2005MNRAS.358L...6K

We show unprecedented resolution of the amplitudes and phases of the principal 2.015-mHz mode and its 4.030-mHz harmonic as a function of atmospheric depth in the roAp star 33 Lib using high-resolution Very Large Telescope (VLT) Ultraviolet-Visual Echelle Spectrograph (UVES) spectra. We show that the pulsation amplitude increases upwards into the Nd iii line-forming layer away from an atmospheric node, and increases downwards from the same node in the Nd ii line-forming layer. We suggest that the Nd iii layer, which arises at, or above, τ∼ 10^–3, overlaps with the magnetoacoustic boundary layer, and that our detailed observations of pulsation amplitude and phase variations with depth provide a first probe of this layer.