Characteristics of middle‐ to low‐latitude Pi2 excited by bursty bulk flows

Abstract

This paper, by using the data of Cluster, TC‐1, GOES, and eight ground stations on 22 October 2004, studied the characteristics of low‐latitude Pi2s generated by an earthward bursty bulk flow (BBF) in the near‐Earth tail plasma sheet. The BBF excited simultaneously two distinct classes of Pi2s: one is long‐period Pi2 (90–130 s) and the other is short‐period Pi2 (∼50 s). The long‐period Pi2 is transient response type Pi2 associated with field‐aligned current produced by the braking of BBFs. The spectrum analysis show that the amplitude spectrum peak of long‐period Pi2 increases with increasing latitude, indicating that the source is at higher latitudes. The time delay for the propagation of Alfven waves from Cluster to the Earth is very close to the time difference between the onset time of the BBFs at Cluster and the starting time of the long‐period Pi2 on the ground. The short‐period Pi2 is a global cavity mode since the Pi2s in H components at eight stations have almost the same starting time, same oscillation period, and same waveform, which are all typical characteristics of cavity mode. The amplitude spectrum peak of short‐period Pi2 at NCK (N42.7) is larger than those at higher‐station UPS (N56.5) and lower‐station CST (N40.8). The polarization analysis at three lower‐latitude stations shows that the polarization underwent two reversals. The major axis of the polarization ellipse points to approximately the north, indicating that the short‐period Pi2s are not excited by nightside current system. TC‐1 observed transverse mode Pi2s. Its period is almost identical with the periods of Pi2 on the ground, indicating they belong to the same wave.