The response of plasma parameters and energy transport in the plasma sheet to interplanetary magnetic field B_z

We use 9 years data of Cluster to study the dependencies of plasma parameters and energy transport in the plasma sheet on the lasting time of northward/southward interplanetary magnetic field (IMF). The plasma parameters and energy transport in the plasma sheet always respond to the change of IMF direction by more or less time. The ion density starts to increase/decrease remarkably at 80 min after northward/southward IMF turning. The ion temperature starts to decrease at 25 min after northward IMF turning, whereas it starts to increase at 80 min after southward IMF turning. The earthward convection velocity within 15 min after northward IMF turning almost equals to that within 15 min period after southward IMF turning. However at time greater than 15 min after southward IMF turning, the earthward convection velocity under southward IMF starts to remarkably increase. The response time (15 min) of magnetospheric convection velocity is well consistent with the response times of nightside ionospheric convection to southward IMF turning. The enthalpy flux is larger than kinetic flux by about three orders of magnitudes, and thus the enthalpy flux plays a dominant role in the plasma sheet energy transport. The enthalpy flux does not weaken immediately after northward IMF turning. The enthalpy flux within 15 min after northward IMF turning is comparable to or even slightly larger than that within 15 min after southward IMF. The enthalpy flux starts to decrease at times greater than 15 min after northward IMF turning, whereas it starts to increase at times greater than 15 min after southward IMF turning. The result that the enhanced energy transport during the 15 min period after northward IMF turning may explain previous observation that substorms frequently occur shortly after northward IMF turning.