  We present time-series echelle spectra of the Pa-beta line of the T Tauri
star SU Aur, observed over three consecutive nights. The line shows strong
variability (~10 per cent) over the velocity range (100 km/s, 420 km/s) in the
red broad absorption component, and weaker variability (~2 per cent) over the
velocity range (-200 km/s, 0 km/s) in the blue wing. The variability in the
velocity range (-200 km/s, 0 km/s) is correlated with that in (200 km/s, 400
km/s), and the variability in these velocity ranges anti-correlates with that
in (0 km/s, 100 km/s). The mean spectrum from the second night shows a
suggestion of a blue-shifted absorption component at about -150 km/s, similar
to that found in the H-alpha and H-beta lines. We find the position of the
subpeak in the red absorption component changes steadily with time, and its
motion modulates on half the rotational period. We also find that the
modulation of the line equivalent width is associated with a half and a third
of the rotational period, which is consistent with the surface Doppler images
of SU Aur. Radiative transfer models of a rotationally modulated Pa-beta line,
produced in the shock-heated magnetospheric accretion flow, are also presented.
Models with a magnetic dipole offset reproduce the overall characteristics of
the observed line variability, including the line equivalent width and the
motion of the subpeak in the red absorption trough.
