  Four spatially resolved near-UV raster scans across the chromospheric disk of
alpha Ori, obtained with the Space Telescope Imaging Spectrograph on the Hubble
Space Telescope, reveal mean chromospheric infall from 1998 January to 1998
April, which reversed to upflow in deeper layers between 1998 September and
1999 March. In 1998 September we detect systematic reversals in the component
maxima of four double-peaked emission lines of Si I (UV 1), Fe II (UV 36), Fe
II (UV 61), and Al II] (UV 1), when scanning across the UV disk. Detailed
modeling of the Si I lambda 2516 resonance line with radiative transport
calculations in spherical geometry constrain the mean radial velocity structure
in the projected slit area (25 by 100 mas) for different aperture positions,
observed off-limb to 157.5 mas. Hence we determine with semi-empirical models
that these spatial reversals of emission line components correspond with
average opposite flow velocities of ~2 km/s across the chromospheric disk. We
determine that the chromospheric velocity field can not be represented by a
unique radial velocity structure across the stellar disk in order to match the
observed peak ratios of this raster scan. These sub-sonic velocities indicate
(local) non-radial movements of chromospheric fluid in confined regions during
a chromospheric oscillation phase, which reverses from global contraction into
expansion over this monitoring period of 15 months.

Astrophysics