues that should be addressed in future investigations: (1) initial conditions—starting with the solidification of the global magma ocean with the onset timing of plate tectonics and geodynamo actions and (2) planetary habitability—how the dynamics of the Earth’s deep interior affects the long-term surface environment change that has been maintained in the Earth’s multisphere coupled system.

scholarly journals The separation of the geomagnetic field originated in the core, in the asthenosphere, and in the crust

1999 ◽  
Vol 42 (2) ◽  
Author(s):  
G. P. Gregori ◽  
W. J. Dong ◽  
X. Q. Gao ◽  
F. T. Gizzi
Keyword(s):  
Geomagnetic Field ◽  
Inner Core ◽  
Spatial Spectrum ◽  
Satellite Measurements ◽  
Core Mantle Boundary ◽  
The Core ◽  
Crustal Field ◽  
Core Boundary ◽  
Internal Sources ◽  
Geomagnetic Anomalies

The separation of the field produced by different internal sources can be accomplished by means of the so-called spatial spectrum of the geomagnetic field of internal origin. It is shown how such a rationale, when suitably interpreted, allows to recognize the field that is originated by electric currents that flow either on the Inner-Core Boundary (ICB), or on the Core-Mantle Boundary (CMB), or on the Asthenosphere-Lithosphere Boundary (ALB). It appears crucial, however, to rely on satellite measurements alone, because ground-based and ship- and air-borne records are severely perturbed by the crustal field. Therefore, it is shown, on the basis of a critical reconsideration of a few key-papers in the literature, that the best approach is to avoid mixing together all kinds of measurements. Satellite data are best suited for recognizing the dynamo field, while ground-based, ship- and air-borne records, which are measured much closer to crustal sources, are best suited, after subtraction of the satellite-derived dynamo field, for inferring the geomagnetic anomalies that are to be associated with crustal sources alone.

Decomposing the geomagnetic field: oscillation modes and characteristics

2020 ◽  
Author(s):  
Cristiana Stefan ◽  
Venera Dobrica ◽  
Crisan Demetrescu
Keyword(s):  
Geomagnetic Field ◽  
Field Model ◽  
Empirical Orthogonal Functions ◽  
Time Span ◽  
Eof Analysis ◽  
Detailed Structure ◽  
Orthogonal Functions ◽  
Oscillation Modes ◽  
Field Oscillation

<p>Using the COV-OBS.x1 (Gillet et al., 2015) main geomagnetic field model, covering the time span 1840–2020, respectively IGRF-13 (1900-2020), we decomposed the geomagnetic field at Earth’s surface in oscillation modes by means of empirical orthogonal functions (EOF) as well into a long term and a cyclic component using HP filtering (Hodrick and Prescott, 1997). Further, the long term component is filtered using a Butterworth filter (1930) with different cut-off periods in order to obtain oscillation at inter-centennial (> 100 years) and sub-centennial (60-90 years) timescales. The EOF analysis shows that the first three oscillation modes are characterized by periodicities of >100 years while modes 4 and 5 are characterized by dominant periodicities of 70-90 year. Although the variance of the modes 4 and 5 is rather small compared to that of the first three modes, these two modes are responsible for the detailed structure of the geomagnetic field. A comparison between the results of both methods is done as well.</p>

scholarly journals A model for the geomagnetic field reversal rate and constraints on the heat flux variations at the core-mantle boundary

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Vincenzo Carbone ◽  
Tommaso Alberti ◽  
Fabio Lepreti ◽  
Antonio Vecchio
Keyword(s):  
Heat Flux ◽  
Geomagnetic Field ◽  
Reversal Rate ◽  
Field Reversal ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core
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