Abstract
Field reversals are some of the most prominent and commonly known
temporal variations of the geomagnetic field. Polarity changes have been
observed in seafloor magnetisation patterns, volcanic records, sediment
sequences, speleothem records, and have been reported in geodynamo
simulations. However, open questions remain concerning the phenomenology
and underlying causes of this process. In particular, there is currently
no consensus about the temporal scales over which geomagnetic reversals
occur. Numerical simulations aimed at understanding Earth’s million-year
evolution have predicted a time scale on the order of thousands of
years. On the other hand, analysis of a lacustrine sequence in the
central Italian Apennines suggests that the most recent geomagnetic
reversal (the Matuyama-Brunhes) took place in as short as 13 years,
requiring VGP latitudinal changes of the order of 10 degrees/yr
[Sagnotti, L. et al. (2015). GJI, 204(2), 798-812]. This extremely
short decadal time scale challenges our current understanding of the
geodynamo and present-day numerical models. Here we derive fluid flows
at the top of Earth’s outer core that optimise either the rate of dipole
decay or directional changes local to the Italian Apennines, subject to
a minimal number of physical ingredients. Specifically, we neglect the
internal dynamics and prescribe a total flow kinetic energy that is
consistent with observational bounds. Our optimal flows can drive an
instantaneous VGP latitudinal change of at most 5 degrees/yr during the
Matuyama-Brunhes transition. Extending the methodology to account for
the spatio-temporal evolution of the magnetic field, we find that this
solution does not drive a global reversal, but only changes that are
highly localised around the Mediterranean and West-African regions.
Solutions that optimise the rate of dipole decay suggest that full
reversal would take at least 380 years.