  Dynamics of a charged relativistic particle in a uniform magnetic field and
an obliquely propagating electrostatic shock wave is considered. The system is
reduced to a two degrees of freedom Hamiltonian system with slow and fast
variables. In this system, the phenomenon of capture into resonance can take
place. Under certain condition, a captured phase point stays captured forever.
This corresponds to unlimited surfing acceleration of the particle. The
preprint is a more detailed version of a comment on the paper by D.Ucer and
V.D.Shapiro (Phys.Rev.Lett., vol. 87, 075001, 2001), intended for the Comments
section of Physical Reviews Letters.
