Speaker
Dr
Andre Giesecke
Description
A simple way to interpret the reversal mechanism of the
Earth's magnetic field has been achieved in theoretical
models based on the interplay between very few magnetic
modes. Motivated by the temporal behavior of elementary
multipolar components of the Earth's magnetic field during
the last reversal 780ka ago, (Leonhardt & Fabian 2007) the
possibility of interacting magnetic modes is examined in a
simple mean field model. Field modes that are suitable
candidates to be involved in the reversal process are
oscillating eigenfunctions of the linear eigenvalue problem
for geodynamo models of alpha^2 type. Regarding the spectrum
of the dynamo operator time-dependent solutions arise at so
called exceptional points where two stationary modes merge
and continue at a single oscillating eigenfunction. In the
present model this behavior essentially involves dipolar and
octupolar modes. The spectrum exhibits further
time-dependent modes of higher order that appear at coupling
points of different radial field modes. In order to couple
odd ("dipolar-like") and even ("quadrupolar-like") modes
equatorial symmetry breaking is required. However, instead
of oscillating eigenfunctions an equatorial asymmetry
results in stationary hemispherical dynamos. This behavior
can be explained by the approximate dipole-quadrupole
degeneration for the unperturbed problem. More complicated
scenarios occur in case of (more realistic) anisotropies of
alpha- and beta-effect or through non-linearities caused by
the backreaction of the magnetic field (magnetic quenching).
