Using the auroral boundary index derived from DMSP electron precipitation data and the Dst index, changes in the size of the auroral belt during magnetic storms are studied. It is found that the equatorward boundary of the belt at midnight expands equatorward, reaching its lowest latitude about one hour before Dst peaks. This time lag depends very little on storm intensity. It is also shown that during magnetic storms, the energy of the ring current quanti®ed with Dst increases in proportion to v À3 e , where v e is the vvalue corresponding to the equatorward boundary of the auroral belt designated by the auroral boundary index. This means that the ring current energy is proportional to the ion energy obtained from the earthward shift of the plasma sheet under the conservation of the ®rst adiabatic invariant. The ring current energy is also proportional to i mg , the total magnetic ®eld energy contained in the spherical shell bounded by v e and v eq , where v eq corresponds to the quiet-time location of the auroral precipitation boundary. The ratio of the ring current energy i to the dipole energy i mg is typically 10%. The ring current leads to magnetosphere in¯ation as a result of an increase in the equivalent dipole moment.