We propose that the magnetosphere reconfiguration induced by magnetic twists in the closed field line region can account for the mode-switching of intermittent pulsars .
We carefully investigate the properties of axisymmetric force-free pulsar magnetospheres with magnetic twists in closed field line region around the polar caps .
The magnetosphere with twisted closed lines leads to enhanced spin-down rates .
The enhancement in spin-down rate depends on the size of region with twisted closed lines .
Typically , it is increased by a factor of \sim 2 , which is consistent with the intermittent pulsars ’ spin down behavior during the ‘ off ’ and ‘ on ’ states .
We find there is a threshold of maximal twist angle \Delta \phi _ { thres } \sim 1 .
The magnetosphere is stable only if the closed line twist angle is less than \Delta \phi _ { thres } .
Beyond this value , the magnetosphere becomes unstable and gets untwisted .
The spin-down rate would reduce to its off-state value .
The quasi-periodicity in spin-down rate change can be explained by long-term activities in star ’ s crust and the untwisting induced by MHD instability .
The estimated duration time of on-state is about one week , consistent with observations .
Due to the MHD instability , there exists an upper limit for the spin down ratio ( f \sim 3 ) between the on-state and the off-state , if the Y-point remains at the light cylinder .