EFFICIENCY OF THE 2ND-ORDER FERMI ACCELERATION AT PARALLEL SHOCK-WAVES

Basing on simplifying assumptions about the structure of non-linear MHD turbulence in the shock vicinity, efficiency of the second-order Fermi acceleration of cosmic ray particles at parallel shock waves is considered. We discuss the range of shock parameters enabling efficient acceleration. With the use of simple hydrodynamical estimates, we show that the sufficient amount of shock energy can be transferred into turbulent wave field. As required, the acceleration time scale can be also quite short. The general opinion that the process under discussion can play a role only in weak shocks is confirmed, and we provide the values specifying the meaning of the word 'weak' in this statement.