Data of Core Angular Momentum Changes

Author(s) : Alexandra Pais (1) and Gauthier Hulot (2)

Sampling time interval : 1 year

Time span from : 1840 to 1990

Modelling hypotheses :

The present series stands for to the predicted LOD decade variations as a result of the axial angular momentum exchange between the liquid core and the mantle. Following Jault, Gire and Le Mouel (1988), only the toroidal zonal coefficients t_1^0 and t_3^0 of the core surface flow contribute. Such a flow is computed for each year in the time interval 1840 to 1990, through inversion of the geomagnetic time-dependent field model of Bloxham and Jackson (1992). The different inversion assumptions are that of an isolating mantle, frozen flux and tangential geostrophy. It is also assumed that the errors in the inverted secular variation (SV) model have remained essentially the same throughout the time period considered. As all that can be predicted are relative changes in the LOD, a value of 2.9 milliseconds has been added to optimize the agreement between this series and the corresponding series obtained from astronomical observations.

Description of the data used for model computation :

The magnetic time-dependent ufm1 geomagnetic field model of Bloxham and Jackson (1992). The spatial dependency of the field is represented using a spherical harmonics expansion truncated at L=14 and the time dependency is represented using a cubic B-spline expansion truncated at N=63.

References to be mentioned when data are used :

Pais, A. and Hulot, G., 'Length of day decade variations, torsional oscillations and inner core superrotation: evidence from recovered core surface zonal flows', Phys. Earth Planet. Int. 118, 291-316, 2000.

Related references :

Jault D., Gire C., and Le Mouel J.-L., "Weastward drift, core motions and exchanges of angular momentum between core and mantle", Nature, 333 , 353-356, 1988.
Bloxham J. and Jackson A., 'Time-dependent mapping of the magnetic field at the core-mantle boundary', J. Geophys. Res., 97, 19537-19563, 1992.
Jackson A., Bloxham J., and Gubbins D., 'Time-dependent flow at the core surface and conservation of angular momentum in the coupled core-mantle system', in: Le Mouel J.-L., Smylie D. E. and Herring T. (Eds.), Dynamics of Earth's Deep Interior and Earth Rotation , IUGG Vol. 12, AGU Geophysical Monograph 72, pp. 97-107, 1993.