  High-resolution spectroscopic time series of rapidly oscillating Ap stars
show evidence for a co-existence of standing and running waves in their
atmospheric layers. With the purpose of understanding these observations we
have carried out a theoretical analysis of the pulsations in the outermost
layers of these stars, starting from the simplest possible model that still
retains all important physical ingredients. In our analysis we considered an
isothermal atmosphere in a plane-parallel approximation. Moreover we assumed
that in the region considered the magnetic pressure is much larger than the gas
pressure and, consequently, that the magnetoacoustic wave has decoupled into
its acoustic and magnetic components. Using the analytical solutions for the
velocity components appropriate to this model we estimate the velocity
component parallel to the line of sight averaged over the visible stellar disk.
Fitting the latter to a function of the form Acos($\sigma$t+phase), with
$\sigma$ the dimensionless oscillation frequency and t the dimensionless time,
we derive the amplitude A and the phase for our model as function of height in
the atmosphere.
