Possible generation mechanisms of the Pi2 pulsations estimated from a global MHD simulation

Abstract:

The plasmaspheric virtual resonance (PVR) and the transient Alfven wave bouncing between the ionospheres in both hemispheres (the transient response, TR) are regarded as the possible generation mechanisms of the Pi2 pulsations. However, the global MHD simulation of a substorm [Tanaka et al., 2010] did not reproduce such wave modes because of insufficient ionospheric reflection of the Alfven wave, numerical transfer of the Alfven wave across the field lines, and no plasmasphere. Furthermore, it is noted that the substorm current wedge (SCW) which is a driver of the TR is not reproduced in the global MHD simulation. In this study, we search the sources of the Pi2 pulsations in the global MHD simulation, namely, the compressional wave in the inner magnetosphere for the PVR and the Alfven wave injected to the ionosphere for the TR. In conclusion, there appears a compressional signal in the inner magnetosphere when the high-speed Earthward flow at the substorm onset surges in the inner edge of the plasma sheet. This simulation result suggests that this compressional wave would be trapped in the plasmasphere as the PVR if the model has the plasmasphere. As for TR, the global MHD simulation provides suddenly increasing field-aligned current (the Alfven wave) associated with sudden appearance of the shear flow which comes from the high-speed flow in the plasma sheet at the onset of the substorm. If the global MHD simulation correctly lets the Alfven wave be reflected in the ionosphere and transmitted along the field line, the TR would be established. In addition to these, we also present the transient slow mode oscillation in the inner boundary of the plasmasheet. This oscillation appears just after the high-speed plasma flow associated with the depolarization injecting to the inner boundary of the plasmasheet. This oscillation has the frequency slower than the typical Pi2 pulsations.