Re: Is past reversal of the poles proven?

The Earth's magnetic field is mainly dipolar in its configuration, so, if
you want a good first approximation of this field, it would be like a
magnet bar, with north and south poles.

Some rocks have minerals that can preserve a magnetic field, e.g., basalt.
As these rocks are formed, they are too hot to mantain the magnetization,
but as they are cooled, they can align their magnetic domains in the
direction of the main magnetic field that is present, in this case, the
Earth's magnetic field. So these rocks act as weak magnets, preserving the
direction of the Earth's magnetic field in the time they were cooled.

The fact that in many places sucessive flood basalts presented
magnetization compatible with the Earth's magnetic field as it is today,
but with some layers showing a reversed field, was the first evidence that
the Earth's magnetic field can somehow reverse its polarity in the
geological time (millions of years). Before trying to explain the
observations by this uncommon process, however, many attempts were made in
order to find some physical explanation, but none of them could explain
this pattern.

As the oceanic floor is composed mainly of basalts, the information about
the magnetic field is "recorded" in the oceanic crust, and shows that the
Earth's magnetic field reverses its polarity frequently in the geological
time. The scientists could not find any periodicity in these reversals yet,
but some physical models that simulate the behavior of the metallic Earth's
core show that the observed pattern can be achieved with no major problems,
at least in the laboratory.

In your question you asked for evidence about the magnetosphere, if I
undestood it correctly. There isn't any way that a change in the magnetosphere
could be recorded geologically.  However, since it reflects dynamically the
Earth's magnetic field, the magnetosphere would reverse polarity as well.

Best regards

Eder C. Molina
eder@iag.usp.br
Dept. of Geophysics
Institute of Astronomy, Geophysics and Atmospheric Sciences
University of Sao Paulo - USP - Brazil