at unusually low latitudes – are a vivid reminder that the solar input side of this balance is variable. In the case of a magnetic storm, a coronal mass ejection impinges on and compresses the magnetosphere, greatly reducing the magnetopause standoff distance.

Of course, Earth’s magnetic field is dynamic and it also varies on a wide range of timescales. At any given time, the geomagnetic field at a radius $r$, colatitude $\theta$ and longitude $\phi$ can be specified by the gradient of the scalar potential ($\Phi$):

$$\Phi(r,\theta,\phi)=r_{e}\sum_{l=1}^{\infty} \sum_{m=0}^{l}
\left(\frac{r_{e}}{r}
\right)^{l+1}   P^{m}_{l}(cos \theta)[g^{m}_{l}\cos m \phi + h^{m}_{l}\sin m
\phi]$$

where $P^{m}_{l}$ are partially normalized Schmidt functions, $r_{e}$ is the radius of Earth and the Gauss coefficients $g^{m}_{l}$ and $h^{m}_{l}$ describe the size of spatially varying fields. The current field is approximately 80% dipolar and when averaged over several hundred thousand years to 1 million years, the axial dipole term ($g^{1}_{0}$) is thought to dominate. The latter assumption– the geocentric dipole hypothesis– forms the basis of tectonic interpretations of paleomagnetic data that have been essential in our understanding of the plate tectonic history of the planet.

Paleomagnetic directions provide the opportunity for tests of the geocentric dipole hypothesis [@Irving1964] and these have 