PX266 Geophysics (2010/11)
Lecture 15 Handout – Earth’s Magnetic Field and Paleomagnetism
Dr. Gavin Bell
Magnetic declination: angle between magnetic and geographical meridians.
Magnetic inclination: angle Iat which local field dips below horizontal, depending
only on latitude
for a cylindrically symmetric (dipole) field.
tan2tan
I
Where are the geomagnetic poles (GMPs) right now?
NGMP: Canadian Arctic, 82°N 248°E.
SGMP: Antarctic Ocean, 65°S 138°E.
These are not antipodal (i.e. opposite each other)! They also drift (see animation on
course web site). The Earth’s field is quite close to but not precisely a dipole field –
there are higher order components which alter the detailed form of the field, including
breaking its N-S and longitudinal symmetry. Actually, symmetry-breaking is required
in the “alpha-omega” theory of the self-exciting geodynamo. The non-dipolar
components vary rapidly (timescale < 100 years). Changes in the geomagnetic field
intensity and magnetic pole locations are known as secular variation.
The Geodynamo
Currents flowing in the core give rise to the geomagnetic field: the Earth is not a
“giant bar magnet” since the iron core is well above its Curie temperature! Both the
liquid outer core and solid inner core can carry electrical currents. The “geodynamo”
which gives rise to the Earth’s field must be self-exciting since the dissipative effects
of ohmic heating have not caused the currents and field to fade away over the Earth’s
geological history. The precise mechanism of the self-sustaining geodynamo is not
known. However, the so-called alpha-omega theory, illustrated below, shows how
convective motion in the outer core (basically powered by cooling of the core) can
lead to a self-sustaining magnetic field.
The inner core may rotate a couple
of degrees longitude per year faster
than the mantle and crust. The
difference in rotation speed feeds
back from the torque on the core
due to the magnetic field. This
omega effect “wraps up” the field
lines helically (b,c) to give a
toroidal field in the core (d).
Convection in the outer core causes
field lines to become helical via the
Coriolis force (e) leading to loops
of field which reinforce the original
dipolar field (f).
