  The rapidly oscillating Ap (roAp) star 10 Aql shows one of the lowest
photometric pulsation amplitudes and is characterized by an unusual
spectroscopic pulsational behavior compared to other roAp stars. More than 1000
spectra were taken during 7 nights over a time span of 21 days with
high-resolution spectrographs at the 8-m ESO VLT and 3.6-m TNG telescopes
giving access to radial velocity variations of about 150 lines from different
chemical species. A comparison of pulsation signatures in lines formed at
different atmospheric heights allowed us to resolve the vertical structure of
individual pulsation modes in 10 Aql which is the first time for a
multiperiodic roAp star. The inferred propagation of pulsation waves in 10 Aql
is qualitatively similar to other roAp stars: pulsation amplitudes become
measurable in the layers where Y and Eu are concentrated, increase in layers
where the Halpha core is formed, reach a maximum of 200-300 m/s in the layers
probed by Ce, Sm, Dy lines and then decrease to 20-50 m/s in the layers where
NdIII and PrIII lines are formed. A unique pulsation feature of 10 Aql is a
second pulsation maximum indicated by TbIII lines which form in the uppermost
atmospheric layers and oscillate with amplitudes of up to 350 m/s. The dramatic
decline of pulsations in the atmospheric layers probed by the strong PrIII and
NdIII lines accounts for the apparent peculiarity of 10 Aql when compared to
other roAp stars. The phase-amplitude diagrams and bisector measurements of the
NdIII 5102 A line reveal a rapid change of phase and amplitude with height for
all three main pulsation modes, indicating the presence of a pulsation node in
the stellar atmosphere. (abridged)
