A Brief Guide To Magnetism, Terrestrial Magnetic Field and Polar Reversals Jack Oughton
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Definitions Curie temperature (Tc) The temperature above which a ferromagnetic material loses its permanent magnetism. In minerals, lightning often flash-heats minerals above their Curie temperatures, effectively resetting the magnetic fields trapped in lava flows.
Dynamo- A system that uses electromagnetic induction to convert mechanical energy (motion) into a magnetic field
Ferromagnetism is believed to be caused by magnetic fields generated by the electrons' spins in combination with a mechanism known as exchange coupling, which aligns all the spins in each magnetic domain.
Magnetism in a nutshell
Ampere suggests in 1820 that magnetic properties of matter were due to tiny atomic currents â€˘The dynamo theory proposed by the German-born American physicist Walter M. Elsasser and the British geophysicist Edward Bullard during the mid-1900s All atoms exhibit magnetic effects
A magnet cannot have a single North pole or a single South pole, which would be called a â€˜monopoleâ€™
The medium in which charges are moving effects magnetic forces
All known magnetic fields are caused by the movement of electrical charges. Electrons in orbit in atoms give rise to magnetic fields, so that every atom is, like the Earth, surrounded by a magnetic field
Importance of the poles â€˘ if you break a bar magnet into two pieces, each piece will again have a North and South pole. If you take one of those pieces and break it again, each smaller piece will have a North and South pole. No matter how small the pieces of the magnet become, each piece must have two poles. â€˘ Without this, the field could not reverse
The Magnetic Field â€˘ A magnetic field is an area of magnetic activity â€˘ The magnetic field at any given point is specified by both a direction and a magnitude(or strength) â€˘ The earth has a magnetic field..
Our Magnetic Field / The Magnetosphere • The Earth's magnetic field near the surface is very similar to a field produced by a magnetic dipole (e.g. a bar magnet). • The best-fit dipole is located at the centre of the Earth pointing south and is not perfectly aligned with the south pole. • Magnetic north is not ‘true north’ • The Earth's magnetic field is created by Earth's partially ionized outer core, which rotates more rapidly than the Earth's surface.
• Ocean water generates electric currents as it moves in the main field, so that the ebb and flow of the tides have a slight magnetic effect. • As gauged by satellites, the main field is roughly 6,000 times stronger than the rock magnetism of the ocean floor, and 30,000 times greater than the influence of the oceanic tides. • Fun fact: it’s believed a "supergiant" asteroid several times larger than the one that likely killed the dinosaurs struck Mars with such force that it shut down the planet's magnetic field*
Cause of the Field – The Dynamo effect • The way earthquake waves spread indicates that the Earth has at its center a dense liquid core, of about 1/2 the radius of the Earth •Inside that, a solid inner core. •It is believed this core is made up of molten iron, perhaps mixed with nickel and sulfur.. •Iron concentrated in the Earth's core because it is heavy--the same reason that, when extracted from its ores, it sinks to the bottom of the blast furnace.
• Scientists aren’t clear about what provides the heat in the Earth's core. • It might come from some of the iron becoming solid and joining the inner core. • Or generated by radioactivity, like the heat of the Earth's crust • Dynamo effect is thought to generate the sun’s powerful magnetism. • The outer core also has "hurricanes"--whirlpools powered by the Coriolis forces of Earth's rotation. • These complex motions generate our planet's magnetism through the dynamo effect. • The source of the field, the outer core, is itself seething, swirling, turbulent. "It's chaotic down there,” • The changes we detect on our planet's surface are a sign of that inner chaos.
what does the magnetic field do ? • Without it, life would die. • It shields our planet against charged atomic particles coming from outer space, and the sun’s solar wind. • Protects our satellites and astronauts, depending on how high they are in orbit • Birds, fish and migratory animals rely on the steadiness of the magnetic field to navigate. •Dr. Kirschvink of the California Institute of Technology discovered such reliance in bees, pigeons, bacteria, salmon, whales and newts, among other animals. •The magnetic sense, he found, usually relies on tiny crystals of magnetite • The same mineral that gets immobilized in cooling lava.
Magnetic Polar Wandering •James Ross located the pole for the first time in 1831 after an exhausting arctic journey during which his ship got stuck in the ice for four years. •In 1904, Roald Amundsen found the pole again and discovered that it had moved at least 50 km since the days of Ross. •The North Magnetic Pole is currently located in northern Canada. It wanders in an elliptical path each day, and moves, on the average, more than forty meters northward each day. •Evidence indicates that the North Magnetic Pole has wandered over much of the Earth's surface in the 4.5 billion years since the Earth formed.
• In 2007 scientists confirmed that the pole is now moving toward Siberia at 34 to 37 miles (55 to 60 kilometers) a year. • It’s fluctuations are thought to be caused by instability in the core
How do we tell there have been shifts in the past?
•Many rocks have built-in magnetism that remembers the direction of the main magnetic field when they formed. •This is first discovered in the 1950s •Scientists towing magnetic sensors behind ships found that the rocky seabed exhibited odd stripes of magnetization. •Molten lava proved to hold tiny mineral grains that acted like compasses, freely aligning themselves with the current field. •As the lava cooled, the tiny compasses froze in place, immobile even if the field shifted.
• Experts called it paleomagnetism and found that the tiny compasses were often made of magnetite, a naturally magnetic mineral. • Paleomagnetic studies showed that the Earth's field reversed in a fairly random way and early patterns were more chaotic. • Eg. During the age of dinosaurs, no flips occurred for roughly 35 million years. • Probably due to early earth’s core being more unstable.
Paleoreversal / ‘flips’
• Average interval between flips is approximately 300,000 years • The last reversal was 78,00 years ago; the BrunhesMatuyama Reversal • In 1995 scientists at the Los Alamos National Laboratory made the first computer simulation of the geodynamo in action, including field reversals. • Dr. Glatzmaier, at Los Alamos, said it showed that the Earth's solid inner core resisted the flipping because the field there could not change as rapidly as it did in the fluid outer core. • "The reversal starts with a small region that gets larger… Most of the time they die away, but other times they continue to grow." • To date, the simulations of millions of years have produced more than a dozen flips. •
• “There have been effects on earth during solar max due to the influx of the sun on the power grid and the communications, and if we’re going to head toward a reversal, all of those problems are going to become more serious.” • Reversals take a few thousand years to complete, and during that time--contrary to popular belief--the magnetic field does not vanish. "It just gets more complicated”. • Magnetic lines of force near Earth's surface become twisted and tangled, and magnetic poles pop up in unaccustomed places. • A south magnetic pole might emerge over Africa, for instance, or a north pole over Tahiti • But it's still a planetary magnetic field, and it still protects us from space radiation and solar storms.
Measuring The Field’s Changes • The Earth’s magnetic field is generally becoming weaker. • a French-Danish team compared results from 2000 with those from an American satellite, Magsat, 20 years earlier • The decline in the field’s strength suggested that “it might disappear completely in a thousand years or so.” • Some of them have theorized this is the first stage of a flip • SWARM project* • The satellites will adopt orbits passing over the Earth’s poles. Swarm A and B will fly side by side, simultaneously measuring the magnetic field from positions up to 150 kilometres apart in the east-west direction near the equator.
â€˘ Swarm C will fly higher, remaining at more than 500 kilometres altitude throughout the mission. â€˘ Swarm C will give simultaneous snapshots of the magnetic field over quite different regions of the Earth, and impressions of the same region at different times of day.
The Reversal In Fiction • Like any disaster scenario, the field’s reversal has inspired a creative outpouring of human paranoia. Most is pseudoscience. Lots of ‘Prophecies’ • "The Core," a Hollywood film, gives a wildly exaggerated portrayal of what would happen if the field vanished. People with pacemakers fall dead. Pigeons fly into people and windows, the sun burns off portions of the sea.
The Orion Prophecy – an example • Author Patrick Geryl believes that in the year 2012 the world will suffer a major disaster that will wipe out billions. • “In my book I scientifically reveal the millennia-old codes of the Maya and the Old Egyptians, which refer to this super-disaster. ” • “Giant sun-flames will send a gigantic wave of particles to the earth. The particles that are spewed out will set the earth's atmosphere "in flames" and have a real destructive effect on the Van Allen belts. Because of the continuous stream of electromagnetism, the magnetic field of the earth will get overcharged. Trillions of particles will reach the poles.” • Not accepted by scientific community.
Summary – Realistic Outlook? • No correlations in the fossil record between flips and mass extinctions. • Homo erectus and our ancestors certainly survived many previous reversals. The thinking ape is certainly older than 78,000 years. • Evidence that field is retained during shift. The poles move but the protective effects are retained • BUT Instabilities could harm our sensitive electronic networks, especially in regions closer to poles eg. Canada • Shifts take thousands of years – we could be at the beginning of one now.
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Published on May 28, 2010
Jack Oughton • ® D ynamo- A system that uses electromagnetic induction to convert mechanical energy (motion) into a magnetic field F er r om...