The spherical harmonic of degree 2, order 0,C(2,0), is due to the flattening of the Earth. Its technical name is 'Earth’s dynamic oblateness'. C(2,0) (also known as 'J2', although they differ by a constant factor: J2=-C(2,0)*sqrt(5)) is only a function of the difference between equatorial and polar radii of the equipotential surface of the Earth's gravity field that best fits mean sea level; more accurately, J2 is a function of the difference in principal moments of inertia. (for more information on spherical harmonics, see the Mathematica article)

A steady decrease in J2 has been observed by artificial satellites since 1979 and with eclipse data for the last 2500 years. The reason for this decrease is postglacial rebound (glacial isostatic adjustment). Ocassionally this steady decrease is temporarily slowed down or reversed for a few years due to ocean and ice mass redistribution, but at the end of those short periods the main trend downward becomes evident again (Cox and Chao, 2002; Dickey et al, 2002).

The data available here were derived by Cheng and Tapley (2004).

There are two types of time series at this site:

a) C(2,0) only, 1976-2009, which is an extension of the time series in Cheng and Tapley (2004).

b) The spherical harmonic coefficients C and S of degree 2, orders 0, 1 and 2 for the time period 4/2002 to present. As opposed to the previously mentioned data, these data use the same background fields as the latest GRACE data RL 05, especially the so-called AOD (atmosphere-ocean dealiasing). The files contain self-explanatory information about the data.

These estimates are obtained from the analysis of Satellite Laser Ranging (SLR) data to five geodetic satellites: LAGEOS-1 and 2, Starlette, Stella and Ajisai. The background gravity model used in the SLR analysis is consistent with GRACE Release-04 (old) or Release-05 processing, including the use of the same Atmosphere-Ocean De-aliasing (AOD) dataset. However, the monthly mean of the AOD model (included in the file) has been restored, so that the coefficients represent the full month's signal (in earlier versions, the coefficients represented the difference between the full coefficients and a standard trend). For the same reason, no rate models have been applied. These data are housed at the Center for Space Research at University of Texas Austin.

ACKNOWLEDGEMENT and CITATION

When using these data, please acknowledge receiving the data from "http://grace.jpl.nasa.gov", and cite

Cheng, M.K., Tapley, B.D., Variations in the Earth's oblateness during the past 28 years, J. Geophys. Res., 109(B09402), 2004

References cited

C.M.Cox, B.F.Chao, Detection of a Large-Scale Mass Redistribution in the Terrestrial System Since 1998 Science 297, p831 (2002).
J.O. Dickey et al: Recent Earth Oblateness Variations: Unraveling Climate and Postglacial Rebound Effects. Science, p 1975 (2002)