Geoscience Reference
In-Depth Information
Fig. 13
Time series of the
EOP 05 C04 precession-
nutation residuals with respect
to the conventional
a pri-
ori
model of Mathews et al.
(
2002
). The daily corrections
are projected to Earth's sur-
face, thus 1 cm corresponds to
∼
2
1
0
0.3mas
−1
dX
dY
−2
2005
2006
2007
2008
2009
2010
2011
Year
apriori
model provide observational evidence of the FCN but also reveal its temporal
variability (Fig.
13
).
Partly due to the presence of the NDFW/FCN eigenmode, different physical prop-
erties of the Earth are determinative when estimating excitation of nutation instead
of slow polar motion variations (Brzezi nski
1994
). As already demonstrated, the
assumption of a decoupled core (Sect.
2.4
) is not tenable at retrograde diurnal fre-
quencies and has to be refined by taking into account the effects of the FCN. Such an
approach is also deeply required in view of steadily improving Earth rotation obser-
vations, that nowadays are available at high temporal resolution and at an accuracy
that is sufficient to detect subdaily geophysically-induced variations in EOP.
As suggested e.g. in Schindelegger et al. (
2011
), the eigenfrequency
σ
f
of the
NDFW resonance can be derived from
T
f
, the period of the FCN in the celestial
frame, and
Q
f
, its Earth-referred quality factor. Both values are taken from the
conventional precession-nutation model (Mathews et al.
2002
)
1
T
f
−
1
1
T
sid
i
2
Q
f
σ
f
=
2
π
−
.
(81)
The numerical values are
T
f
=−
.
=
=
.
9973d,
describing the length of the sidereal day. Evaluation of Eq. (
81
) yields a frequency
of resonance at
430
2d,
Q
f
20000 and
T
sid
0
00232 cpsd.
Given the introduction above, we can now devote ourselves to the derivations of
Brzezi nski (
1994
), which start from the frequency-dependent equation of motion of
Sasao and Wahr (
1981
)
σ
=−
1
.
