Uncertainties
In order to understand the workings of the dynamo mechanism,
some basic data need to be known. Unfortunately, it is very
hard to know anything about the interior of the Earth. Many
important quantities (such as the magnetic field strength
and electrical conductivity
in the core) are not known with much accuracy. (Merrill 300)
We are fortunate enough to have a long standing record of
the magnetic field encoded into magnetic lava
(paleomagnetism)
but this carries its own problems.
Uncertainties in measurement of the field
- Non-uniform data points
Ideally it would be nice to know the magnetic field of every
point on earth at every time. The reality is that both
paleomagnetic records and modern observatories are distributed
non-uniformly across the globe. This leads to inaccuracies
when a global magnetic field is inferred based upon these points. (Merrill 40)
- Non-uniqueness of interior field
Even if there was a perfect knowledge of the magnetic field
at every single point on the Earth's surface this would still
not be enough to reconstruct the field in the Earth's interior.
There are many interior field configurations that would not even
reach the surface at all. (Merrill 41)
- Time averaging of lava
Depending on the thickness of a lava flow it can cool over a
period of a few years to a thousand years. The result is
that the recorded field represents data that is sometimes
averaged over a large period of time. (Merrill 92)
- Electrical screening of mantle
One would hope that at least the present day field could be
monitored in real-time. Unfortunately, since the mantle is
electrically conducting it absorbs all of the rapid field
changes. Any fluxuation that occurs over a period of less
than a year gets filtered out. (Merrill 47)
Uncertainties in the properties of the interior
- Conductivity in mantle
The conductivity of the Earth's mantle and the distribution
of metallic regions therein is important because it acts to screen
rapid changes from reaching the surface and because it represents
the boundary conditions for the dynamo. Unfortunately
only very rough estimates of these quantities are known. (Merrill 275-277)
- Viscosity of core
The viscosity of the fluid responsible for dynamo action
is of course of vital importance. Again there is great
uncertainty - estimates of viscosity vary by 11 orders of
magnitude (Merrill 274).
- Strength and structure of internal magnetic field
It is not possible to directly measure the magnetic field inside
of the core. Neither the strength (Merrill 300) nor the structure (Merrill 42)
is known for sure.
Uncertainties in the theory
- Strength of field
The dipole component of the magnetic field has always been within
10% and 200% of the present value but there is not general agreement
on why it is this strong. (Backus 287)
- Period between reversals
The time between polarity reversals is about 100,000 years but was
much longer 40 million years ago. There is no explanation for this. (Backus 287)
- Speed of reversal
It takes 2000 to 10000 years for the polarity to change during
a reversal. It it not known what factors affect this. (Backus 288)
- Why dipole is primary component
The dipole component is stronger than the
higher moments.
The theory does not necessarily explain this. (Backus 287)