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The Earth’s Magnetic Field. Source: NASA http://www.nasm.si.edu/research/ceps/etp/earth/img/dipole_earth.gif. Source: NASA http://www.nasm.si.edu/research/ceps/etp/earth/img/magneto.gif. spher. harmonic analyses. magnetic reversals, secular variation. Earth’s Magnetic Field.
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Source: NASA http://www.nasm.si.edu/research/ceps/etp/earth/img/dipole_earth.gif
Source: NASA http://www.nasm.si.edu/research/ceps/etp/earth/img/magneto.gif
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.htmhttp://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm
http://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htmhttp://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm
http://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htmhttp://geophysics.ou.edu/solid_earth/notes/mag_earth/earth.htm
Mathematical description: • Spherical harmonic analyses
Short-term changes: • Careful measurements since 1600 • Navigational purposes
Paleomagnetism – history of the earth’s magnetic field • Earth is dipole field • Relationship between Inclination and latitude tan I = 2 tan λ
1 basalt gabbro Sedimentary rocks granite 10-1 sandstone shale 10-2 Magnetic susceptibility, κ (SI units) limestone dolomite Volcanic rocks 10-3 10-4 10-5 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 Median values and ranges of the magnetic susceptibility in common rock types Modified from Lowrie, 1997
Generation of the Magnetic Field • No big fat bar magnet • Dynamic processes inside the outer core
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: NASA http://photojournal.jpl.nasa.gov/jpeg/PIA02008.jpg