It has been demonstrated recently that the diffusive shock acceleration the
ory based on the assumption that the velocity distribution function of part
icles is close to isotropy is incapable of explaining observed spectra of i
ons accelerated at shocks in corotating interaction regions. In the present
paper the shock drift acceleration of protons suffering multiple reflectio
ns at shock fronts is studied as applied to the forward and reverse shocks
detected at Ulysses on day 20, 1993, and on day 22, 1993. The velocity dist
ribution function is considered to be anisotropic near the shock fronts, an
d the magnetic moment of particles is supposed to be an adiabatic invariant
which is valid in the case of weak scattering.