Schmucker, 1970 (Scripps) - MTNet

Schmucker: Geomagnetic Variations 75
outweigh slightly the field from deep induction currents in the mantle. Without
the dampening effect of a core ( P = 0) we would have obtained T) s = 2.67.
We infer from(5. 60) that the internal shell term Is leads in phase relative
to the external term E by tan-1[ l/Tl s] , while the corresponding core term Ic
lags in phase by tan-lT) s, when Sc is close to its upper real limit. Thus, induction
anomalies which originate from nonuniformities in the shell will lead
in phase relative to those of deep origin (cf. fig. 35).
Figure 34 illustrates the complexity of the mutual dependence of shell and
core induction. The induction curves for T)n = 0 refer to a shell without core.
With increasing conductivity of the core the curves are shifted toward higher
0.2 10 4,,(".)(:. d A 10 100 1000
Fig. 34. Induction curves for a
special shell-core model, shown
for various induction parameters
1)n of the uniform core. Upper
diagram: in-phase components of
the internal part, shown separately
for shell and core induction
in accordance with
equation 5.60. Lower diagram:
out-of-phase components of the
internal part.
values of the shell-induction parameter, wmch implies that higher conductivities
in the shell are needed to produce the same induced field from the
shell (= dampening effect of the core). For T) n = co the ultimate curves for a
perfect conductor at the indicated depth are reached. We see that the core
induction is suppressed at the same rate as the shell induction is built up
(= shielding effect of the shell); Is and I c preserve their characteristic opposite
arguments except for small values of T)n and 41TWTA.