The Earth’s Magnetic Field

The Earth’s Magnetic Field

Source: NASAhttp://www.nasm.si.edu/research/ceps/etp/earth/img/dipole_earth.gif

Source: NASAhttp://www.nasm.si.edu/research/ceps/etp/earth/img/magneto.gif

spher. harmonic analyses

magnetic reversals, secular variation

Earth’s Magnetic Field

• Mostly a dipole field (modern and paleomagnetic evidence)

• Can change direction (polarity)

• Secular variation (small-scale changes in direction and magnitude)

Magnetic Units:

• Magnetic field – Tesla

field mostly described in micro-Tesla

or

gamma (γ) 1γ = 10-9 T

http://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm

http://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm

http://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm

Mathematical description:

• Spherical harmonic analyses

Short-term changes:

• Careful measurements since 1600

• Navigational purposes

http://gsc.nrcan.gc.ca/geomag/nmp/early_nmp_e.php

Paleomagnetism – history of the earth’s magnetic field• Earth is dipole field

– Relationship between Inclination and latitude

tan I = 2 tan λ

Identification of magnetic minerals in rocks

Rocks are assemblages of diamagnetic,

paramagnetic, and ferrimagnetic minerals.

Concentration of ferrimagnetic minerals is

usually very small (< 1 %)Microphotograph of a thin section of norite

dolomite

limestone

sandstone shale

Sedimentary rocks

Volcanic rocks

granitegabbro

basalt1

10-1

10-2

10-3

10-4

10-5Mag

neti

c su

scep

tibi

lity

, κ (

SI

unit

s)

Median values and ranges of the magnetic susceptibility in common rock types

Modified from Lowrie, 1997

Magnetic Polarity

Duration of a Polarity Chron

Generation of the Magnetic Field

• No big fat bar magnet

• Dynamic processes inside the outer core

Early (Simplistic) Model

Ultimate driver of magnetic field ?

Magnetic Field of Other Planets

• Mercury – yes, but weak• Venus – no, or very weak• Mars – no clear evidence for field, but

might have had field in the past• Jupiter – yes, very strong (20,000 times

earth)• Saturn – yes 500 times earth• Uranus – yes, strong tilt with respect to

rotational axis

Source: NASAhttp://photojournal.jpl.nasa.gov/jpeg/PIA02008.jpg