  When a magnetically-dominated super-fast magnetosonic GRB jet leaves the
progenitor star the external pressure support may drop and the jet may enter
the regime of ballistic expansion during which its magnetic acceleration
becomes highly ineffective. However, recent numerical simulations suggested
that the transition to this regime is accompanied by a sudden "burst" of
acceleration. We confirm this finding and attribute the acceleration to the
sideways expansion of the jet - the magnetic energy is converted into the
kinetic one in the strong magnetosonic rarefaction wave, which is launched when
the jet loses its external support. This type of acceleration, the rarefaction
acceleration, is specific to relativistic jets because their energy budget can
still be dominated by magnetic energy even in highly super-fast magnetosonic
regime. Just like the collimation acceleration of externally confined
magnetized jets, it is connected with the geometry of magnetic flux sufaces. In
both cases, in the acceleration zone the poloidal field lines diverge faster
than in the monopolar configuration. On the other hand, whereas the collimation
acceleration keeps the product of jet opening angle and Lorentz factor somewhat
below unity, the rarefaction acceleration allows to make it significantly
larger, in agreement with the standard model of jet breaks in afterglow light
curves.
