Plasma sheet Pi2 pulsations associated with bursty bulk flows

Abstract

There is a debate as to whether magnetic Pi2 pulsations (0.006-0.025 Hz) observed in the plasma sheet and on the ground are triggered by the earthward moving bursty bulk flows (BBFs) from midtail or distant tail or are caused by the substorm plasma instabilities initiated in the near-Earth plasma sheet. To resolve this debate, we conducted case and statistical studies of enhanced plasma sheet Pi2 pulsations using Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations from X~-30 RE to X~-8 RE. The case studies show that for both substorm-related and quiet time BBFs, enhanced Pi2 oscillations are observed in front of BBFs from midtail to the near-Earth plasma sheet. The oscillations intensify 1-4 min prior to the substantial flow enhancement, and usually initiate within several narrow discrete frequency bands, with lower frequencies intensifying earlier. We further examined the wave properties using low-frequency MHD fitting method and found that one or more discrete waves for each event are magnetosonic waves. Our statistical study of 364 BBFs shows that Pi2 pulsations are commonly associated with BBFs throughout the plasma sheet. The lowest-frequency oscillations usually appear below 0.01 Hz, and the strongest power oscillations appear at ~0.01 Hz or higher frequencies. We found that over 75% of the discrete oscillations are magnetosonic waves, and that 70%-75% of them are slow mode. These suggest that plasma sheet Pi2 pulsations are likely triggered by BBFs in the entire plasma sheet regardless of substorm onsets.