PHOTO: ESA/DTU SPACE, UC SANTA CRUZ, ALAMYis liquid. Heat from the inner core,
along with the rotation of the Earth,
causes the liquid iron in the outer core to
move around. “It moves about a tenth of
a millimetre per second – that’s incredibly
fast for a geological process,” notes Gubbins.
Although the exact mechanism is unknown,
the Earth acquired a small magnetic field early
in its history. This created electric currents
in the core. Moving electric currents in turn
create a magnetic field, which acts to create
more electric current. This self-sustaining
process is known as a dynamo, and it is what
powers the Earth’s magnetic field to this day.
So what could be causing the strength of
this field to decrease so rapidly over the past
few centuries? Modelling the process using
advanced computer simulations shows that
small magnetic anomalies can appear at theboundary between the core and the mantle
(the layer between the core and the crust).
These are regions where the core’s magnetic
field points the ‘wrong’ way. “These anomalies
come and go, but once in a while they
grow,” says Gary Glatzmaier, who runs these
computer simulations at the University of
California, Santa Cruz. “This acts against the
main magnetic field and reduces its strength.
Eventually the old polarity gets destroyed,”
Glatzmaier adds.
There is evidence that this might already
be happening. There is a region of the Earth’s
magnetic field, close to South America, where
the magnetic flux is reversed. Known as the
South Atlantic Anomaly (SAA), the strength
of the field is 30 per cent weaker there. It is
getting weaker all the time and has grown
substantially so in just the last century.The
white spots
indicate where
satellites were affected
by increased radiation
due to the South Atlantic
Anomaly weakening
our magnetic
fieldGary
Glatzmaier
has delved
into the heart of
our planet using
computer
simulationsPOLE REVERSAL