  In the framework of the dynamical Chern-Simons gravity, we study the scalar
field perturbations of the Reissner-Nordstr\"{o}m-Melvin spacetime, which
describes a charged black hole permeated by a uniform magnetic field. In the
presence of the magnetic field, the scalar field acquires an effective mass
whose square takes negative value in the half domain of the angular direction.
This inevitably introduces the tachyonic instability and associated spontaneous
scalarization as long as the coupling constant between the scalar field and the
Chern-Simons invariant exceeds a threshold value. We study the object pictures
of the time evolutions of the scalar field perturbations at the linear level,
and find that the presence of the magnetic field will dramatically change the
waveforms and associated ringdown modes. Nonlinear evolutions for the unstable
perturbations are also performed in the decoupling limit, which demonstrate the
scalar cloud as the final fate. Influences of the coupling constant and the
black hole charge on the wave dynamics are also studied.
