Geology Reference
In-Depth Information
note the presence of a prominent intensity low,
F
23,000 nT, in Argentina (Fig.
4.27
), which
is known as
South Atlantic anomaly
(Heirtzler
2002
). This field low is associated with a region
of intense radiation in the space near the Earth,
which causes damage to spacecrafts in low
Earth orbit and is a hazard for astronauts. The
magnetic inclination map (Fig.
4.28
)showsa
roughly dipolar trend, especially in the tropical
belt. Conversely, the declination plot of Fig.
4.29
shows a much more complex pattern. Recall that
a pure tilted dipole generates a field with two
agonic lines
(contours of zero
D
) and a single
dip equator
(where
I
D
0). The contour lines of
Fig.
4.29
show three agonic lines: one through
the Americas, one that encompasses Europe,
western Asia, and northeast Africa, and one that
crosses the northwestern Pacific, southeast Asia,
and western Australia.
In the next chapter, we shall see that the IGRF
is an essential tool for the modelling of marine
magnetic anomalies, because it allows to separate
the crustal components of the observed field from
the contribution of the Earth's core.
the magnetic field lines are close to being
vertical and the field intensity increases as
approaching the Earth;
6. How does the ring current influences the
Earth's magnetic field, assuming that it is
formed by protons concentrated in a thin band
having 1 km
2
cross-section at distance
r
D
4
R
e
with density
n
p
D
4nAm
2
?
References
Blakely RJ (1996) Potential theory in gravity and mag-
netic applications. Cambridge University Press, Cam-
bridge, UK, p 441
Buffet BA (2000) Earth's core and the geodynamo. Sci-
ence 288:2007-2012. doi:
10.1126/science.288.5473.
2007
Butler RF (1992) Paleomagnetism: magnetic domains to
geologic terranes. Blackwell Scientific Publications,
Boston, p 238
Cain JC, Wang Z, Schmitz DR, Meyer J (1989) The
geomagnetic spectrum for 1980 and core-crustal sepa-
ration. Geophys J Int 97(3):443-447
Campbell WH (2003) Introduction to geomagnetic fields,
2nd edn. Cambridge University Press, Cambridge,
p 337
Campbell WH, Schiffmacher ER, Kroehl HW (1989)
Global quiet day field variation model WDCA/SQ1.
Eos Trans AGU 70:66-74
Cande
Problems
SC,
Kent
DV
(1995)
Revised
calibration
of
the
geomagnetic
time
scale
for
the
late
Creta-
1. Write a computer program to convert from
geographic to geomagnetic coordinates and
vice versa;
2. Determine an expression for the field com-
ponents starting from the spherical harmonic
expansion (
4.93
);
3. Download the MS Excel worksheet CK-
GTS2004.xls, containing a combined time
scale Cande and Kent (
1995
) and Gradstein
et al. (
2004
). Plot the frequency distribution
and the cumulative frequency distribution of
the lengths of polarity intervals. What kind of
distribution results?;
4. Perform a Fourier analysis of the time scale
CK-GTS2004.xls, plotting the power density
as a function of the frequency of inversions;
5. Describe quantitatively the motion of a
charged particle in the auroral zone, where
ceous
and
Cenozoic.
J
Geophys
Res
100(B4):
6093-6095
Courtillot V, Le Mouël JL (1988) Time variations of the
earth's magnetic field-from daily to secular. Annu Rev
Earth Planet Sci 16:389-476
Creel KM (1977) Geomagnetic secular variations during
the last 25 000 years: an interpretation of data obtained
from rapidly deposited sediments. Geophys J R Astron
Soc 48:91-109
Dobson DP, Brodholt JP (2000) The electrical conduc-
tivity and thermal profile of the Earth's mid-mantle.
Geophys Res Lett 27(15):2325-2328
Elsasser WM (1939) On the origin of the Earth's magnetic
field. Phys Rev 55:489-498
Finlay
CC
et
al
(2010)
International
geomagnetic
reference
field:
the
eleventh
generation.
Geophys
J
Int
183(3):1216-1230.
doi:
10.1111/j.1365-246X.
Glatzmaier GA, Roberts PH (1995) A three-dimensional
convective dynamo solution with rotating and finitely
conducting inner core and mantle. Phys Earth Planet
Inter 91(1):63-75
