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Lecture 16

Earth Sciences 2123A/B Lecture 16: Lecture 16- Earth Magnetic

7 Pages

Earth Sciences
Course Code
Earth Sciences 2123A/B
Catherine Neish

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Lecture 16 – Earth’s Magnetic field History of Geomagnetism  Lodestone – mineral that is natural magnet – ex: magnetite o Early Chinese compass – spoon points south in 1000 AD o Compass used for geomancy for a long time before it was used for navigation  Study of geomagnetism has a longer history than other branches of geophysics partly due to its uses in navigation th o Used for marine navigation by 11 Chinese ships used floating compasses o Columbus used one – notice North Star was not exactly over True North o Original thought that magnetic field was extra-terrestrial origin but William Gilbert in 1600 solved mystery of the compass  Geomagnetism was discovered by William Gilbert and it was used for navigation o Personal physician to Elizabeth I o He published De Magnete in 1600 – a 6 volume treatises documenting his experiments with magnetism o Proposed that Earth behaves like a giant bar magnet with the magnetic axis approximately North-South o Died in 1603 of bubonic plague o Earth behaves like a giant bar magnet with magnetic axis approximately North -South o Gauss – field generated inside the Earth  Magnetic Dipole – pair of magnetic pole of equal strength but opposite sign o No evidence for magnetic monopole o Magnetic field lines of force – shows the orientation of a small bar magnet paced at that location in space  To a first approximation geomagnetic field is like the field of a dipole (bar magnet) with geomagnetic axis titled about 11.5 degree from the geographic axis Measuring Earth’s Magnetic field generated?  A magnetic need that is free to swing under the influence of a magnetic force will align itself parallel to the local magnetic field line  Not permanent bar magnetic because of high temperature destroys magnetism – it exceeds Curie temperature  Magnetic mineral – iron (Fe) has a Curie temperature of 770  Its more than 3000 in the outer core o Inclination (i) – angle measured from horizontal angle at which point into or away from earth  Dip angle o Declination (D) – angle measured from geographic north o Intensity – magnetic field strength (Units Nano Tesalas)  The geomagnetic field cannot be generated by a permanent bar magnet at its center because high temperature destroys magnetism  When ferromagnetic materials exceed a certain temperature – known as Curie point – they lose their permanent magnetism Maxwell’s Equation  Changing magnetic field – it creates an electric field  Moving electric charges create a magnetic field o Only when the charges are moving What causes Earth’s magnetic field  If it were not continually generated – earth’s field would decay in 20,000 years  Dynamo – a machine for converting mechanical energy to electrical current flow  Self-exciting dynamo – the earth converts mechanical energy to electrical current flow and magnetic field  Requires movement of electrically conductive liquid Fe in the core  Geomagnetic field is a dynamic field – unlike a bar magnet o Sourced with Earth’s outer core 1. Small initial magnetic field is presented 2. Disk begins to rotate- moving an electrical conductor through a magnetic field causes electrons to flow 3. Electrons flow through a coil of a wire that produces a magnetic field in the same direction as the original field 4. Magnetic field is enhanced Thermal convection in the outer core  Heat release inner to the outer core o Freezing of the inner core o Radioactive heating may also be contributing Compositional convection in the outer core  Compositional Convection – inner core is made of solid Fe and is growing due to freezing of the liquid outer core  Pure Fe freezing out to grow the inner core while the lighter elements remain in the outer core  Due to their low density, they rise in the outer core creating convection cells  Dominant cause for convection Summary  Geomagnetic field is a dynamic field unlike a bar magnet – which is static/ permanent  Source of the field is in the Earth’s outer core – motion metal o 90% Fe Ni with 10% Si, O generates field is process called Geodynamo  Cannot measure the geodynamo can only model – using magnetohydrodynamic equations and measure its effect on and above the Earth Lecture 2 Computer Models  In 1995 the first computer model was published (the Glatzmaier Roberts Model) which successfully simulated the geodynamo including spontaneous reversal  Snapshot of magnetic field lines produced by G-R’s computer program  Their model gained a lot of credibility since it was the first model to show a Polarity Reversal Earth’s field is the sum of 3 parts 1. Main Magnetic Field – 99.2% 2. External Magnetic Field – sun - <0.6% 3. Induced Magnetic Field - <0.2% External Magnetic Field  Like the Earth, the sun has a magnetic field generated through internal convection  The 11-year sunspot cycle is due to the periodic magnetic polarity reversals whose effects can influence Earth’s climate o Solar wind – charged particles ejected from the sun o Magnetosphere – zone where changed particles are deflected by Earth’s magnetic field o Magnetotail- portion of magnetosphere that is pushed away from the sun by the solar wind  It can affect electronics etc. – astronauts almost Van Allen Belt  Van Allen radiation Belts discovered in 1958 – the first American spacecraft  Main discovery was that there are 2 radiation belts that can destroy spacecraft and harm people- so these orbits must be avoid Aurorae  It also produces the Northern and Southern Lights  Electrons hit air molecules and are excited as the reach earth o The molecules give off light as they calm down o An oxygen rich atmosphere will be green o Nitrogen would have excited atmosphere would be blue or purplish  Saturn also has Aurora because it had a magnetic field – like Jupiter Jupiter and Saturn  Jupiter and Saturn have magnetic fields  The interior of Jupiter and Saturn are most molecular hydrogen and helium  At some depth, under immense atmospheres becomes metallic and conducts electricity  Their magnetic fields are powered by convection generated from internal heat and aligned with the rotation axis Mars and Venus  Neither Mars nor Venus have substantial magnetic fields  This is puzzling since they are like Earth in many ways except neither planet exhibits plate tectonics  Mars is smaller than Earth, so it lost its internal heat more quickly, solidifying any initially liquid core  Venus may lack convection in the mantle – it has no mechanism to dissipate the heat Review Question  Convection is likely happening in the mantle and outer core of the earth Earth’s Magnetic Field Changes Over Time How do we measure changes in the geomagnetic /field?  Since the 1960s the earth magnetic field had been observed intermittently by Satellites (300-
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