Text of Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement...
Magnetism and Paleomagnetism Chapter outline Magnetic field and the dipole Magnetic measurement (washing) Magnetic remenance Magneto-stratigraphy
Earths PRIMARY magnetic field with solar wind blowing on it. The solar wind is high kinetic energy charged particles emitted from Sun. The solar wind deforms the earths primary magnetic field: note close field line spacing on Sun side and wide field lines on non-sun side of earth. What causes auroa-borealis ?
Relation between spin axis POLE that define true north and magnetic POLE that approximately defines the primary magnetic field dipole orientation If the magnetic and spin axis poles change, then WHERE is the real north pole ? Use the stars, whose motion with respect to our planet is too small to be measured, which can provide a reference frame. Note this is how we discover precession of the earths spin axis (2000 yrs ago!). Note: we can see black holes moving around the gallactic center. The earths primary magnetic field can be approximated as a big bar magnet. But, the big bar magnet is only a good metaphor. This dipole field approximation is very useful for predicted magnetic field near earths surface to facilitate the study of paleo-magnetism. WHY ? What does arrow direction manifest ?
Magnetic dipole (dipoles as a concept in general) A dipole has two parameters: Direction of the axis in 3-space (vector) and the polarity of the north/south pole. A scalar magnetic dipole strength in Amps/m*2. The Earths dipole is 10*22 Amp/m*2. Like electric charges, for magnetic fields, the same poles repulse and opposite poles attract. Note a compass is a magnetic dipole. Note the compass S-pole is attracted towards the N-pole and the compasses N-pole is attracted towards the S-pole.
All magnetic fields derive from moving electric charges (current) To make an electro-magnetic, wrap a wire around a magnetic conductor (nail) and hook up a battery to permit electrical current to flow. The direction of current flow give polarity of magnetic dipole. B field around a wire with current flow I.
So where is the moving electric charge to make magnetism? Two places: When charged particles move in a fluid (gas or liquid): e.g., the earths outer core or in gas nebula clouds in intra gallatic space or a current in a wire. An electron and proton have a magnetic dipole which is an intrinsic property required by quantum mechanics. In certain ferromagnetic substance, such as iron, the unpaired outer electrons in the high F orbitals do an extraordinary thing, they will all line up when the temperature (thermal agitation) is small enough (the curie temperature). Its called exchange interaction.
Earths Geodynamo that makes primary magnetic field Liquid iron in outer core can both conduct electricity AND convective flow! Thus it can create a spiraling flow (tangent yellow cylinder around inner core above) that produce a self-reinforcing dynamo that generates the earths primary magnetic field. When the flow reverse, the polarity of magnetic field reverses.
Geodynamos in other solar system planets? Mercury: Little magnetic dynamo, 1% earths field strength. Venus: Field at least 100,000 less than earths field. Why? The planet almost certainly has a liquid iron core like the earth. But, Venus only rotates once every 220 days. Mars: No primary field now, but evidence for magnetic remanence. Small planetary radius means the liquid iron core solidified in first Ga. Jupiter: largest dynamo of planets, 14 times stronger field than earth. Dynamo is core of liquid hydrogen. Saturn, Uranus, Neptune: all have magnetic dynamos and strong fields. Jupiter Aureo Borealis
History of magnetic force 700 BC Greeks found loadstone which is a highly magnetized rock (due to magnetite) 400 Chinese discovery that loadstone whittled into a needle points about north-south. 1175 Compass make it to Europe (Venice) and spawns the Age of Discovery. 1269 Peregrinus, a French Crusader, describes a floating compass and concept of poles. 1601 William Gilbert publishes De Magneta saying earth is like a huge bar magnet. Start of the scientific method with Francis Bacons publications. 1745 Leyden Jar is made that can store and discharge electricity. 1770 Ben Franklin does a lot of electrical experiments (e.g., the kite). 1800 Volta makes first battery: greatly increase amount of current available to experimenters. 1820 Oersted, by accident, finds that a changing electric field (current) deflects a compass. This provides the first link between electric and magnetic phenomena. 1882 Maxwell discovers theory of electromagnetism (light is just an EM-wave!!) 1905 Einsteins special relatively leads to understanding of magnetic field as relativistic effect of moving charge when speed of electromagnetic waves is finite (c).
What is a charge and its field? A charge is a quantity that is the source of a field that extends into space. For gravity, the charge is mass (kg) and for electromagnetism the charge is electric (coulombs). The field strength is proportional the amount of charge (kg or coulombs). The closer the field line are together; the stronger the field locally is. The field can perform the miracle of action at distance: i.e., apply a force and do work on another object proportional to the objects charge. It took physicists until 1890 or so to accept the concept that a force field that can do work without two object touching.
Compare field charges: mass, electric, magnetic? Gravity (mass) charge Only one sign: positive! Always attractive! Field is spherical symmetric and varies as: 1/r*2 Electric (Coulomb) charge Two signs: plus or minus. Same sign repulsive force; opposite sign attractive force. Field is spherical symmetric and varies as: 1/r*2 Magnetic charge NO SUCH THING!! All magnetism is relativistic effect of moving (accelerating) charge.
The Earths PRIMARY magnetic field interacts with rocks to provide a REMANENT magnetic field record. Provides a fossil compass record used to ascertain conditions of the formation of the rocks Can be used to track the movements of the rocks Can also be used to investigate the subsurface for mineral exploration Understanding its origin due to flow of conductive iron liquid in outer core is fundamental to understanding evolution of earths atmosphere. Earths Magnetic Field
Paleomagnetism utilizes the fossil magnetism preserved in rocks Can be used to measure the movements of the rocks Can be due to plate movements Can result from tectonic tilting Requires an understanding of how rocks acquire a remanent magnetization Requires access to the rocks Paleomagnetism & Rock Magnetism
Magnetic Field A magnet (dipole) produces a magnetic field The field lines map out the direction and magnitude of the force (torque) that a compass (a bar magnet which is a magnetic dipole).
Dipole Magnetic Field Where the field lines are dense (close), the magnetic field is strong MKS Units of a magnetic field is Tesla (T) On the surface of the Earth the magnetic field ranges from 60,000 nT at the pole to 30,000 nT at the equator Current flow through loop (b) makes magnetic field dipole. The bar magnetic is a form of fossil remanent magnetism where the current flow is derived from the electrons.
Magnetic Field A Magnetic field can be produced by a magnet or a current in a coil The Earths magnetic field is more complicated It is produced by electrical currents in the liquid outer core
Earths Magnetic Field Geodynamo Electrical currents produced by convective currents of convective fluids in the liquid outer core Not fully understood We will call it a magnetic dipole Means that the source volume is far from where we measure the field
The Earths magnetic field does not align with the Earths rotational axis Presently tilted 11.5 Magnetic North differs from geographic (true) N Termed declination Earths Magnetic Field
The Earths magnetic field lines intersect the surface of the Earth at an angle At the poles, it is nearly vertical At the equator, it is nearly horizontal Termed inclination Can be measured with a compass Positive when points down Negative when points up Earths Magnetic Field
Where the axis of the Earths magnetic field intersects the surface of the Earth is called the north and south magnetic poles Magnetic equator and magnetic latitude are similarly defined The Earths magnetic field is symmetric about the magnetic axis Earths Magnetic Field
The magnetic inclination and magnetic latitude are related by Earths Magnetic Field
From observatory records going back a few hundred years, we know that the magnetic axis continually changes direction Slow and somewhat irregular Called secular variation Earths Past Magnetic Field
From paleomagnetic (fossilized magnetic remanence) records in rocks, we find that the Earths magnetic axis wobbles about the rotational axis Completes a cycle in around a couple of thousand years Averaged over several thousand years, the Earths magnetic field is a geocentric, axial dipole Using average inclination to