The Geomagnetic Field - Quantitative Plate Tectonics - page 121

Fig. 4.16 Theoretical 24-h plot of Sq field components at 0 ı N, 15 ı E, calculated using the SQ1 model of Campbell
et al. ( 1989 ). The horizontal axis shows local time (in min after midnight) on June 1st 2005

prevalenceofthe24,12,8,and6hharmonics
shows that the driving mechanism of the source
currents depends strongly from the Earth's rota-
tion. In fact, it is known that this field results from
enhancement of conductivity of the E region of
the ionosphere, an intermediate layer between 90
and 140 km altitude, induced by solar radiation
(e.g., Hitchman et al. 1998 ).
In general, at high latitudes the Sq field does
not give a significant contribution to the external
field, whereas at the magnetic equator the highest
magnitude is of a few tens of nT.

This isotherm is typically more than 20 km
depth in stable continental regions, but may
be as shallow as 2 km in young oceanic
regions. Although both remnant and induced
magnetization contribute to the crustal field, only
the former is important for plate kinematics. In
paleomagnetic studies performed on continents,
the remnant components of magnetization
are isolated directly on rock samples through
laboratory procedures. Conversely, in the case
of marine geophysics, it is not a simple task
to separate these components from a data
set of total field measurements, thereby, our
interpretations often rely on the hypothesis (more
or less justified) that the induced component is
small. In general, igneous rocks have the highest
Koenigsberger ratios (often between 5 and 50, see
Tab le 3.1 ) , and in the important case of MORBs
Q may reach values as high as 160. Therefore,
in the oceanic regions, where it is not generally
possible to separate the remnant magnetization
from the induced component, the crustal field
can be considered with good approximation as
the product of time independent TRM at the
historical scale.
In most places, the crustal field is less than
1 % of the total magnetic field, but locally may
represent up to 20 % of the observed field. The
first global maps of the crustal field were built

4.6 Crustal Magnetic Field

The magnetization of crustal rocks represents
an important source for the Earth's internal
field, originating near-surface anomalies between
3,600 and C 8,500 nT. Conversely, both the
mantle and the unconsolidated sediments are
essentially non-magnetic. The crustal sources are
located in regions where the temperature field is
below the Curie point of the magnetic minerals
(see Sect. 3.5 ) . Therefore, the lower boundary
of the so-called magnetic basement ,whichis
the region of crustal rocks having relatively
large magnetic susceptibilities, approximately
coincides with the Curie isotherm for magnetite.