Auroral substorm response to solar wind pressure shock

Two cases of auroral substorms have been studied with the Polar UVI data, w hich were associated with solar wind pressure shock arriving at the Earth. The global aurora activities started about 1-2 min after pressure shocks ar rived at dayside magnetopause, then nightside auroras intensified rapidly 3 -4 min later, with auroral substorm 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 ons et possibly result from hydromagnetic wave produced by the pressure shock. The fast-mode wave propagates across the magnetotail lobes with higher loca l Alfven velocity, magnetotail was compressed rapidly and strong lobe field and cross-tail current were built in about 1-2 min, and furthermore the su bstorm was triggered due to an instability in current sheet.