  AIMS. We investigate the stability and dynamics of accretion shocks in CTTSs,
considering the case of beta >= 1 in the post-shock region. In these cases the
1D approximation is not valid and a multi-dimensional MHD approach is
necessary.
  METHODS. We model an accretion stream propagating through the atmosphere of a
CTTS and impacting onto its chromosphere, by performing 2D axisymmetric MHD
simulations. The model takes into account the stellar magnetic field, the
gravity, the radiative cooling, and the thermal conduction (including the
effects of heat flux saturation).
  RESULTS. The dynamics and stability of the accretion shock strongly depends
on the plasma beta. In the case of shocks with beta > 10, violent outflows of
shock-heated material (and possibly MHD waves) are generated at the base of the
accretion column and strongly perturb the surrounding stellar atmosphere and
the accretion column itself (modifying, therefore, the dynamics of the shock).
In shocks with beta ~ 1, the post-shock region is efficiently confined by the
magnetic field. The shock oscillations induced by cooling instability are
strongly influenced by beta: for beta > 10, the oscillations may be rapidly
dumped by the magnetic field, approaching a quasi-stationary state, or may be
chaotic with no obvious periodicity due to perturbation of the stream induced
by the post-shock plasma itself; for beta ~ 1 the oscillations are
quasi-periodic, although their amplitude is smaller and the frequency higher
than those predicted by 1D models.
