Statistical Study of the Relationship Between Pi1/2-Band Wave Powers and Firehose Instability Criterion During Fast Flows in the Magnetotail Plasma Sheet

Fast flows play a significant role in the transport of magnetic flux, mass, and energy in the terrestrial plasma sheet. Abundant wave activities occur during fast flows, however, what determines their wave powers is not fully understood. Here, we statistically investigate the relation of Pi1-band (10–40 s) and Pi2-band (40–150 s) mean wave amplitudes (MWAs) to the threshold of firehose instabilities during fast flows based on observations of the Magnetospheric Multiscale mission. To reduce the effect of dipolarization processes on the MWAs, we only analyze the fast flows with an average B_Z < 3 nT, where B_Z is the Z component of the magnetic field in the geocentric solar magnetospheric coordinate system. The compressional and transverse components of the magnetic field fluctuations during fast flows are mostly comparable for each frequency range. The probability of reaching firehose unstable condition tends to be larger for the faster fast flow. The Pi1/2-band MWAs tend to be larger as the probability of the firehose unstable condition near the neutral sheet is larger for the fast flows with a certain average velocity near the neutral sheet, although they have a weak relationship. Our results suggest that the firehose instability is a possible factor to affect the Pi1/2-band MWAs during the fast flow in the magnetotail current sheet.