Geoscience Reference
In-Depth Information
Notice that the parameter L is measured in Earth radii R e . For example, if the field
line exits the Earth's surface .r D R e / at a magnetic latitude D 60 ı , then L D 4.
This implies that the field line crosses the equatorial plane at r D 4R e .
The field near the ground surface, however, is not quite dipole since it contains
abnormal regions. Such a region can be as high as several hundreds or thousands
kilometers. There is known a number of the so-called world magnetic anomalies
such as Brazilian, Siberian, and Canadian anomalies. Moreover, there are a lot of
local anomalies on the Earth surface. The magnitude of the local anomalies is of the
order of 2 10 7 T, and their sizes vary from unity up to several hundreds kilometers.
For the Kursk magnetic anomaly (Russia) the deviation from the main geomagnetic
field reaches a value of about 10 5 T. The influence of magnetized rocks, which
contain ferrimagnetic minerals on the basis of iron oxide, can be the major cause of
such local anomalies.
Empirical corrections to the field of the main dipole are described through a
number of additional dipoles situated inside the core and at the boundary between
the core and mantle. The observed magnitudes of the dipole harmonics decrease in
accordance with a power law.
The geomagnetic field exhibits slow random variations with characteristic period
from 10 to 10 4 years that is referred to as the secular variations. During the main
period of the secular variations, that is 8 10 3 years, the main dipole moment
can change by 1.5-2 times in magnitude. On average the dipole moment vector
is approximately directed along the Earth spin axis. Therefore, the Earth rotation
may greatly affect the evolution of the geomagnetic field. The result of the secular
variations is that the geomagnetic pole exhibits precession around the geographic
pole with the period of the order of 1:2 10 2 years.
Study of the residual magnetization of sedimentary which contain ferrimagnetic
minerals has shown that the geomagnetic field keeps the magnitude close to the
modern value at least during 2:5 billion years, that is comparable with the geologic
age of the Earth, that is 4:6 billion years. On average during each 10 5 -10 7 years
the random fluctuations of the geomagnetic field reach the critical levels in a way
that the geomagnetic field changes the initial direction to inverse one. When the
northern and southern poles are traded their places the period of the poles inversion
continues 10 3 -10 4 years. The numerical modeling has shown that during this period
the structure of the geomagnetic field is far from the dipole and the field pattern
becomes complicated. It should be emphasized that these changes cannot reduce to
the simple rotation of the vector of magnetic dipole.
It is interesting to note that such inversion of the geomagnetic field has not been
observed during the last 7:8 10 5 years. The geomagnetic field was first measured at
the beginning of nineteenth century and since then it has decreased by 10 %! Maybe
the next inversion of magnetic poles is already close?
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