  A two-dimensional second-order topological superconductor exhibits a finite
gap in both bulk and edges, with the nontrivial topology manifesting itself
through Majorana zero modes localized at the corners, i.e., Majorana corner
states. We investigate a time-reversal-invariant topological superconductor in
two dimension and demonstrate that an in-plane magnetic field could transform
it into a second-order topological superconductor. A detailed analysis reveals
that the magnetic field gives rise to mass terms which take distinct values
among the edges, and Majorana corner states naturally emerge at the
intersection of two adjacent edges with opposite masses. With the rotation of
the magnetic field, Majorana corner states localized around the boundary may
hop from one corner to a neighboring one and eventually make a full circle
around the system when the field rotates by $2\pi$. In the end we briefly
discuss physical realizations of this system.
