Astronomy 311: Lecture 3 - Geomagnetism• Magnetism is a very important force in physics: one half of electromag-netism.• Originally discovered in rocks called ”lodestone” (a type of iron), one hastwo ”poles” in magnetism, a north and south pole. Historically, t he righttype of rocks were found to point toward the North Pole.• Iron filings sprinkled around a simple bar magnet fall into a well definedshapes following the magnetic field.• A field in physics is a model to help scientists think about how to deal with”action at a distance”.• In magnetism, the field lines are directed from north to south mag neticpole.• Like poles repel and unlike p oles attra ct.• Some objects are permanently magnetics whilst others retain their mag-netism for a short time.• A f undamental observation is that wires carrying currents have a magneticfield.• Further, motion in a magnetic field produces an electrical current (genera-tor) and a n electrical current in a magnetic field produces motion (electricmotor).• In fact electromagnetism is a fundamental physics force: light is an electro-magnetic wave.• So perhaps at formation there were some lumps of rock that had magnetism.In contracting, the proto-Earth started to spin and maybe some of its in-ner parts were already molten and had some charged particles. The initialrotation in the Earth’s initial magnetic field led to some current. This cur-rent in a mag netic field leads to motion and so on. This is a self-sustainingdynamo.• Based on the Earth’s size, if this ”self-sustaining” feature was not present,any initial magnetism wo uld have died away in only 20,00 0 years.• Magma coming thro ugh volcanos, after its solidifes retains the polarizationof the magnetic field when it started to cool.1• This paleomagnetic record indicates the Earth’s magnetic field has existedfor 3 × 109years at least. Thus there has to be some renewal process. Thetemperature in the core (say 6000 K) is too hot to retain a permanent mag-net without renewal. Heating a permanent magnet destroys its magnetism(Curie temperature).• Solid inner core about the size of the moon, temp 6000K, consisting ofmainly Fe.• Convection is a very efficient way of transporting heat.• Initially when you heat a gas or a liquid, the heat travels through by con-duction or even radiation. But if sufficient heat is input, the temperaturegradient gets very steep and the only way heat can be transported fastenough is convection - the fluid boils over. Steep temperature gra dients instars/planets lead to convection.• The temperature gra dient in the Earth’s outer core is too steep to satsifythe ”Shcwarzschild criterion”: its convectively unstable.• Also, as the core cools, iron in the fluid outer core solidifies onto the innercore and leaves behing lighter elements which rise up - convection again.• Additional heating due to radioa ctive decay by potassium-40.• Consider children sitting around a roundabout with the roundabout in mo-tion. They roll a ball to each other. To the children, as they roll the ballto someone, it appears to travel in a curved path due to the motion of theroundabout. To someone outside, the ball moves in a straightline.• Thus people in a ”rotating frame of reference” postulate this curved parthas due to a fo r ce, a fictitous force called the Coriolis force. For example,as air moves from high to low pressure in the Northern hemisphere, it isdeflected to the right by the Coriolis force, the result of the Earth’s rotation.In the Southern hemisphere, air is deflected to the left. Someone not in theEarth’s rotating frame of reference would see the air moving in a straightline.• Thus in the outer core, light fluid does not rise straight up and heavy liquiddoes not sink straight down but follows curved trajectories.• This fluid is electrically conducting. Its curved path in the ambient existingmagnetic field produces an electromotive force by Faraday’s law of inductionwhich drives large electric currents.• These electric currents produce new magnetic fields through Amperes lawthat compensate for the decay of ambient magnetic field (Lenz’s law).2• The detailed analysis of this problem is a complicated problem in 3D mag-netodydrodnamics.• Simulations show that a dipolar magnetic field like the Earth;s can be main-tained by convection driven by the Earth’s interior reservoir of heat energy.• Inside the core, the field is very complicated but outside its very much likea bar magnet field ie dipole dominated. Outside the core the field is muchweaker.• These models find that the core rotates slightly faster than the mantle andthe Earth’s crust: perhaps by as much as 2-3 degrees of longitude per year.• This could be how the drift in the precise location of the Earth’s magneticNorth Pole comes about.• After a period of some 36,000 years, the model underwent a flip in polarity.• The inner and outer core have different polarities and the outer core isalways trying to inva de the inner core and change its polarity. More oftenthan not it can change a small part of the inner core’s polarity before thecore re-establishes itself with the existing polarity. However, sometimeswhen the inner core field is low, the outer core field ”invades” the innercore and flips it causing a general polarity reversal.• If you have a uniform heat flux at the core-mantle boundary, then the flipshappen about every 100000-3 00000 years apart taking about 1-2000 yearsto occur.• Evidence of this magnetic pole reversal is in the geologic record: magmaretains the polarity of the prevalent magnetic field when it cooled.• Some rocks in Oregon have suggested that at times 16 × 106years ago, themagnetic polarity was moving by about 6 degrees/day.• The paleomagnetic record proves such reversals happened many times overthe past billion years.• Some reversals occured within a few 10,000 years whilst at other timesthere are no reversals for millions of years. The last reversal happendapproxiametly seven hundred thousand years ag o.• Currently there is evidence that the strength of the Earth’s magnetic fieldhas declined by 1 0% in say a few hundred years. This is a much greater de-cline than would be expected if the dynamo inside the Earth’s core stoppedwor king.3• For example, the paper, ”Paleomagnetic Study of a Reversal of the Earth’sMagnetic Field” by Dunn J. R., Fuller M., Ito H., and Schmidt