A statistical analysis of Pi2‐band waves in the plasma sheet and their relation to magnetospheric drivers

We use the Cluster data from 2001 to 2009 to investigate the occurrence of Pi2-band waves in the plasma sheet. To study the generation mechanisms of these waves, we examine the association between Pi2-band waves and dynamic processes in the plasma sheet (fast flows and substorm activity) and the direction of the solar wind velocity. For a total of 80 large-amplitude Pi2-band waves in the plasma sheet, we find that Cluster records fast flows during 62 events, 11 waves without fast flows occur during substorm time, 3 events occur when the solar wind velocity significantly changes its direction, and 4 events are not associated with any of the above activities. Most of the observed Pi2-band waves are predominantly compressional, while 2 events are transverse. Based on this statistical study, we suggest that fast flows maybe the main driver of Pi2-band waves/oscillations in the plasma sheet, especially considering that most of these waves are compressional. The relatively small number of other events indicates that other mechanisms also play a role in creating Pi2-band waves/oscillations in the plasma sheet but are relatively rare. In all wave events of this study, the plasma pressure and magnetic pressure vary in antiphase, suggesting that these waves have the slow-mode feature.