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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.

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Presentation on theme: "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."— Presentation transcript:

1 Announcements & Reminders 1. ES 123 Essay: Due Friday November 24, 3:00 p.m. (B&GS 10) 2. Lab final: Tuesday December 5, 10: :20 a.m. room NS Course final: Saturday December 9, 2:00 - 4:00 p.m. NS-1 FORMAT: Multiple choice (45 questions)

2 Earth's magnetic field: Introduction

3 The magnetic compass ! The loadstone compass was discovered in China ca 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

4 William Gilbert and the Birth of Magnetism William Gilbert, ! 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

5 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)

6 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.

7 Secular Variation in Earth's magnetic field since 1600

8 Secular Variation in Earth's magnetic field since 1600

9 Secular Variation in Earth's magnetic field since 1600

10 Secular Variation in Earth's magnetic field since 1600

11 Secular Variation in Earth's magnetic field since 1600

12 Secular Variation in Earth's magnetic field since 1600

13 Secular Variation in Earth's magnetic field since 1600

14 Secular Variation in Earth's magnetic field since 1600

15 Secular Variation in Earth's magnetic field since 1600

16 Migration of the magnetic north pole ! At the magnetic north pole, the inclination is 90 o (straight down) ! It has been in the Canadian arctic for centuries, but is migrating towards Siberia at 40 km per year

17 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

18 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

19 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

20 What causes Earth’s magnetic field? ! 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) Self-exciting dynamo Elsasser, 1958

21 What causes Earth’s magnetic field? Core-mantle boundary 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

22 Computer Model of the Geodynamo Core-mantle boundary 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

23 Glatzmaier-Roberts model ! Snapshot of magnetic field lines produced by G-R’s computer program


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