Is there evidence of multiple poles (higher order than dipole) in earth's magnetic field?

Question

There is this slightly uncombed simulation model of a geomagnetic field reversal, showing multiple poles, a quadrupole or even higher order magnetic field. Earth's magnetic field is mostly described as a dipole and navigation wouldn't work without that, (considerable) local perturbations left aside. But a multipole field is a theoretically valid configuration, as long as no N or S pole remains solo (quadru-, sextu, octupole ...). Some bodies in the solar system may have multipole fields.

But for earth's magnetic field, is there actual evidence of multiple poles at a time, from geomagnetic chronolgy, ocean floor or ophiolites, magma flows ? These may be transient events, happening during reversals.

Answer 1

It's common for dynamo models to predict a large reduction in the dipole field during a reversal. Proving it in the paleomagnetic record is another matter entirely. To characterize the geometry of the magnetic field, you need several simultaneous measurements, preferably with a good spatial distribution. Instead what you get are relatively infrequent, asynchronous measurements. See this article by Fabio Florindo for more details. For what it's worth, though, a model by Olson et al. (2011) with multipolar fields during the transition predicts some observations that are consistent with the Matayama-Brunhes transition, including reductions in the intensity of the field and variations in the virtual geomagnetic pole path (VGP) from one site to another (see page 124 of this book for a definition of a VGP).

Answer 2

The simple answer is YES.

The Earth's magnetic field is generated by a self-exciting dynamo in the fluid outer core. The interaction between electric currents and liquid motion sustain this field which is largely oriented along the rotation axis, since this rotation exerts constraints on the convection pattern. The dynamo field is largely dipolar but since the fluid circulation and current system is not perfectly symmetrical there arise smaller, non-dipole components which also emerge at the Earths surface.

Geophysical models describe two magnetic poles: first, the points where the north and south poles of the main (model) dipole outcrop at the Earth's surface and secondly, the points where the (observable) magnetic inclination of the field is vertical - the dip poles.

There is of course only one N and one S pole for the main dipole. These do not correspond to any special places on the Earth's surface - they are simply optimum mathematical solutions to the field that we observe and model. On the other hand, there can be several N and several S dip poles since the non-dipole field is complex and the field may also be perturbed by local geology.

All these theoretical and observable poles move with time, so there is no point is erecting a pillar or more glamorous monument to mark their position since the next day they will have wandered to different places.

During a geomagnetic reversal the main dipole decays to zero and all that is left is the weak and irregular, non-dipole field giving rise to numerous dip poles scattered over the Earth's surface.