Time-series spectroscopy of the rapidly oscillating Ap star HR 3831

We present time-series spectroscopy of the rapidly oscillating Ap (roAp) st ar HR 3831. This star has a dominant pulsation period of 11.7 min and a rot ation period of 2.85 d. We have analysed 1400 intermediate-resolution spect ra of the wavelength region 6100-7100 Angstrom obtained over one week, usin g techniques similar to those we applied to another roAp star, alpha Cir. We confirm that the H alpha velocity amplitude of HR 3831 is modulated with rotation phase. Such a modulation was predicted by the oblique pulsator mo del, and rules out the spotted pulsator model. However, further analysis of H alpha and other lines reveals rotational modulations that cannot easily be explained using the oblique pulsator model. In particular, the phase of the pulsation as measured by the width of the H alpha line varies with heig ht in the line. The variation of the H alpha bisector shows a very similar pattern to that observed in alpha Cir, which we have previously attributed to a radial node in the stellar atmosphere. However, the striking similarities between the two stars, despite the much shorter period of alpha Cir (6.8 min), argues a gainst this interpretation unless the structure of the atmosphere is somewh at different between the two stars. Alternatively, the bisector variation i s a signature of the degree l of the mode and not the overtone value n. High-resolution studies of the metal lines in roAp stars are needed to unde rstand fully the form of the pulsation in the atmosphere.