  We present Halpha spectropolarimetry observations of a sample of 10 bright T
Tauri stars, supplemented with new Herbig Ae/Be star data. A change in the
linear polarisation across Halpha is detected in most of the T Tauri (9/10) and
Herbig Ae (9/11) objects, which we interpret in terms of a compact source of
line photons that is scattered off a rotating accretion disk. We find
consistency between the position angle (PA) of the polarisation and those of
imaged disk PAs from infrared and millimetre imaging and interferometry
studies, probing much larger scales. For the Herbig Ae stars AB Aur, MWC 480
and CQ Tau, we find the polarisation PA to be perpendicular to the imaged disk,
which is expected for single scattering. On the other hand, the polarisation PA
aligns with the outer disk PA for the T Tauri stars DR Tau and SU Aur and FU
Ori, conforming to the case of multiple scattering. This difference can be
explained if the inner disks of Herbig Ae stars are optically thin, whilst
those around our T Tauri stars and FU Ori are optically thick. Furthermore, we
develop a novel technique that combines known inclination angles and our recent
Monte Carlo models to constrain the inner rim sizes of SU Aur, GW Ori, AB Aur,
and CQ Tau. Finally, we consider the connection of the inner disk structure
with the orientation of the magnetic field in the foreground interstellar
medium: for FU Ori and DR Tau, we infer an alignment of the stellar axis and
the larger magnetic field direction.
