We present time-series spectroscopy of the rapidly oscillating Ap star HR 3831 .
This star has a dominant pulsation period of 11.7 minutes and a rotation period of 2.85 d. We have analysed 1400 intermediate-resolution spectra of the wavelength region 6100–7100Å obtained over one week , using 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 model , and rules out the spotted pulsator model .
However , further analysis of H \alpha and other lines reveal rotational modulations that can not 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 height 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 against this interpretation unless the structure of the atmosphere is somewhat different between the two stars .
Alternatively , the bisector variation is a signature of the degree \ell of the mode and not the overtone value n .

High-resolution studies of the metal lines in roAp stars are needed to understand fully the form of the pulsation in the atmosphere .