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Introduction to Earth System

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:

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Introduction to Earth System

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  1. Introduction to Earth System Solid Earth part Rocco Malservisi roccom@lmu.de Phone: 2180 4201

  2. 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)

  3. The Earth’s Magnetic Field B = (X, Y, Z) Or B = (F, I, D ) OrB = (D, H, Z) F: intensity I: inclination D: declination H: Horizontal component The seven elements of the (local) magnetic fieldin the geographic coordinate system I. Geomagnetic field – Local Geomagnetic Field Vector

  4. 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

  5. The place where the axis of the dipole intersect the surface Of the earth are called geomagnetic poles

  6. Magnetic Observatory http://www.ngdc.noaa.gov/seg/geomag/icons/Obs1999_lg.gif FUR http://www.geophysik.uni-muenchen.de/observatory/geomagnetism

  7. 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:

  8. 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

  9. 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

  10. 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.

  11. 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

  12. Geomagnetic Field Intensity Other units: Gauss=100000nT g=10000nT I. Geomagnetic field – Worldwide Variation of F

  13. Geomagnetic inclination (IGRF) I. Geomagnetic field – Worldwide Variation of I

  14. Worldwide Distribution of Geomagnetic Declination according to IGRF 2000 I. Geomagnetic field – Worldwide Variation of D

  15. 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?

  16. Temporal (diurnal and secular) variations 10 nT / hour other secular variation: reversal From Butler, Palaeomagnetism, 1992. Magnetic storm Slide I. Geomagnetic field – Temporal Variations Compare:X = 3 nT/km * 33 km/h => 100 nT/hY = 0.3 nT/km * 33 km/h => 10 nT/h,Thus, diurnal changes matter !

  17. other secular variation: reversal From Butler, Palaeomagnetism, 1992. Slide I. Geomagnetic field – Temporal Variations

  18. Where the magnetic field came from? 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

  19. Where the magnetic field came from? Self Sustaining dynamo

  20. Where the magnetic field came from? From Fowler, 2004

  21. 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.

  22. 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

  23. External Field MAGNETOSPHERE IONOSPHERE Meloni, 1993

  24. Interaction with Solar Wind Meloni, 1993

  25. PaleoMagnetic Field:Magnetization of Rocks DRM Detrital Remanent Magnetization TRM Thermal Remanent Magnetization

  26. 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.

  27. 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.

  28. A tape recorder “An essay of GeoPoetry” Submarine Lava flow at ridge From www.ridge2000.org/science/tcs/epr06activity.php

  29. Dating the Magnetic Reversal

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