APOD and General Astronomy Discussion Forum
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dougettinger wrote:Alternating polarity reversals of the magnetic dipole fields of Mars, Earth and to some extent on the Moon have occurred. Why ? Do the spinning solid cores inside a liquid core (existing or previously existing) actually stop spinning and reverse direction causing this global reversal of polarity ?
http://en.wikipedia.org/wiki/Geomagnetic_reversal wrote:
<<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. 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. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7–15 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength. Present computational methods have used very strong simplifications in order to produce models that run to acceptable time scales for research programs.
The rate of reversals in the Earth's magnetic field has varied widely over time. 72 million years ago (Ma), the field reversed 5 times in a million years. In a 4-million-year period centered on 54 Ma, there were 10 reversals; at around 42 Ma, 17 reversals took place in the span of 3 million years. In a period of 3 million years centering on 24 Ma, 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on 15 million years ago. These eras of frequent reversals have been counterbalanced by a few "superchrons" – long periods when no reversals took place.>>
Our planet's magnetic field is in a constant state of change, say researchers who are beginning to understand how it behaves and why.
...
To understand what's happening, we have to take a trip ... to the center of the Earth where the magnetic field is produced.
At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it "the inner core." It's really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2o of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core."
Earth's magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. 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 a process called the dynamo effect.
Using the equations of magnetohydrodynamics, a branch of physics dealing with conducting fluids and magnetic fields, Glatzmaier and colleague Paul Roberts have created a supercomputer model of Earth's interior. Their software heats the inner core, stirs the metallic ocean above it, then calculates the resulting magnetic field. They run their code for hundreds of thousands of simulated years and watch what happens.
What they see mimics the real Earth: The magnetic field waxes and wanes, poles drift and, occasionally, flip. Change is normal, they've learned. And no wonder. The source of the field, the outer core, is itself seething, swirling, turbulent. ... The changes we detect on our planet's surface are a sign of that inner chaos.
They've also learned what happens during a magnetic flip. 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. Weird. But it's still a planetary magnetic field, and it still protects us from space radiation and solar storms.
Simple Mechanism for Reversals of Earth’s Magnetic FieldAccording to a new model, small fluctuations in convective flow in Earth’s core can explain how the Earth’s magnetic field reverses.
We show that a model, recently used to describe all the dynamical regimes of the magnetic field generated by the dynamo effect in the VKS experiment [1], also provides a simple explanation of the reversals of Earth's magnetic field, despite strong differences between both systems.
Neufer, I had not realized that as many as 51 reversals have been catalogued for the Earth. In checking with Extinction Events, I found no correlations with the times for magnectic reversals. Obviously, geomagnetic reversals are not influenced by factors outside the Earth's surface.http://en.wikipedia.org/wiki/Geomagnetic_reversal wrote:
<<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. 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. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7–15 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength. Present computational methods have used very strong simplifications in order to produce models that run to acceptable time scales for research programs.
The rate of reversals in the Earth's magnetic field has varied widely over time. 72 million years ago (Ma), the field reversed 5 times in a million years. In a 4-million-year period centered on 54 Ma, there were 10 reversals; at around 42 Ma, 17 reversals took place in the span of 3 million years. In a period of 3 million years centering on 24 Ma, 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on 15 million years ago. These eras of frequent reversals have been counterbalanced by a few "superchrons" – long periods when no reversals took place.>>
http://www.nasa.gov/vision/earth/lookingatearth/29dec_magneticfield.html wrote:
Earth's Inconstant Magnetic Field NASA News - 2003 Dec 29
<< [Magnetic] 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.
It is all too complex to state that anything is "obvious" here.dougettinger wrote:Neufer, I had not realized that as many as 51 reversals have been cataloged for the Earth. In checking with Extinction Events, I found no correlations with the times for magnetic reversals. Obviously, geomagnetic reversals are not influenced by factors outside the Earth's surface.
http://en.wikipedia.org/wiki/Geomagnetic_reversal wrote:
<<Others believe that geomagnetic reversals are triggered by external events
which directly disrupt the flow in the Earth's core. Such processes may include
1) the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones,
2) the initiation of new mantle plumes from the core-mantle boundary, and
3) possibly mantle-core shear forces resulting from very large impact events.
Supporters of this theory hold that any of these events could lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field. Because the magnetic field is stable in either the present North-South orientation or a reversed orientation, they propose that when the field recovers from such a disruption it spontaneously chooses one state or the other, such that a recovery is seen as a reversal in about half of all cases.>>
neufer wrote:"External events" cover a lot of different situations:http://en.wikipedia.org/wiki/Geomagnetic_reversal wrote:
<<Others believe that geomagnetic reversals are triggered by external events
which directly disrupt the flow in the Earth's core. Such processes may include
1) the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones,
2) the initiation of new mantle plumes from the core-mantle boundary, >>
Dips in Earth's gravitational field tied to 'slab graveyards'
Scientists have unearthed a new explanation for several low-gravity spots detected around the world. They’re blaming the anomalies on vast “slab graveyards” that lie buried deep near the planet’s core.
When these slabs of rock were buried long ago, they released water that reduced the density of overlying rock, Caltech geophysicist Michael Gurnis and his colleagues reported online May 9 in Nature Geoscience. Low-density rock has less mass, and so less gravitational pull.
Scientists had previously noticed that gravity’s tug is smaller where tectonic plates, or large sections of Earth’s crust, once plunged below the surface, Gurnis says. The team’s new findings, he notes, provide insight into the causes of super-low values measured in four regions — in particular, areas south of Asia, along the coast of Antarctica south of New Zealand, in the northeastern Pacific and in the western Atlantic.
Beneath those four areas, seismic waves travel slower than normal at shallow depths —1,000 kilometers or less — but faster than normal deeper down, says Gurnis. Because sound waves travel faster through dense material (for example, water compared with air) the difference in speed with depth told the team that lighter, less dense material is lying over a layer of very dense material. Most likely, the top layer’s relative buoyancy stems from its water-rich composition, the researchers contend.
The water could have been injected in the mantle as a result of slow-motion tectonic collisions between lighter and heavier plates. As dense slabs slipped downward through the mantle, water in those rocks — as well as water in the seafloor sediments that had accumulated above those slabs — was released. “When that water was added to the surrounding rocks, it lowered their melting point and reduced their density,” buoying them upward, Gurnis explains.
The dehydrated remains of the dense tectonic plates now lie deep in the mantle, where they boost the speed of seismic waves, says Gurnis.
The notion that upwelling deep in the mantle could be linked to water carried downward in long-buried tectonic slabs is new, says Carolina Lithgow-Bertelloni, a geophysicist at University College London. A similar upwelling of water-rich material near subduction zones has long been associated with volcanism at Earth’s surface, she notes.
Previously, studies suggested that hot spots along the core-mantle boundary create buoyant plumes of rock that rise within Earth’s mantle (SN: 7/9/05, p. 24). In the new study, however, lowered density arises from presumed changes in the composition of rock, not its temperature.
http://www.sciencenews.org/view/generic/id/59045/title/Gravity_lows_mark_burial_sites_of_ancient_tectonic_plates_ wrote:
Dips in Earth's gravitational field tied to 'slab graveyards'
Science News; 05 June 2010; Vol.177 #12 (p. 13)
<<Scientists have unearthed a new explanation for several low-gravity spots detected around the world. They’re blaming the anomalies on vast “slab graveyards” that lie buried deep near the planet’s core. When these slabs of rock were buried long ago, they released water that reduced the density of overlying rock, Caltech geophysicist Michael Gurnis and his colleagues reported in Nature Geoscience. Low-density rock has less mass, and so less gravitational pull. The notion that upwelling deep in the mantle could be linked to water carried downward in long-buried tectonic slabs is new, says geophysicist Carolina Lithgow-Bertelloni. A similar upwelling of water-rich material near subduction zones has long been associated with volcanism at Earth’s surface, she notes.>>
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