  Convection in an infinite fluid layer is often modelled by considering a
finite box with periodic boundary conditions in the two horizontal directions.
The translational invariance of the problem implies that any solution can be
translated horizontally by an arbitrary amount. Some solutions travel, but
those solutions that are invariant under reflections in both horizontal
directions cannot travel, since motion in any horizontal direction is balanced
by an equal and opposite motion elsewhere. Equivariant bifurcation theory
allows us to understand the steady and time-dependent ways in which a pattern
can travel when a mirror symmetry of the pattern is broken in a bifurcation.
Here we study symmetry-breaking instabilities of convection with a square
planform. A pitchfork bifurcation leads to squares that travel uniformly, while
a Hopf bifurcation leads to a new class of oscillations in which squares drift
to and fro but with no net motion of the pattern. Two types of travelling
squares are possible after a pitchfork bifurcation, and three or more
oscillatory solutions are created in a Hopf bifurcation. One of the three
oscillations, alternating pulsating waves, has been observed in recent
numerical simulations of convection in the presence of a magnetic field. We
also present a low-order model of three-dimensional compressible convection
that contains these symmetry-breaking instabilities. Our analysis clarifies the
relationship between several types of time-dependent patterns that have been
observed in numerical simulations of convection.
