Announcements & Reminders 1. ES 123 Essay: Due Friday November 24, 3:00 p.m. (B&GS 10) 2. Lab final: Tuesday December 5, 10:30 - 11:20 a.m. room NS-145 3. Course final: Saturday December 9, 2:00 - 4:00 p.m. NS-1 FORMAT: Multiple choice (45 questions)
Earth's magnetic field: Introduction
The magnetic compass The loadstone compass was discovered in China ca. 1000 AD Used as the basis for marine navigation Columbus found that the compass needle changes direction at different locations The mystery of the compass was solved by William Gilbert in 1600 http://www.phy6.org/earthmag/demagint.htm
William Gilbert and the Birth of Magnetism Personal Physician to Elizabeth I Published De Magnete in 1600, a 6- volume treatise documenting his experiments with magnetism Proposed that Earth behaves like a giant bar magnet Died in 1603 of bubonic plague William Gilbert, 1544-1603 http://www.phy6.org/earthmag/demagint.htm
Measuring Earth’s magnetic field Three measurements Inclination = angle measured from horizontal Declination = angle measured from geographic north Intensity = magnetic field strength (units: Teslas, nanoTeslas) http://earth.leeds.ac.uk/dynamicearth/plates_move/longterm/pmag2.htm
Temporal Variations in Earth’s magnetic field Secular Variations - Long-term (decades) changes caused by fluid motion in Earth's outer core. Diurnal Variations - Daily variations related to interaction of the geomagnetic field with solar wind. Magnetic Storms - Intense (1000 nT), irregular and unpredictable variations associated with solar flares. http://apollo.lsc.vsc.edu/classes/met130/notes/chapter2/aurora_magnetosphere.html
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Secular Variation in Earth's magnetic field since 1600 http://swdcwww.kugi.kyoto-u.ac.jp/igrf/
Migration of the magnetic north pole At the magnetic north pole, the inclination is 90o (straight down) It has been in the Canadian arctic for centuries, but is migrating towards Siberia at 40 km per year http://gsc.nrcan.gc.ca/geomag/nmp/northpole_e.php
The magnetosphere Solar wind = charged particles ejected from the Sun Magnetosphere = a zone of charged particles trapped by Earth’s magnetic field Magnetotail = portion of a magnetosphere that is pushed away from the sun by the solar wind http://apollo.lsc.vsc.edu/classes/met130/notes/chapter2/aurora_magnetosphere.html
The Van Allen radiation belts Two regions of charged particles trapped in magnetosphere Corkscrew-paths, bouncing between magnetic poles Outer belt is recharged by the solar wind Aurorae occur where outer belt intersects the lower atmosphere http://www.physics.upenn.edu/~thomsone/phys151/
Characteristics of Earth’s magnetic field Nearly dipolar Approximately aligned with Earth’s rotation axis Changes slowly with time Spontaneously reverses every ~ 200,000 years Is at least 3 Ga old
What causes Earth’s magnetic field? Self-exciting dynamo Elsasser, 1958 If it were not being continually generated, Earth’s field would decay in 20,000 years Solution: Self-exciting geodynamo Dynamo: A machine for converting mechanical energy to electrical current flow (and magnetic field)
What causes Earth’s magnetic field? Inner core It has long been speculated that the geodynamo is caused by convection in the outer core Convection is driven by: a) heat, and b) crystallization of the inner core Core-mantle boundary http://www.es.ucsc.edu/~glatz/geodynamo.html
Computer Model of the Geodynamo Inner core In 1995, the first computer model was published (Glatzmaier-Roberts model) which successfully simulated the geodynamo, including spontaneous reversal Since then, several other groups have also succeeded Core-mantle boundary http://www.es.ucsc.edu/~glatz/geodynamo.html
Glatzmaier-Roberts model Snapshot of magnetic field lines produced by G-R’s computer program http://www.es.ucsc.edu/~glatz/geodynamo.html