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-145
3. 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
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,

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 90o (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?
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)

21. 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

22. 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

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