  Fast changes of Earth's magnetic field could be explained by inviscid and
diffusion-less quasi-geostrophic (QG) Magneto-Coriolis modes. We present a
hybrid QG model with columnar flows and three-dimensional magnetic fields and
find modes with periods of a few years at parameters relevant to Earth's core.
For the simple poloidal magnetic field that we consider here they show a
localization of kinetic and magnetic energy in the equatorial region. This
concentration of energy near the equator and the high frequency make them a
plausible mechanism to explain similar features observed in recent geomagnetic
field observations. Our model potentially opens a way to probe the otherwise
inaccessible magnetic field structure in the Earth's outer core.
