射电天文研究室知识库

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 regions 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 the. region with twisted closed lines. Typically, it is increased by a factor of similar to 2, which is consistent with the intermittent pulsars' spin-down behavior during the "off" and "on" states. We find that. there is a threshold of maximal twist angle Delta phi(thres) similar to 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 the star's crust and the untwisting induced by MHD instability. The estimated duration of on-state is about 1. week, consistent with observations. Due to the MHD instability, there exists an upper limit for the spin-down ratio (f similar to 3) between the on-state and the off-state, if the Y-point remains at the light cylinder.