Introduction to Earth System
Solid Earth part
Rocco Malservisi
roccom@lmu.de
Phone: 2180 4201
Magnetic Field is a vector
It has an intensity (can be measured looking
At the oscillation of a compass)
And a direction
The direction change with the position
Magnetic Pole:
The place where the compass is pointing
Down
Magnetic Equator:
The place where the compass is horizontal
The Magnetic Poles are close to the geographical
Poles but do not coincide (~11 off)
The Earth’s Magnetic Field
B = (X, Y, Z)
Or
B = (F, I, D )
Or
B = (D, H, Z)
F: intensity
I: inclination
D: declination
H: Horizontal component
The seven elements of the (local) magnetic field
in the geographic coordinate system
I. Geomagnetic field – Local Geomagnetic Field Vector
The Earth’s Magnetic Field
From this:
Magnetic pole is the point where
H=0 D= +- 90
Magnetic Equator the point where
D=0
F: intensity
I: inclination
D: declination
H: Horizontal component
Where 3000nT<H<6000nT erratic zone (compass work badly)
Where H<3000nT unusable zone (compass does not work)
I. Geomagnetic field – Local Geomagnetic Field Vector
The place where the axis of the dipole intersect the surface
Of the earth are called geomagnetic poles
Magnetic Observatory
http://www.ngdc.noaa.gov/seg/geomag/icons/Obs1999_lg.gif
FUR
http://www.geophysik.uni-muenchen.de/observatory/geomagnetism
Geomagnetic models: Interpolation of the observations
Using spherical harmonics.
If we do not have electrical charges and magnetic sources
We can have a potential. (otherwise it is not conservative
so no potenzial)
Gauss in 1830 thought that he can divide the field in internal
And external having 2 potentials.
The internal goes as r^-n the external as r^n
The potential can be expressed as:
a n 1 m
v i  a   gn cosm  hnm sin mPnm cos 
 
n1 m 0 r

n

n


v e  a  ar g cosm  h sin m Pnm cos 
n
n1 m 0
*m
n
*m
n
a n 1 m
v i  a   gn cosm  hnm sin mPnm cos 
 
n1 m 0 r

n

n


v e  a  ar g cosm  h sin m Pnm cos 
n
n1 m 0
*m
n
*m
n
This is not only an interpolation scheme but also the solution for
 the physical problem of the magnetic field due to an internal or
external source!
The coefficients are called Gauss coefficients
The internal field coef. start from r^-2 because we do not have
the magnetic monopole
The internal field represent 90% of the total field

a n 1 m
v i  a   gn cosm  hnm sin mPnm cos 
 
n1 m 0 r

n

n


v e  a  ar g cosm  h sin m Pnm cos 
n
n1 m 0
*m
n
*m
n
Table 1. Spherical Harmonic Coefficient (in nT) of Terrestrial Magnetic Field (IGRF 1985)
Coefficient
Degree (m)
Order (n)
1
2
3
4
______________________________________________________
4
169
gnm
3
835
-426
2
1691
1244
363
______________________________________________________
1
-1903
2045
-2208
780
gno
0
-29877 -2073
1300
937
1
5497
-2191
-312
233
______________________________________________________
2
-309
284
-250
hnm
3
-296
68
4
-298
a n 1 m
v i  a   gn cosm  hnm sin mPnm cos 
 
n1 m 0 r

n

n


v e  a  ar g cosm  h sin m Pnm cos 
n
n1 m 0
*m
n
*m
n
The internal field represent 90% of the total field

The coef. g with n=1 m=0 give the magnitude of the dipole
aligned with the rotation axis.
The coef. with n=1 give the magnitude of the dipole is the largest
one ~85% of the field. It is inclinated of ~11 degrees.
For n>12 the coef. are neglegible.
N=2 quadrupole etc… can be controlled by regional features.
The numerical interpolation of the data is called
Geomagnetic models. Every 5 yr a new model is released by
The international community now we have the IGRF 2005
From Press, 1992.
90% of spatial field distribution can be explained by a simple dipolar field
I. Geomagnetic field – Geocentric inclined dipole
Geomagnetic Field Intensity
Other units: Gauss=100000nT g=10000nT
I. Geomagnetic field – Worldwide Variation of F
Geomagnetic inclination (IGRF)
tan I = 2 tan 
I. Geomagnetic field – Worldwide Variation of I
Worldwide Distribution of Geomagnetic Declination according to IGRF 2000
I. Geomagnetic field – Worldwide Variation of D
The dipolar field is called the MAIN FIELD
It changing slowly (this is why we update the model
Every 5 yr by IAGA)
The external field can change quickly.
How does the field change:
http://www.geophysik.uni-muenchen.de/observatory/geomagnetism
Big diurnal variation and annual variation what can cause it?
Temporal (diurnal and secular) variations
other secular variation: reversal
10 nT / hour
From Butler,
Palaeomagnetism, 1992.
Magnetic storm
Slide
I. Geomagnetic field – Temporal Variations
other secular variation: reversal
From Butler,
Palaeomagnetism, 1992.
Slide
I. Geomagnetic field – Temporal Variations
Where the magnetic field came
from?
a) Dipole inside the Earth
can not have reversal
b-c) Uniformly magnetic mantle
Or core, mantle of silicate
too hot
d) Current in the core
Most likely
Where the magnetic field came
from?
Self Sustaining dynamo
Where the magnetic field came
from?
From Fowler, 2004
based on the size and electrical
conductivity of the Earth's core, the
field, if it were not continually being
generated, would decay away in only
about 20,000 years since the
temperature of the core is too high to
sustain permanent magnetism.
http://www.es.ucsc.edu/~glatz/geodynamo.html
The convection in the fluid outer core is thought to be
driven by both thermal and compositional buoyancy
sources at the inner core boundary that are produced as
the Earth slowly cools and iron in the iron-rich fluid alloy
solidifies onto the inner core giving off latent heat and the
light constituent of the alloy. These buoyancy forces cause
fluid to rise and the Coriolis forces, due to the Earth's
rotation, cause the fluid flows to be helical. Presumably
this fluid motion twists and shears magnetic field,
generating new magnetic field to replace that which
diffuses away.
Since the
mechanism of
generation of the
magnetic field is
influenced by the
rotation the dipole is
mainly oriented
along the rotation
axis and people use
the magnetic pole as
past proxy for the
rotation axis
External Field
MAGNETOSPHERE
IONOSPHERE
Meloni, 1993
Interaction with Solar Wind
Meloni, 1993
PaleoMagnetic Field:
Magnetization of Rocks
DRM
Detrital
Remanent
Magnetization
TRM
Thermal
Remanent
Magnetization
Gesteinsmagnetisierung:
Curie Temperatur: etwa 580 Grad C für Magnetit
680 Grad C für Hämatit
Blocking Temperatur:
Typische Schmelztemperaturen liegen allerdings bei
1100 – 800 Grad C, also wesentlich höher.
Das heißt, Gesteine können eine Magnetisierung im
Umfeld annehmen, und diese bei Abkühlung unter die
Blocking-Temperatur auf geologische Zeiträume hinweg
behalten.
Wir unterscheiden:
Thermoremanente Magnetisierung: TRM
Depositionale Magnetisierung (in Sedimenten): DRM
Chemoremanente Magnetisierung: CRM
DRM entsteht durch die geordnete Ablagerung magnetischer
Minerale in Sedimentgesteinen zur Zeit der Deposition.
CRM entsteht durch das langsame Mineralwachstum nach der
Ablagerung oder Erstarrung.
A tape recorder
“An essay of GeoPoetry”
Submarine Lava flow at ridge
From
www.ridge2000.org/science/tcs/epr06activity.php
Dating the Magnetic Reversal