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Date: Wed, 9 Feb 2011 15:44:56 -0600
To: "Lemoine Jean-Michel" <Jean-Michel.Lemoine@cnes.fr>, <malkin@gao.spb.ru>,
        <zmalkin@zmalkin.com>, <ids.analysis.forum@ids-doris.org>
From: "John C. Ries" <ries@csr.utexas.edu>
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Subject: RE : [ids.analysis.forum] TR : Long term trends in the gravity
  field (forwarded)

Hi Jean-Michel,

>The shape of our C21/S21 solution is quite similar to the shape of 
>C21/S21 computed from IERS2010 mean pole (see plot).
>This although the mean pole that we are using is quite different 
>(green line on the plot).

It's good to see that the model does capture the overall trend. 
That's about all we should expect. It is intended only to provide 
something in the case that reliable observations of C21/S21 are not 
available.

>
>It displays a small positive bias compared to the C21/S21 computed 
>from IERS2010 mean pole:
>+2.e-11 for C21, +3.e-11 for S21
>
>Converted into x and y in mas, this gives -4.9 mas for x and +7.4 
>mas for y w.r.t. IERS2010 mean pole.

The idea of inferring C21/S21 from the mean pole rests on one 
important assumption....that the mean rotational axis and the 
principal inertia axis are closely aligned when averaged over a 
sufficiently long time period. It's not clear just what the 
appropriate averaging interval is, and we probably should not expect 
the predicted C21/S21 to align perfectly. The amount of departure may 
also provide clues to the orientation of the core, since the rotation 
axis is based only on measurements from the crust. This is discussed 
on our recently published paper, though the technical details about 
the core are a little beyond me, so I cannot add much about that.

Anyway, I'd say the model is does a pretty good job of the capturing 
the long-term behavior of C21/S21, certainly better than the older 
straight line model.

Best regards,
John


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