  We have carried out the first survey of the pulsational line profile
variability in rapidly oscillating Ap (roAp) stars. We analysed high
signal-to-noise time-series observations of ten sharp-lined roAp stars obtained
with the high-resolution spectrographs attached to the VLT and CFHT telescopes.
We investigated in detail the variations of Pr III, Nd II, Nd III and Tb III
lines and discovered a prominent change of the profile variability pattern with
height in the atmospheres of all studied roAp stars. In every investigated star
profile variability of at least one rare-earth ion is characterized by unusual
blue-to-red moving features - a behaviour inexplicable in the framework of the
standard oblique pulsator model of slowly rotating roAp stars. Using analysis
of the line profile moments and spectrum synthesis calculations, we demonstrate
that unusual oscillations in spectral lines of roAp stars arise from the
pulsational modulation of line widths. This variation occurs approximately in
quadrature with the radial velocity changes, and its amplitude rapidly
increases with height in stellar atmosphere. We propose that the line width
modulation is a consequence of the periodic expansion and compression of
turbulent layers in the upper atmospheres of roAp stars. Thus, the line profile
changes observed in slowly rotating magnetic pulsators should be interpreted as
a superposition of two types of variability: the usual time-dependent velocity
field due to an oblique low-order pulsation mode and an additional line width
modulation, synchronized with the changes of stellar radius. Our explanation of
the line profile variations of roAp stars solves the long-standing
observational puzzle and opens new possibilities for constraining geometric and
physical properties of the stellar magnetoacoustic pulsations.
