  This paper examines the outflows associated with the interaction of a stellar
magnetosphere with an accretion disk. In particular, we investigate the
magnetospheric ejections (MEs) due to the expansion and reconnection of the
field lines connecting the star with the disk. Our aim is to study the
dynamical properties of the outflows and evaluate their impact on the angular
momentum evolution of young protostars. Our models are based on axisymmetric
time-dependent magneto-hydrodynamic simulations of the interaction of the
dipolar magnetosphere of a rotating protostar with a viscous and resistive
disk, using alpha prescriptions for the transport coefficients. Our simulations
are designed in order to model: the accretion process and the formation of
accretion funnels; the periodic inflation/reconnection of the magnetosphere and
the associated MEs; the stellar wind. Similarly to a magnetic slingshot, MEs
can be powered by the rotation of both the disk and the star so that they can
efficiently remove angular momentum from both. Depending on the accretion rate,
MEs can extract a relevant fraction of the accretion torque and, together with
a weak but non-negligible stellar wind torque, can balance the spin-up due to
accretion. When the disk truncation approaches the corotation radius, the
system enters a "propeller" regime, where the torques exerted by the disk and
the MEs can even balance the spin-up due to the stellar contraction. The MEs
spin-down efficiency can be compared to other scenarios, such as the Ghosh &
Lamb, X-wind or stellar wind models. Nevertheless, for all scenarios, an
efficient spin-down torque requires a rather strong dipolar component, which
has been seldom observed in classical T Tauri stars. A better analysis of the
torques acting on the protostar must take into account non-axisymmetric and
multipolar magnetic components consistent with observations.
