We simulate the ring current evolution during the magnetic storm caused by Earth passage of the January 1997 magnetic cloud. Compared to previous studies, we include for the first time energy diffusion caused by wave-particle interactions. The modeled Dst index agrees reasonably well with the measured one, corrected for magnetopause currents and currents induced in the solid Earth. We compare H+ distributions calculated from our model with those measured by the HYDRA instrument on the POLAR spacecraft and find that: a) the agreement between theory and data at large L shells (L>5.5) is very good; b) although the agreement at low L shells is improved when scattering by EMIC waves is included, the result is not entirely satisfactory, suggesting that either transport in a more realistic magnetospheric electric field or additional loss processes should be considered. ¿ 1998 American Geophysical Union