Geology Reference
In-Depth Information
magnetic fields. The more realistic mechanism
through which Earth's rotation and convective
fluid motions in the outer core generate and
sustain the main geomagnetic field is known
as the
geodynamo
. The fundamental laws of
this model are the MHD induction Eq. (
4.14
)
and the equations of motion of fluid mechanics
basic operating principle of a self-sustained
magnetic field is conceptually simple and can be
described through an analog model formed solely
by a solenoid and a rotating disk. In the next
sections, we shall focus on the time variations of
the magnetic field generated by the geodynamo
mechanism.
with an axis passing through the Earth's centre.
In fact, a better fit to the observed geomagnetic
field can be obtained using a magnetic dipole
located
400 km from the centre of mass in
the direction of the positive
z
-axis. This model
field is knows as the
eccentric dipole field
.The
magnitude of the observed geomagnetic field
varies considerably along the Earth's surface and
in time. The field vectors are sub horizontal
close to the Equator, where
F
30,000 nT,
whereas they tend to be vertical in proximity
of the geomagnetic poles, with magnitudes of the
order of
60,000 nT. Regarding the variability
in time, at the scale of months and years we
observe that the inclined geocentric dipole, which
represents about 90 % of the main field, has no
fixed direction. The magnetic moment of this
dipole precesses irregularly about the geographic
North Pole, thereby determining variations of
declination and inclination, and has variable
magnitude. Although there is some superposition
between the spectrum of time variations
associated with external sources and the
variability of the core field, in general the former
are much more rapid and have much smaller
magnitude. The secular variation of the Earth's
magnetic field was first recognized by Gellibrand
in 1634, who observed that the declination in
London had decreased by more than 7
ı
since the
first measurement in 1580. Figure
4.6
shows the
secular variations of declination and inclination
recorded in London since the sixteenth century.
In general, the geomagnetic field variations are
not constant in time and have variable amplitude
from place to place.
There are three basic phenomena associated
with the secular variation of the geomagnetic
field, which affect both the dipolar component,
over long time intervals, and the non-dipolar part
of the field on the short period. They are:
4.3
Secular Variation of the Core
Field
The magnetic field generated within the external
Earth's core by MHD processes is the
main
geomagnetic field
, as it represents the major
component of the magnetic field observed at the
Earth's surface. The convective currents flowing
in the external core have an estimated velocity
of the order of 10 km/year (e.g., Buffet
2000
,
and references therein). The resulting field is
subject to time variations that are significant
at the scale of months. This variability of the
main field is said
secular variation
and is of
the order of 80 nT/year. The conventions used
in geomagnetism to designate the components
of the main field,
F
, have been illustrated in
would be generated by a magnetic dipole placed
at the Earth's centre and inclined by
11
ı
with
respect to the Earth's spin axis. As mentioned
the Earth's surface at the geomagnetic poles. If
the field were perfectly dipolar, and in absence
of crustal and external contributes, then the
inclination at the geomagnetic poles would be
I
D˙
90
ı
. The points at the Earth's surface
where the
observed
field is effectively vertical are
called
magnetic poles
. At present, the distance of
these poles from the geomagnetic poles is several
hundreds km and they are by no means aligned
(a) A continuous change of direction of the mag-
netic dipole moment and the consequent mi-
gration of the geomagnetic poles;
(b) Variations of magnetic moment magnitude;
(c) A westward drift of some non-dipolar fea-
tures of the field at a rate of
0.4
ı
longitude
per year;
