Some aspects of the relation between Pi 1‐2 magnetic pulsations observed at L = 1.3‐2.1 on the ground and substorm‐associated magnetic field variations in the near‐Earth magnetotail observed by AMPTE CCE

The relation between Pi 1–2 pulsations on the ground and substorm‐associated magnetic field variations in space has been studied using data obtained on the ground at low‐latitude conjugate stations ( L = 1.3–2.1) and in the near‐Earth magnetotail by the AMPTE CCE spacecraft. The ground‐based data were acquired in a campaign period from July 20 to September 16, of 1986, during which the apogee of CCE (8.8 R E ) was located between 2330 and 0230 hours magnetic local time. Of 16 clear magnetic field dipolarizations observed at CCE, all had a corresponding Pi 2 pulsation on the ground, with a time lag of +1 to −7 min. For most (13) of these cases, the time lag was equal to or shorter than 2 min. However, we also found Pi 2 pulsations that do not accompany a dipolarization at CCE. These results are consistent with previous observations, which showed that Pi 2 pulsations are a global indicator of the expansion phase onset of a substorm, whereas dipolarizations occur in a limited region in the near‐Earth magnetotail. One of the 16 events, which occurred on August 28, 1986, is studied in detail because a 13‐s Pi 1 pulsation was observed on the ground in addition to an ordinary Pi 2 pulsation. For this event, CCE also observed a ∼13‐s oscillation at ∼8.1 R E in the mid‐plane of the magnetotail near midnight (Takahashi et al., 1987). We suggest that field line resonance driven by a quasi‐monochromatic oscillation in the near‐Earth tail is the cause of the Pi 1 pulsation observed on the ground. The commonly observed Pi 2 pulsations could be attributed to other wave excitation mechanisms including transient response of the magnetospheric cavity to a substorm‐associated impulse.