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Next: Appendix: Alfvénic Travel Time Up: Phase skipping and Poynting Previous: Discussion

Conclusions

We have examined the Poynting flux and phase skipping properties of the magnetic pulsations. The Poynting flux for Pc3-4 waves has bursty appearance. This result and the traveling wave-like structure of the Poynting vectors tell us that the Pc3-4 waves in the outer magnetosphere have an impulsive energy source. However, the waves are continuous in appearance since the bursts of wave energy feed into the magnetosphere repeatedly. This conclusion departs from the traditional paradigm that most of the pulsations in the magnetosphere are field line resonances, but it should be pointed out that the observations presented in this study are mainly located outside the geosynchronous orbit, which may not represent the situation in the inner magnetosphere. Nevertheless, the wave impulses are capable of traveling across L shells, and they may explain the observations that Pc3 waves often have the same wave frequency at multiple stations located over a wide range of latitudes with quite different resonant frequencies [e.g., Chi et al., 1994].

We also examined a Pc5 event which does show a clear field line resonance structure, in contrast to the traveling wave structure for Pc3-4 waves. It is concluded that, in the outer magnetosphere, the field line resonance is important for some low-frequency pulsations such as Pc5 waves, but it is not a dominant effect for Pc3-4 activities.

It is also found that the phase skipping phenomenon and the directional change of Poynting flux are related. The energy impulses of pulsations alter the direction of the Poynting flux and also abruptly change the phase of wave signals. This finding helps confirm that the phase skipping phenomenon that has been previously studied in the literature only by its magnetic signatures is present in the electric field also and is caused by the impulsive nature of the energy source. It is possible that one wave impulse could cause multiple phase skips in observations due to the Alfvén component of the wave energy being reflected by the ionosphere.


next up previous
Next: Appendix: Alfvénic Travel Time Up: Phase skipping and Poynting Previous: Discussion