The Earth’s Magnetic Field!

1. The Earth’s Magnetic Field! Geomagnetic reversal

2. Geomagnetic Reversal • A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged. • Over very long periods, geomagnetic reversal seems to have occurred with a frequency of 1 to 5 times per million years • The last reversal happened approximately 780,000 years ago. • Earth's magnetic field is fading. Today it is about 10 percent weaker than it was when German mathematician Carl Friedrich Gauss started keeping tabs on it in 1845, scientists say • The earth's magnetic field has reversed approximately 170 times over the last 100 million years.

3. Causes of Reversals • Scientific opinion is divided on what causes geomagnetic reversals. • Many scientists believe that reversals are an inherent aspect of the dynamo theory of how the geomagnetic field is generated. • Dynamo theory describes the process through which motion of a conductive body in the presence of a magnetic field acts to regenerate that magnetic field. • In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core. • In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation.

5. Observing Past Fields • past record of geomagnetic reversals was first noticed by observing the magnetic stripe "anomalies" on the ocean floor • Since the sea floor spreads at an essentially constant rate, this results in broadly evident "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from simply towing a magnetometer along the sea floor.

6. This diagram shows just how the earths ocean floor conveniently records the direction of the earths polarity.

7. Old Age Dating • The previous method mentioned works great for detecting the polarity of earth up to about 180 million years ago • Most sedimentary rocks incorporate tiny amounts of iron rich minerals, whose orientation is influenced by the ambient magnetic field at the time at which they formed. • Under favorable conditions, it is thus possible to extract information of the variations in magnetic field from many kinds of sedimentary rocks. • However it is very difficult to find rocks that are in good enough condition to perform this procedure

8. History of Reversals • A long period of time during which there were no magnetic pole reversals, the Cretaceous Long Normal lasted from about 120 to 83 million years ago. • An interesting trend can be seen when looking at the frequency of magnetic reversals approaching and following the Cretaceous Long Normal. • The frequency steadily decreased prior to the period, reaching its low point (no reversals) during the period. • Following the Cretaceous Superchron the frequency of reversals slowly increased over the next 80 million years ago, to the present. • There has been two other long periods of time that a reversal did not take place

9. What's Happening Now? • At present, the overall geomagnetic field is becoming weaker at a rate which would, if it continues, cause the current field to temporarily collapse by 3000-4000 AD. • The Earth's magnetic north pole is drifting from northern Canada towards Siberia with a presently accelerating rate -- 10km per year at the beginning of the 20th century, up to 40km per year in 2003. • It is unknown if this drift will continue to accelerate.

10. Consequences • Some speculate that a greatly diminished magnetic field during a reversal period will expose the surface of the earth to a substantial and potentially damaging increase in cosmic radiation. • However, Homo erectus and their ancestors certainly survived many previous reversals. There is no uncontested evidence that a magnetic field reversal has ever caused any biological extinctions. • Although the inspection of past reversals does not indicate biological extinctions, present society with its reliance on electricity and electromagnetic effects (e.g. radio, satellite communications) may be vulnerable to technological disruptions in the event of a full field reversal.

11. Aurora • Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. • Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by the earth's magnetic field. • Indeed, satellites show aurora electrons to be guided by magnetic field lines, spiraling around them while moving earthwards.

14. Animals and Earth’s Magnetic Field • Many animals depend on the earths magnetic field for a sense of direction while migrating • Turtles, frogs, fruit fly’s, birds and everything in-between use the polarity for direction • Blind mole rats use the field to determine how deep they are under the ground, turtles do the same under water. • If the poles of the earth were to switch, what problems would these animals face? • One would think they would be able to adapt as they must have already survived 170 reversals just to be alive today.

15. The End