Report of the Special Bureau for the Core.
Dehant V.^{1} (Head) and by alphabetic order: Cardin Ph.^{2},
Chulliat A.^{3}, Defraigne P.^{1}, de Viron O.^{1},
Dormy E.^{3}, GreffLefftz M.^{3}, Hinderer J.^{4},
Holme R.^{5}, Hulot G.^{3}, Jackson A.^{6}, Jault D.^{2},
Kuang W.^{7}, Legros H.^{4}, Le Mouel JL.^{3}, Noir J.^{2},
Pais A.^{8}, Van Hoolst T.^{1}
^{1} Royal Observatory
of Belgium (ROB), Av. Circulaire 3, B1180 Brussels, Belgium
^{2} Laboratoire de Géophysique Interne
et Tectonophysique (LGIT) de Grenoble, France
^{3} Institut de
Physique du Globe de Paris (IPGP), Laboratoire de Géomagnétisme, France
^{4} Ecole et Observatoire de Physique
du Globe de Strasbourg (EOPGS), France
^{5} University
of Liverpool, United Kingdom
^{6} School of
Earth Sciences, University of Leeds, United Kingdom
^{7} Goddard
Space Flight Center (GSFC), USA
^{8} University
of Coimbra (UMBC), Portugal
Introduction
Our activities are mainly related to the
scientific developments and results concerning the angular momentum exchange
between the core and the mantle and the physical mechanisms (torques) at the
CoreMantle Boundary (CMB) and at the Inner Core Boundary (ICB). This angular momentum exchange induces
variations in all three components of Earth rotation. Because there are no direct observations of the core flow that
could lead to wellconstrained models as do exist for the ocean and atmosphere,
only calculations of core angular momentum (CAM) can be obtained from core
flows derived from variations in the magnetic field observed at the Earth
surface. Further, these flows depend on
poorly constrained additional assumptions to resolve formal nonuniqueness in
the solution. On our website (www.astro.oma.be/SBC/main.html), we have
collected data of core angular momentum. We have also given explanations
related to the different assumptions that may be used for the core angular
momentum computations. In addition, we
included a review of the candidate coupling mechanisms that are currently under
investigation for explaining this angular momentum exchange (possible torques
at the CMB).
Data of Core angular momentum computations
We have put 9 series of CAM on our
website, with their physical assumptions and the references to be mentioned.
The assumptions used are based on our
understanding of the physical process involved. First of all, based on order of
magnitude evaluation at decade time scale, only the induction term is retained
in the magnetic induction equation, equivalent to the frozenflux approximation
(the changes in the magnetic field lines follow the flow line changes).
Secondly, dealing with more unknowns than equations, it is necessary to further
consider an additional constraint. In the literature, one finds four additional
constraints (1) toroidal field flow, (2) piecewise steady flow, (3) flow steady
in a drifting frame, (4) tangential geostrophic flow. The series put on the web are mainly related to last hypothesis.
Further, these additional assumptions are
poorly constrained. Consequently, the associated calculated changes of the
Earth's angular momentum are quite uncertain. Still, the LengthOfDay (LOD)
changes calculated from these core flows agree remarkably well with the
observed LOD changes.
The list of these CAM series is:

Jackson’s three different core angular momentum (CAM)
based on the torsional oscillations using the hypothesis of fully
timedependent geostrophic flow and the surface magnetic field UFM1 of Bloxham
and Jackson [1992, J. Geophys. Res., 97, 1953719563] (for three different
smoothings) [Jackson, 1997, PEPI 103, 293311]

Petrov’s three different core angular momentum (CAM)
based on the torsional oscillations using (a) the IGMF surface magnetic field
and the geostrophic flow approximation, (b) the surface magnetic field and the
quasisteady flow approximation, and (c) the LOD observation, the surface
magnetic field and the geostrophic flow approximation [Petrov and Dehant, not
published];

Jault’s core angular momentum (CAM) based on the
torsional oscillations using the tangential geostrophic flow and the surface
magnetic field [Jault, Gire, and Le Mouël, 1988, Nature 333, 353356];

Pais’ core angular momentum (CAM) based on the
torsional oscillations using the tangential geostrophic flow and the surface
magnetic field UFM1 of Bloxham and Jackson [1992, J. Geophys. Res., 97,
1953719563] [Pais and Hulot, 2000, PEPI 118, 291316];

Boggs’ core angular momentum (CAM) based on the
torsional oscillations using the tangential geostrophic flow and the surface
magnetic field UFM1 of Bloxham and Jackson [1992, J. Geophys. Res., 97,
1953719563] [Hide, Boggs, and Dickey, 2000, Geophys. J. Int. 143, 777786].
Bibliography
Additionally, we have built a bibliography
of material relevant to the core that
presently contains more than thousand
references.
Book on “Core dynamics, structure and
rotation”
Following an Union Session at the end of
2000 at the American Geophysical Union that we had organised, we have decided
to edit a volume. The idea of the
session was to bring together specialists of the core from different
disciplines: seismology, geochemistry, geomagnetism, geodynamo and geodesy
(Earth rotation). The book has the same
philosophy and will be published at the end of 2002. The title of the book is “Core dynamics, structure and rotation”,
and the editors are V. Dehant, K. Creager, S. Karato, and S. Zatman. Our book is dedicated to Stephen Zatman who
died in an accident in July 2002.
Stephen did a lot of research in the frame of LOD variations and the
Earth core.
Change of Head
Following the proposition of the present
Head of the SB for the core, V. Dehant, she will be replaced for the next term
(starting in July 2002) by her colleague Tim Van Hoolst.