A statistical analysis of the association between fast plasma flows and Pi2 pulsations

What is the energy source for Pi2 pulsations? Some researchers have suggested that Pi2 pulsations are caused by plasma flows. Others have suggested that Pi2s are caused by a plasma instability initiated in the near-Earth region. In this study we use Time History of Events and Macroscale Interactions during Substorms (THEMIS) data to examine the relationship between plasma flows and Pi2s. We first identify plasma flows in the tail and then associate these flows with Pi2s. We find that the overall probability of association is nearly 70%. If we further examine the spatial distribution of association we find that in the near-Earth region (8-15 R-E) 90% of the flows are associated with Pi2s. This suggests either that plasma flow in the tail is the cause of Pi2 on the ground or that both are caused by the same process. More importantly, as the flows approach the Earth, the increasing magnetic field causes the flow speed to decrease, that is, the flows are decelerated. In this process of flow braking, the local magnetic field becomes more dipolar. The region of strongest flow braking corresponds to the region of strongest association between flows and Pi2s (8-15 R-E). The GSM X component of plasma flows become zero at around 8 R-E where the most significant magnetic dipolarization occurs. We also find that most plasma flows are associated with a sudden enhancement of the westward electrojet. These results suggest that plasma flows, Pi2 pulsations, and magnetic field dipolarization are essential components of the substorm expansion onset.