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Magnetic pole not coincident with rotational axis



 
 
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  #11  
Old June 8th 11, 09:35 PM posted to sci.astro
Dr J R Stockton[_115_]
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Posts: 3
Default Magnetic pole not coincident with rotational axis

In sci.astro message ,
Tue, 7 Jun 2011 16:19:12, Yousuf Khan
posted:

Even though the force has now been officially united and is called the
Electromagnetic force, the magnetic part is a much more difficult force
to analyse, as it acts in a rotational direction, while the electrical
part acts radially. Prior to the 20th century, they were still
considered separate forces, until the connection to electricity was
found out.


If you consider the 20th century to have started around 1825, yes.
Read http://en.wikipedia.org/wiki/Magnetic_field.

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  #12  
Old June 8th 11, 10:41 PM posted to sci.astro
Darwin123
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Posts: 247
Default Magnetic pole not coincident with rotational axis

On Jun 6, 11:03*pm, Frisbieinstein wrote:
Why is the magnetic pole of many heavenly bodies not aligned with the
axis of rotation?

As best as I can tell no one really knows. *According to Wikipedia is is because of "non-dipole moments,"

Wikipedia has a slightly more detailed description. Here is a link
to the Wikipedia article on dynamo theory.
http://en.wikipedia.org/wiki/Dynamo_theory
“In geophysics, dynamo theory proposes a mechanism by which a
celestial body such as the Earth or a star generates a magnetic field.
The theory describes the process through which a rotating, convecting,
and electrically conducting fluid can maintain a magnetic field over
astronomical time scales.
History of theory
In 1905, shortly after composing his special relativity paper, Albert
Einstein described the origin of the Earth's magnetic field as being
one of the great unsolved problems facing modern physicists.[citation
needed] Since then, there have been many studies of the geodynamo
problem based on historical measurements of the Earth's field.
Walter Maurice Elsasser (March 20, 1904 - October 14,1991) was a
German-born American physicist considered a "father" of the presently
accepted dynamo theory as an explanation of the Earth's magnetism. He
proposed that this magnetic field resulted from electric currents
induced in the fluid outer core of the Earth. He revealed the history
of the Earth's magnetic field through pioneering the study of the
magnetic orientation of minerals in rocks.
In order to maintain the magnetic field against ohmic decay (which
would occur for the dipole field in 20,000 years) the outer core must
be convecting. The convection is likely some combination of thermal
and compositional convection. The mantle controls the rate at which
heat is extracted from the core. Heat sources include gravitational
energy released by the compression of the core, gravitational energy
released by the rejection of light elements (probably sulphur, oxygen,
or silicon) at the inner core boundary as it grows, latent heat of
crystallization at the inner core boundary, and radioactivity of
potassium, uranium and thorium.[1]
At the dawn of the 21st century, numerical modeling of the Earth's
magnetic field has not been successfully demonstrated, but appears to
be in reach. Initial models are focused on field generation by
convection in the planet's fluid outer core. It was possible to show
the generation of a strong, Earth-like field when the model assumed a
uniform core-surface temperature and exceptionally high viscosities
for the core fluid. Computations which incorporated more realistic
parameter values yielded magnetic fields that were less Earth-like,
but also point the way to model refinements which may ultimately lead
to an accurate analytic model. Slight variations in the core-surface
temperature, in the range of a few millikelvins, result in significant
increases in convective flow and produce more realistic magnetic
fields.[2][3]
Dynamo theory describes the process through which a rotating,
convecting, and electrically conducting fluid acts to maintain a
magnetic field. This theory is used to explain the presence of
anomalously long-lived magnetic fields in astrophysical bodies. The
conductive fluid in the geodynamo is liquid iron in the outer core,
and in the solar dynamo is ionized gas at the tachocline. Dynamo
theory of astrophysical bodies uses magnetohydrodynamic equations to
investigate how the fluid can continuously regenerate the magnetic
field.
It was actually once believed that the dipole, which comprises much of
the Earth's magnetic field and is misaligned along the rotation axis
by 11.3 degrees, was caused by permanent magnetization of the
materials in the earth. This means that dynamo theory was originally
used to explain the Sun's magnetic field in its relationship with that
of the Earth. However, this theory, which was initially proposed by
Joseph Larmor in 1919,[4] has been modified due to extensive studies
of magnetic secular variation, paleomagnetism (including polarity
reversals), seismology, and the solar system's abundance of elements.
Also, the application of the theories of Carl Friedrich Gauss to
magnetic observations showed that Earth's magnetic field had an
internal, rather than external, origin.
There are three requisites for a dynamo to operate:
An electrically conductive fluid medium
Kinetic energy provided by planetary rotation
An internal energy source to drive convective motions within the fluid.
[5]
In the case of the Earth, the magnetic field is induced and constantly
maintained by the convection of liquid iron in the outer core. A
requirement for the induction of field is a rotating fluid. Rotation
in the outer core is supplied by the Coriolis effect caused by the
rotation of the Earth. The coriolis force tends to organize fluid
motions and electric currents into columns (also see Taylor columns)
aligned with the rotation axis. “
and even in the Earth and Sun the
origin of the magnetic field is not understood

????????????????????????????
I listed some experimental studies in a separate post.
  #13  
Old June 9th 11, 01:27 AM posted to sci.astro
Frisbieinstein
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Posts: 55
Default Magnetic pole not coincident with rotational axis

On Jun 8, 3:23*pm, "Mike Dworetsky"
wrote:


There are still vigorous arguments over whether these fields are "fossil
remnants" from the stellar collapse during formation, or generated by some
sort of internal dynamo. *If the latter, it is strange that the strongest
fields are found in the slowest rotators.


In neutron stars the strongest fields are found with slow rotators.
The reason is that some neutron stars form a dynamo, others do not.
Those that form a dynamo have rotational energy transformed to the
magnetic field, while the fast rotators have only the fossil field.

I seem to recall that fast rotators tend to not form a dynamo because
the speed of rotation exceeds the speed of convection.

  #14  
Old June 9th 11, 01:29 AM posted to sci.astro
Frisbieinstein
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Posts: 55
Default Magnetic pole not coincident with rotational axis

On Jun 9, 5:41*am, Darwin123 wrote:
“and even in the Earth and Sun the
origin of the magnetic field is not understood


????????????????????????????
I listed some experimental studies in a separate post.


I should have written "not completely understood." According to my
admittedly cursory search of the literature it is still not
established in which layer of the sun the magnetic field is generated.

 




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