Institute of Geophysics and Planetary Physics, University of California, Los Angeles
It is recognized that the continuous pulsations (Pc) in the magnetosphere are in general composed of many wave packets and that these wave packets are separated by phase skips. Previous observations by multiple spacecraft and ground magnetometer stations suggest that the phase skipping and packet structure of pulsation signals are most likely due to impulsive wave sources or the beating of waves. However, the magnetic field data alone have not led to an understanding of the propagation of these wave bursts. By using both the electric field and magnetic field data of the ISEE 1 spacecraft, we find that the Poynting flux of Pc3-4 pulsations in the outer magnetosphere has an impulsive nature. The direction of time-average Poynting flux changes every several wave cycles, and the phase skips in wave signals, both in the magnetic field and electric field, are often found between two adjacent Poynting flux bursts. Furthermore, the phase difference between the electric wave and the magnetic wave indicates that Pc3-4 waves are generally not in exact resonance, counter to the traditional field line resonance paradigm. These observational results provide us with strong evidence that the ``continuous'' pulsations in the Pc3-4 band are in fact maintained by a series of pulses, and the phase skips in the wave signals are its natural consequence. The direction of wave energy propagation and its implications are also discussed.