  Several recent studies have demonstrated how large-scale vortices may arise
spontaneously in rotating planar convection. Here we examine the dynamo
properties of such flows in rotating Boussinesq convection. For moderate values
of the magnetic Reynolds number ($100 \lesssim Rm \lesssim 550$, with $Rm$
based on the box depth and the convective velocity), a large-scale (i.e.
system-size) magnetic field is generated. The amplitude of the magnetic energy
oscillates in time, nearly out of phase with the oscillating amplitude of the
large-scale vortex. The large-scale vortex is disrupted once the magnetic field
reaches a critical strength, showing that these oscillations are of magnetic
origin. The dynamo mechanism relies on those components of the flow that have
length scales lying between that of the large-scale vortex and the typical
convective cell size; smaller-scale flows are not required. The large-scale
vortex plays a crucial role in the magnetic induction despite being essentially
two-dimensional; we thus refer to this dynamo as a large-scale-vortex dynamo.
For larger magnetic Reynolds numbers, the dynamo is small scale, with a
magnetic energy spectrum that peaks at the scale of the convective cells. In
this case, the small-scale magnetic field continuously suppresses the
large-scale vortex by disrupting the correlations between the convective
velocities that allow it to form. The suppression of the large-scale vortex at
high $Rm$ therefore probably limits the relevance of the large-scale-vortex
dynamo to astrophysical objects with moderate values of $Rm$, such as planets.
In this context, the ability of the large-scale-vortex dynamo to operate at low
magnetic Prandtl numbers is of great interest.
