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Le Champ Magnétique Terrestre : Structures Spatiales et Variations Temporelles vues par les Ondelettes

Abstract : The aim of this work is a better description and understanding of the spatial structures and temporal variations
of the Earth's magnetic field.

In the first part, we described and used two methods to model the geomagnetic field.

The first used method is the spherical harmonics analysis. Using measurements
provided by the magnetic observatories and the recent satellites, Ørsted and CHAMP, and
applying various selection criteria to these datasets,
we computed candidate models for the DGRFs 1995 and 2000.

The second developed method is the description of the geomagnetic field using the wavelet frames.
The chosen wavelets are the Poisson ones which derive from the same generating function as the spherical harmonics.
The comparisons between wavelet frames and spherical harmonics, as well as the representations of scalar functions
on the sphere, illustrate the flexibility of the method: the development in wavelet frames really suit to the data distribution.
The same model contains small spatial scales in zones covered by a dense dataset
and larger spatial scales in areas with sparsed measurements. Vector fields derived from harmonic potentials,
as magnetic and gravity field, are developed on wavelet frames.

In the second part, we study the temporal variation of the main geomagnetic field, known as
secular variation. Its more enigmatic characteristic is the occurrence,
at irregular intervals, of abrupt changes of its tendency: the geomagnetic jerks.
These events are detectable by wavelet analysis in observatory monthly mean series.
Unfortunately, the observatory distribution is uneven at the Earth's surface. To circumvent this problem
and to find a solution for geomagnetic jerks study all over the Globe, the use of continuous field models,
like comprehensive model, is suitable.

We studied, by simple methods, the temporal and spatial
distributions of jerks detected in these models, at any point
of the sphere over the four last decades. We focused on jerks around 1971, 1980 and 1991
and highlighted "blind" zones, where no jerk is clearly detected.
In the last chapter, we present a possible relation between the geomagnetic jerks and a physical parameter
at the surface of the core, the absolute dynamic pressure.

The use of the wavelets, as tools of modelling and/or analysis, makes it possible to explore all the spatial
and/or temporal scales of the geomagnetic field.
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Submitted on : Wednesday, January 12, 2005 - 2:05:36 PM
Last modification on : Monday, December 14, 2020 - 9:56:55 AM
Long-term archiving on: : Friday, April 2, 2010 - 9:27:17 PM


  • HAL Id : tel-00008021, version 1



Aude Chambodut. Le Champ Magnétique Terrestre : Structures Spatiales et Variations Temporelles vues par les Ondelettes. Géophysique [physics.geo-ph]. Institut de physique du globe de paris - IPGP, 2004. Français. ⟨tel-00008021⟩



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