Formation of the geomagnetic field

[cats0410]

Hi All,

I´m quite new to physics (from a biology background) and am hoping someone can give me a quick explanation of what causes the Earth to have a magnetic field. I have read about the dynamo theory but am finding it difficult to visualise or is it as simple as moving currents make an electric field?

Thanks for your help

 

[Andre]

We do not know what causes the Earth to have an magnetic field. We only have hypotheses about that, which are supported by observations.

I'll try to elaborate more later today - priorities.

 

[Xnn]

The Earth's core is made of a combination of liquid and solid metal. The liquid portion moves due to convection and the Earth's rotation. This movement produces electrical currents which in turn produce the magnetic field.

We know that the Earth's magnet field is not produced like a conventional magnet since conventional magnets loose their strength at the high temperatures found inside the earth. Also, the movement of the liquid metal varies over time, as does the strength of the Earth's magnetic field.

There are also electrical currents produced in the upper reaches of the atmosphere that induce a level of magnetism. These fields are also influenced by storms on the sun.

 

[Andre]

A magnetic field is directly related with moving electrical charges according to the Lorentz law.

Hence the dynamo hypothesis due to Earth rotation has reigned for a long time, rotating Earth driving magnetism, but that could not explain the numerous magnetic pole reversals and Earth magnetic excursions.

http://www.psc.edu/science/glatzmaier.html succeeded in modelling an Earth core model in which the Earth magnetic field is assumed to be the resultant of several counter rotating convection cells, which basically tend to cancel each other out. Turbulent variation in those convection cells would explain why the Earth magnetic field can suddenly collapse and or reverse in polarity, just due to changes in the balance of the counter rotating convection cells in the outer core.

However it still remains an hypothesis, since there is no way to test it from a prediction

 

[Saul]

The Earth's core is made of a combination of liquid and solid metal. The liquid portion moves due to convection and the Earth's rotation. This movement produces electrical currents which in turn produce the magnetic field.

We know that the Earth's magnet field is not produced like a conventional magnet since conventional magnets loose their strength at the high temperatures found inside the earth. Also, the movement of the liquid metal varies over time, as does the strength of the Earth's magnetic field.

There are also electrical currents produced in the upper reaches of the atmosphere that induce a level of magnetism. These fields are also influenced by storms on the sun.

The self dynamo mechanism is hypothesized to create both the geomagnetic field and the solar field. In both cases recent observations are becoming increasingly difficult for the self dynamo theory to explain.

In the case of the Earth recent observations indicate the South Atlantic magnetic anomaly is drifting west at 0.3 degree/year. The South Atlantic magnetic anomaly is a region of the planet's core that is attempting to change magnetic polarity. The question is how can core processes that are 10^5 times slower create an organized reversal pattern on the surface that is moving at 0.3 degree/year.

Another observation that is peculiar is that the geomagnetic field appears to varying intensity with a cyclic of 30 kyr and 100 kry. (i.e. It is not changing random.) As 100 kyr and 30kyr cycles are similar to orbital cycles, some researchers have searched for a mechanism whereby orbital changes somehow affects the planet's core.

http://arxiv.org/abs/0902.2873

Temporal Variations of Strength and Location of the South Atlantic Anomaly as Measured by RXTE

The evolution of the particle background at an altitude of approx. 540km during the time interval between 1996 and 2007 is studied using the particle monitor of the High Energy X-ray Timing Experiment on board NASA's Rossi X-ray Timing Explorer. A special emphasis of this study is the location and strength of the South Atlantic Anomaly (SAA). The size and strength of the SAA are anti-correlated with the 10.7 cm radio ux of the Sun, which leads the SAA strength by approx. 1 year re-ecting variations in solar heating of the upper atmosphere. The location of the SAA is also found to drift westwards with an average drift rate of about 0.3 degrees/yr following the drift of the geomagnetic field configuration. Superimposed to this drift rate are irregularities, where the SAA suddenly moves eastwards and where furthermore the speed of the drift changes. The most prominent of these irregularities is found in the second quarter of 2003 and another event took place in 1999. We suggest that these events are previously unrecognized manifestations of the geomagnetic jerks of the Earth's magnetic field.

From archeomagnetic data Pinto et al. (1992) have shown that the SAMA (South Atlantic Magnetic Anomaly) has been slowly drifting westwards for at least 1400 years. This long-term drifting was recently confirmed by Mandea et al. (2007), using data calculated via the GUFM model (Jackson et al., 2000) and the CHAOS model (model from the satellites CHAMP, Orsted and SAC-C, Olsen et al., 2006).

Is the geodynamo process intrinsically unstable?

http://eprints.whiterose.ac.uk/416/

Recent palaeomagnetic studies suggest that excursions of the geomagnetic field, during which the intensity drops suddenly by a factor of 5 to 10 and the local direction changes dramatically, are more common than previously expected. The `normal' state of the geomagnetic field, dominated by an axial dipole, seems to be interrupted every 30kyr to 100 kyr; it may not therefore be as stable as we thought.

Recent studies suggest that the Earth's magnetic field has fallen dramatically in magnitude and changed direction repeatedly since the last reversal 700 kyr ago (Langereis et al. 1997; Lund et al. 1998). These important results paint a rather different picture of the long-term behaviour of the field from the conventional one of a steady dipole reversing at random intervals: instead, the field appears to spend up to 20 per cent of its time in a weak, non-dipole state (Lund et al. 1998).

 

[Saul]

This picture summary provides some historical information concerning geomagnetic theory.

http://www.scostep.ucar.edu/comics/books/ste_geomag_e.pdf

 

[Xnn]

The self dynamo mechanism is hypothesized to create both the geomagnetic field and the solar field. In both cases recent observations are becoming increasingly difficult for the self dynamo theory to explain.

In the case of the Earth recent observations indicate the South Atlantic magnetic anomaly is drifting west at 0.3 degree/year. The South Atlantic magnetic anomaly is a region of the planet's core that is attempting to change magnetic polarity. The question is how can core processes that are 10^5 times slower create an organized reversal pattern on the surface that is moving at 0.3 degree/year.

Actually, the drift rate of the South Atlantic Anomaly is very close to the rotation differential between the Earth's core and its surface, estimated to be between 0.3 and 0.5 degrees per year.

 

[Saul]

Actually, the drift rate of the South Atlantic Anomaly is very close to the rotation differential between the Earth's core and its surface, estimated to be between 0.3 and 0.5 degrees per year.

http://en.wikipedia.org/wiki/Earth's_rotation

The Earth of course rotates counterclockwise. The earth’s core also rotates counterclockwise with a rotation speed that is roughly 0.4 degrees faster than the crust.

If the South Atlantic Anomaly was created by core based activities it seems the South Atlantic Anomaly would be moving to the east as the core moves ahead of the crust.

The South Atlantic Anomaly is however moving to the west and has been moving to the west for the last 1400 years.

Magnetic field anomalies also move to the the west.

Something is puzzling here.

http://www.livescience.com/environment/050825_earthcore.html

Earth's Core Rotates Faster than Surface, Study Confirms,

By analyzing the minute changes in travel times and wave shapes for each doublet, the researchers concluded that the Earth's inner core is rotating faster than its surface by about 0.3-0.5 degrees per year.

 

[Xnn]

http://en.wikipedia.org/wiki/Earth's_rotation

Something is puzzling here.

Agreed!

I understand that the SAA cycles every 12 hours too.
So, there is an aspect of it that involves the Sun.

 

[Choragos]

http://en.wikipedia.org/wiki/Earth's_rotation

The Earth of course rotates counterclockwise. The earth’s core also rotates counterclockwise with a rotation speed that is roughly 0.4 degrees faster than the crust.

If the South Atlantic Anomaly was created by core based activities it seems the South Atlantic Anomaly would be moving to the east as the core moves ahead of the crust.

The South Atlantic Anomaly is however moving to the west and has been moving to the west for the last 1400 years.

Magnetic field anomalies also move to the the west.

Something is puzzling here.

http://www.livescience.com/environment/050825_earthcore.html

You're assuming one current loop yielding an approximate dipole, which is not the case (though close enough for everyday use). There's a complex mix of them, so it's not too surprising that the movement you describe isn't as you think it should be. What do you mean by "magnetic field anomalies also move to the west"? To what anomalies are you referring? (I'm a crustal geophysicist so the discussion of whole-earth processes is slightly out of my area of expertise)