ULF
WAVES
PATHWAYS

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ULF Waves

Ultra low frequency waves (period 1-1000 sec) are a common occurrence during all forms of magnetic activity. These waves provide a means for the magnetospheric plasma to release energy stored in the particle populations (instabilities) and are also the normal modes of oscillation of various cavities within the magnetospheric system. ULF waves are useful as probes of the magnetospheric environment because they carry information about their source regions and the plasma through which they propagate. Prof. McPherron's group was among the first to study the properties of these waves with spacecraft measurements and has developed a number of techniques for such studies. One example of such studies is the proof that a class of ULF waves characterized by azimuthal linear polarization that is observed on the dayside of the magnetosphere at synchronous orbit is produced by harmonic oscillation of magnetic field lines [button to IGPP List]. The period of oscillation is characteristic of individual field lines and depends on both the length of a field line and the distribution of plasma along it. Up to seven or more harmonics of the fundamental may be observed at any one point, but the period of oscillation changes continuously as one moves through the magnetosphere. The magnetosphere is thus divided into regions dominated by waves of different period [button to IGPP List]. The source of the energy for these waves appears to be waves generated in the solar wind upstream of the bow shock.

In contrast to the above, another class of waves is characterized by constant frequency as a function of position within the magnetosphere. These waves tend to have a strong compressional component and are thought to be produced by oscillation of the resonant cavity formed by the volume of space between the magnetopause and the plasmapause. A study of a rare event exhibiting these properties was described in the Ph. D. dissertation of Dr. Ming Cao. A paper describing this event is under review [button to IGPP List]. Again, the solar wind is thought to be the cause of these waves, but by processes such as buffeting of the magnetopause.