The Magnetic Field

LATE CRETACEOUS
Helen
Your assetation that the origin of the Earth's magnetic field is unknown was true when you and I were in grade school. However, science has demonstrated how it occurs and it is widely accepted among scientists.

Have you ever heard of a little publication called Scientific American? http://www.sciam.com/askexpert/geology/geology9/geology9.html

Jupiter's magnetic field (caused by flowing liquid metallic hydrogen) is so huge it even protects it's moons (Serious scientists even entertain the notion of Europa harboring life).

The following brief excerpts from NOAA's "spaceweather.com" website reveal some of the story:

AURORA BLAST: A fast-moving coronal mass ejection that billowed away from the Sun on Nov. 4th 2001 swept past our planet at 0150 UT on Nov 6th (8:50 p.m. EST on Nov 5th). The impact triggered a severe geomagnetic storm -- now subsiding -- and widespread auroras reaching as far south as Alabama and Texas in the United States.

* * *

RADIATION STORM: Our planet remains inside a stream of energetic solar protons accelerated by Sunday's X1-class solar flare and coronal mass ejection (CME). The ongoing radiation storm reached severe (S4) levels on Monday, but is subsiding now that the CME passed our planet.


JOHN2001
As to some of the comments regarding the Earth's magnetic field and the mechanism which generates it, I would point readers to: http://www.igpp.lanl.gov/Geodynamo.html

Recent computer modeling suggests the Earth's magnetic field is generated by convection currents within the Earth's fluid outer core (rather than the earth's rotation, as suggested by some posters) is the source of the Earth's magnetic field.


LATE CRETACEOUS
The Earth's rotation influences the fluid dymanics of liquid metals (mostly iron) in the outer core. If the Earth did not rotate the liquid would flow in a convection current. Rotation sets up a symmetry, roughtly aligned with the axis of rotation. This produces a spiralling flow orientated parallel to the axis. So, yes it is the flow of liquid metal which generates electricity and therefore a magnetic field (flowing electricity always has a magnetic field), but it is the rotation of the planet that creates the pattern and sets up planet wide polarity. Without rotation the magnetic field would not be so polarized. Get it?


JOHN2001
Of course the rotation of the earth is *necessary* for the particular convection effects, but it is not a *sufficient* condition for the generation of the field.

To say that A causes B implies that you are stating a necessary and sufficient condition, which, in this case, you are not.


JOE MEERT
Indeed! The generation, sustenance and reversal of the dipole field is one of the great mysteries of geoscience. There are a number of models for the generation of the main field (dynamos), but all of them are incomplete. If you want to look at a fairly up-to-date review of the many models, I suggest "The Magnetic field of the Earth" by Merrill, McElhinny and McFadden (1998). It gives a good overview of the many models. I can't find the reference right now, but there are a number of time-scaling problems with the recent supercomputer models. The importance of convection in the outer core and differential rotation of the inner/outer core for generating the main field are not well constrained.


HELEN
Have you ever heard of a little publication called Scientific American? http://www.sciam.com/askexpert/geology/geology9/geology9.html[/b]

Yes, LC, I've heard of it....vaguely....once or twice in my life..... I'm SURE I have....I even have a subscription somewhere around here….

The dynamo model is presumed. It is not KNOWN. There are some interesting models concerning it and the possible reasons for field reversal. But whether or not it is widely accepted is simply not the point. Truth is not truth because the majority votes that way.

The dynamo model may be correct. I am not arguing that. What I am saying is that at this point it is simply not a known thing. That's not bad! It's only bad if someone states it IS known.

It's a model that is being worked with. That's good, too.

But not knowing something is not a slam against any kind of science. It simply means we are finite human beings and still discovering things.

On the other hand, I could have saved all these words and simply pointed out the posts above!

 

[Administrator: the following is from another thread where it sidetracked into this material which is related to this thread.]

THOMAS CASSIDY
From 20 years experience as Nuclear Operations Supervisor at one of the largest Nuclear Generating stations in the world.

Although the magnetic field of an electrical generating plant is a bit different from the magnetic field of the earth, I did a bit of study and found the following in several texts on the subject, including a layman's explanation in the EB.

Observations of the magnetic field of the Earth indicate that more than 90 percent of this field arises from sources internal to the planet. A variety of mechanisms for generating this field have been proposed, but at present only the geomagnetic dynamo is seriously considered. In the dynamo mechanism, fluid motion in the core moves conducting material across an existing magnetic field and creates an electric current. This current produces a magnetic field that also interacts with the fluid motion to create a secondary magnetic field with the same orientation as the original field. The two fields together are stronger than the original. The additional energy in the amplified field comes at the expense of a decrease in energy in the fluid motion.

Several conditions must be satisfied for the fluid motion to produce a magnetic field. First, the fluid must be electrically conducting. Second, a magnetic field must be present, possibly as a relic of the initial formation of the body. Third, some force must introduce twists into the fluid motion so that the initial magnetic field becomes distorted by the motion. For the Earth, liquid iron is conducting, an initial magnetic field is likely, and the Coriolis force introduces twists.

The simplest possible poloidal magnetic field is dipolar. Such a field could be produced by a single loop of electric current circulating around the Earth's rotation axis in the equatorial plane. The slight electric resistance of the conducting Earth, however, would long ago have dissipated this current if it were not continuously regenerated.
This generation process is complex and depends on both radial motion and rotation of the fluid core.


JOE MEERT
[Earlier] You mentioned something about the magma/crust.
The differential rotation that is most significant in this discussion is the one between the inner and outer core rather than the outer core and magma/crust. Although we do not precisely know how the earth's magnetic field is generated, the dynamo theory is the most popular explanation. the total earth's magnetic field is made up of both internal and external forces. The dipole portion of the internal field makes up the majority, but not the entire field.
The higher order fields (especially quadrupolar and octupolar) make up a significant percentage of the total internal field. I didn't quite get what you were getting at with your reference to the magma/crust. I also failed to comprehend your (seeming) suggestion that the earth's field has not reversed itself.


THOMAS CASSIDY
Joe, sorry, but I assumed you, being a geologist, would understand the dynamics of fluid movements in the core.

Thermal heating in the core is the process that drives fluid motion. For many years it was thought that this heating was caused by radioactive elements dissolved in the liquid core. Recent work suggests that freezing of the liquid core is more important. Seismic studies have shown that the center of the Earth is a solid sphere of iron with an approximate radius of 750 miles. This sphere is surrounded by an outer core of liquid iron. With time, the inner surface of the liquid core freezes onto the outer surface of the solid core. Energy released in the freezing process heats the surroundings to a high temperature. The heat flows in all directions, raising the temperature of adjacent regions. Because heat cannot be lost from the interior, it eventually flows through the mantle and crust to the surface. There, it is radiated into space as infrared radiation. This process establishes a radial temperature distribution that decreases toward the surface. If heat is generated too rapidly for conduction to carry it away convection takes over and we have Mt. Shasta, Mt. St. Helens, Lassen Peak, Mount Mazama, and Mounts Rainier and Hood, and other US volcanos. Not to mention Taal, and Mt. Pinatubo. I was in the Philippines during/after the eruption. Interesting time.

[Administrator: Meert’s response to Cassidy was to criticize the way he wrote his material in the post. Cassidy responded that if the discussion was to degenerate to that level of response on Meert’s part, it was a waste of Cassidy’s time to continue.]