THE SIZE OF THE AURORAL BELT DURING MAGNETIC STORMS

Using the auroral boundary index derived from DMSP electron precipitat ion data and the Dst index, changes in the size of the auroral belt du ring magnetic storms are studied. It is found that the equatorward bou ndary of the belt at midnight expands equatorward, reaching its lowest latitude about one hour before Dsf peaks. This time lag depends very little on storm intensity. It is also shown that during magnetic storm s, the energy of the ring current quantified with Dsr increases in pro portion to L-e(-3), where L-e is the L-value corresponding to the equa torward boundary of the auroral belt designated by the auroral boundar y index. This means that the ring current energy is proportional to th e ion energy obtained from the earthward shift of the plasma sheet und er the conservation of the first adiabatic invariant. The ring current energy is also proportional to E-mag, the total magnetic field energy contained in the spherical shell bounded by L-e and L-eq, where L-eq corresponds to the quiet-time location of the auroral precipitation bo undary. The ratio of the ring current energy E-R to the dipole energy E-mag is typically 10%. The ring current leads to magnetosphere inflat ion as a result of an increase in the equivalent dipole moment.