Two cases of auroral substorms have been studied with the Polar UVI data, which were associated with solar wind pressure shock arriving at the Earth. The global aurora activities started about 1-2 min after pressure shocks arrived at dayside magnetopause, then nightside auroras intensified rapidly 3-4 min later, with auroral sub-storm onset. The observations in synchronous orbit indicated that the compressing effects on magnetosphere were observed in their corresponding sites about 2 min after the pressure shocks impulse magnetopause. We propose that the auroral intensification and substorm onset possibly result from hydromagnetic wave produced by the pressure shock. The fast-mode wave propagates across the magnetotail lobes with higher local Alfven velocity, magnetotail was compressed rapidly and strong lobe field and cross-tail current were built in about 1-2 min, and furthermore the substorm was triggered due to an instability in current sheet.
