  We report the results of a synoptic study of the photometric and
spectroscopic variability of the classical T Tauri star AA Tau on timescales
ranging from a few hours to several weeks. Emission lines show both infall and
outflow signatures and are well reproduced by magnetospheric accretion models
with moderate mass accretion rates and high inclinations. The veiling shows
variations that indicate the presence of 2 rotationally modulated hot spots
corresponding to the two magnetosphere poles. It correlates well with the HeI
line flux, with B-V and the V excess flux. We have indications of a time delay
between the main emission lines and veiling, the lines formed farther away
preceding the veiling changes. The time delay we measure is consistent with
accreted material propagating downwards the accretion columns at free fall
velocity from a distance of about 8 Rstar. We also report periodic radial
velocity variations of the photospheric spectrum which might point to the
existence of a 0.02 Msun object orbiting the star at a distance of 0.08 AU.
During a few days, the variability of the system was strongly reduced and the
line fluxes and veiling severely depressed. We argue that this episode of
quiescence corresponds to the temporary disruption of the magnetic
configuration at the disk inner edge. The radial velocity variations of inflow
and outflow diagnostics in the Halpha profile yield further evidence for large
scale variations of the magnetic configuration on a timescale of a month. These
results may provide the first clear evidence for large scale instabilities
developping in T Tauri magnetospheres as the magnetic field lines are twisted
by differential rotation between the star and the inner disk.
