Earth’s Magnetic Field
Earth’s Magnetic Field ! Description ! Geometry–! Inclination and declination ! Strength ! Dipole and non-dipole ! Variability ! Geocentric Axial Dipole hypothesis ! Rock Magnetism ! Physics of Magetization–! induced and remnant magnetization–! Magnetization hysteresis–! TRM, DRM, CRM, and RMs ! Determination of GMP, VGP, paleo-pole. paleo-latitudes, and APW ! Geophysical exploration: measuring magnetic fields ! Origin ! Energy sources ! MHD ! Geodynamo ! Reversals ! Measurements ! Flux-gate ! Proton precession ! SQUIDMagneticField ofthe Earthat magnetic dip poles !intensity is 70,000 nT !at magnetic equator !intensity is 25,000 nT!1 nT 1 gamma (cgs)!Note direction relative tosurface!
Earth's magnetic field!At any location the magnetic field is a vector with magnitude (intensity) direction!inclination angle relative to horizontal!declination angle btw magnetic N geographic N!magnetics ! B and H: Same except for constant in free space (B µoH), differ insidemagnetic material (B µoHex Jint). Both have been called “magneticfield”. We will call H the “Magnetizing Field” ! Total field vector B has vertical component Z and horizontal componentH. (not to be confused with the magnetizing field)–! Inclination, I tan-1 Z/H–! tan I 2 tan (latitude)–! Declination of H measured relative to true north ! Measured using–! Flux-gate (component) or proton precession (total field) - .1 nT–! SQUID (Superconducting Quantum Interference Device) all three components tovery high precision (aT – 10-18 T) refrigerator magnet 10-2 TEarth Magnetic Variability ! External (solar and magnetosphere)–! Magnetospheric substorms, daily solarvariation , solar storms, sun spot cycle–! 1 nT to 100 nT ! Internal (secular variation)–! Intensity (-5% in last 100 years)–! declination/inclination due to GMP movement ! Reversal Chronology
Robinson & Coruh!
Robinson & Coruh!
Origin of Earth’s Field ! Ideas that don’t work:–! Permanent Magnetization–! Residual Currents–! Thermoelectric effect–! Gyromagnetic effect–! Hall effect–! Induced from external sources–! More ! Only plausible remaining idea: DynamoDynamo Theory: The Equations ! Maxwell’s equations (E&M relationships) ! Ohm’s Law (relationship between current &applied field) ! Navier-Stokes (fluid flow equations) ! Gravitational equation ! Heat Productions and transport ! Equation of state ! Continuity equationMagnetic Induction Equation ! Maxwell’s equation plus Ohms law ! Kinematic approachTime dependenceDecayBuild up due to fluid motion
Rock Magnetism ! Requires “unpaired” electrons in material ! Paramagnetism and ferromagnetism ! Magnetic hysteresis–! Induced magnetism–! Remnant magnetismRemnant Magnetism ! Thermal ! DepositionalSecondary Remanent magnetism ! Isothermal remanent magnetism IRMStrong fieldeg.lightning strike ! Chemical remanent magnetism CRM (formation of newminerals) ! Viscous Remanent Magnetism VRM expose a rock toa field for a long time ! VRM adds noise to other magnetic signals in rocks, but can beremoved by exposing the rock to an AC field of increasing strength.
Researchers looked at older samples for next 50 years!!frequently found reverse magnetization!!striking absence of 'in-between' orientations!!huge majority were either normal or reversed!Magnetization of Ocean Crust!Historical development!Remanent magnetization of igneous rocks was studied in early 1900's!!Brunhes, Matuyama, others found weak but stable remanence!!concluded this was locked in when sample was last heated!!! thermoremanent magnetization!!!occurs at unique Curie temperature! 100 dates/polarities from terrestrial!volc rocks around world identified!4 major epochs 3 shorter 'events'!Robinson & Coruh!!most studies were restricted to late Pleistocene/archeological samples!!(Cox et al, 1967)Soon after, symmetric mag anomalies were detected on either side of Mid-Atl Ridge!!obviously non-random controlling process!!Hess ('62) put it together in suggesting crust forms by solidifying at MOR!Though not understood, studies extended to magnetic reversals in marine sediments!Coring in 1950's focussed on deep sea sediments for many reasons!Same terrestrial reversal time scale detected in deep sea sediments!!" thermoremanence!! physical alignment of detrial grains having remanent magnetism!!called 'detrital remnant magnetization'!Measured with 'spinner magnetometer'!!oriented sample placed on rotating spindle!!external field is masked!!permanent mag of spinning sample !!!creates current in surrounding coil!!strength of current coil/sample !!!orientation reveal original field!Symmetric magnetic lineations across the Mid-Atlantic Ridge caused by alternating total field!!(Cox et al, 1967)
Magnetization of ocean floor basalts ! ocean floor basalts retain a high TRM ! large quantity of magnetite grains and small grain sizepromote high TRM ! ocean floor basalts are produced by continuous mid-oceanridge volcanism ! basalt crystallizes fast,forms Layer 2, with ahigh TRM ! strips of Layer 2 withalternating normal andreversed polarity form ! progressively age awayfrom the spreading centre.Magnetization of ocean floor basalts ! in strips that formed during normal polarity, the remnantmagnetization adds to the induced field ! this produces a positive magnetic anomaly ! in strips that formed during reversed polarity, the remnantmagnetization subtracts from the induced field ! this produces a negative magnetic anomaly ! when the anomalies arecontoured, the effect is toproduce stripes ofalternating high (positive)and low (negative)magnetic intensity ! the ocean floor "magneticstripes".Ocean crust as a tape recorder of thermoremanent magnetization!!. vertical reversal time scale is laid out horizontally!!(Cox et al, 1967)(Vine, 1966)!
Magnetization of ocean floor basaltsSoon the reversal time scale was extended to 11 Ma!!!!e.g., the East Pacific Rise out to 500 km, 11 Ma! ! shape of magnetic anomalies best expressed on a linegraph:(Vine, 1966)!Ocean Floor mapObserved modeled anomalies!!EPR, Juan de Fuca model – good match !! !100km 4Ma !! !25km/Ma!! !25*105cm/106yr!! !2.5cm/yr!!note the 8 km source layer!!500 Gammas 500 nT!(Vine and Wilson, 1965)! ! oldest oceanfloor ! fast versusslow sea-floorspreading ! subductedmid-oceanridges ! Iceland ! back arcareas young ! former ocean ridges (Labrador Sea, NE Indian Ocean).
!Determination of GMP, VGP, paleo-pole. paleo-latitudes, and APW !Geophysical exploration: measuring magnetic fields Magnetic Field of the Earth at magnetic dip poles ! intensity is 70,000 nT ! at magnetic equator ! intensity is 25,000 nT!
Experiment 18: Earth’s Magnetic Field 99 A typical compass is constrained to 2 dimensions and rotates to point to Earth’s magnetic south pole, which is (approximately) geographic north. Earth’s magnetic field, however, is a 3 dimensional phenomenon. It has components that point into and out of the earth, not just along the surface.
In magnetic particle testing, we are concerned only with ferromagnetic materials. 3.2.2.4 Circular Magnetic Field. A circular magnetic field is a magnetic field surrounding the flow of the electric current. For magnetic particle testing, this refers to current flow in a central conductor or the part itself. 3.2.2.5 Longitudinal Magnetic Field.
3. Magnetic Field Lines and Magnetic Flux - The field lines point in the same direction as a compass (from N toward S). - Magnetic field lines are not “lines of force”. - Magnetic field lines have
Dec 06, 2013 · the Earth’s magnetic field are, respectively: -38056 732 nT, -28024 521 nT, 26700 607 nT. The horizontal component of the Earth’s magnetic field was determined to be 38708 604 nT, and finally the total magnetic field of the
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0.8 M Earth 1 M Earth 0.1 M Earth Planet Radius (R Earth): 0.95 R Earth 1 R Earth 0.5 R Earth Distance from Sun (DEarth): 0.7 D Earth 1 D Earth 1.5 D Earth Average Surface Temperature: 456 oC 10 oC -95 oC Atmosphere: T
Figure 1: In a record head, magnetic flux flows through low-reluctance pathway in the tape's magnetic coating layer. Magnetic tape Magnetic tape used for audio recording consists of a plastic ribbon onto which a layer of magnetic material is glued. The magnetic coating consists most commonly of a layer of finely ground iron
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