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The Earth’s Magnetic Field

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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The Earth’s Magnetic Field

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  1. The Earth’s Magnetic Field

  2. Source: NASA http://www.nasm.si.edu/research/ceps/etp/earth/img/dipole_earth.gif

  3. Source: NASA http://www.nasm.si.edu/research/ceps/etp/earth/img/magneto.gif

  4. spher. harmonic analyses

  5. magnetic reversals, secular variation

  6. 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)

  7. Magnetic Units: • Magnetic field – Tesla field mostly described in micro-Tesla or gamma (γ) 1γ = 10-9 T

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

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

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

  11. Mathematical description: • Spherical harmonic analyses

  12. Short-term changes: • Careful measurements since 1600 • Navigational purposes

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

  14. Paleomagnetism – history of the earth’s magnetic field • Earth is dipole field • Relationship between Inclination and latitude tan I = 2 tan λ

  15. 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

  16. Magnetic Polarity

  17. Duration of a Polarity Chron

  18. Generation of the Magnetic Field • No big fat bar magnet • Dynamic processes inside the outer core

  19. Early (Simplistic) Model

  20. Ultimate driver of magnetic field ?

  21. 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

  22. Source: NASA http://photojournal.jpl.nasa.gov/jpeg/PIA02008.jpg

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