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tion between Earth and atmosphere can only be provided by the torque approach.
By explicitly looking at the normal and tangential surface forces, the torque method
gives insight into the specific processes that lead to a change in angular momen-
tum of the solid Earth (Seitz and Schuh
2010
). In particular, it allows studying the
origin and spatial location of those processes in an active way, noting how meteo-
rological systems crossing particular mountain ranges effect the angular momentum
exchange.
Consequently, when studying geophysically-induced variations of ERP, the torque
approach represents a source of additional information, even though its actual rotation
corrections are not as precise and less reliable than the corresponding estimates
from the AAM method. Both approaches may be applied in parallel in order to also
learn about their respective reliability and accuracy (de Viron et al.
1999
). Finally,
there is the prospect of improved circulation models in the future, which will be of
great benefit for further advancing the value of torque estimates for Earth rotation
studies.
3 Angular Momentum Functions and Earth Rotation Parameters
Comparison of numerically evaluated angular momentum functions and time series
of Earth rotation (orientation) parameters is the core task of excitation studies and can
be basically accomplished by means of the linearized differential equations system
derived in Sect.
2.4
. Equations (
30
) and (
31
) allow us to relate small changes in the
angular momentum of a certain geophysical fluid to the associated equatorial and
axial variations of Earth's rotation axis (Gross
2007
). However, as already anticipated
in Sect.
2.3
, a practical application of the equation system requires incorporating the
actually observed axis as reference direction. The following portions of the review
will deal with this problem. For the case of polar motion studies, the conventional
model will be introduced in Sect.
3.1
, before being extended for the effects of the
FCN in Sect.
3.2
.
3.1 Polar Motion, Standard Model
The polar motion of Earth's instantaneous rotation axis
i
m
2
is a relative
quantity which requires specification of the underlying reference system. Within geo-
detic practice, this reference system is realized empirically by a network of globally
distributed, rotating reference points, from which observations are made. As soon
as such a body-fixed reference system (the TRS) is established, the instantaneous
rotation pole cannot be defined freely but instead follows from kinematical con-
siderations. Those considerations shall be the focal point of the present section. In
particular, the motion of the instantaneous rotation axis will be related to that of the
CIP, the position of which is usually reported by modern Earth rotation measurement
m
ˆ
=
m
1
+
