  Many types of stars show periodic variations of radius and brightness, which
are commonly referred to as `stellar pulsations'. Observed pulsational
characteristics are determined by fundamental stellar parameters. Consequently,
investigations of stellar pulsations provide a unique opportunity to verify and
refine our understanding of the evolution and internal structure of stars.
However, a key boundary condition for this analysis -- precise information
about the geometry of pulsations in the outer stellar envelopes -- has been
notoriously difficult to secure. Here we demonstrate that it is possible to
solve this problem by constructing an `image' of the pulsation velocity field
from time series observations of stellar spectra. This technique is applied to
study the geometry of nonradial pulsations in a prototype magnetic oscillating
(roAp) star HR 3831. Our velocity map directly demonstrates an alignment of
pulsations with the axis of the global magnetic field and reveals a significant
magnetically induced distortion of pulsations. This observation constitutes a
long-sought solution of the problem of the pulsation geometry of roAp stars and
enables very stringent tests of the recent theories of stellar magneto-acoustic
oscillations.
