VLF Waves in the Foreshock

R. J. Strangeway* and G. W. Crawford**

*Institute of Geophysics and Planetary Physics,
University of California at Los Angeles

**Radio Atmosphere Science Center
Kyoto University at Kyoko 611, Japan
Now at SRI International, Menlo Park, California 94025, U. S. A.


Adv. Space Res., vol 15, (8/9)29-(8/9)42, 1995
Copyright 1995 by COSPAR


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Summary and Conclusions

      VLF waves observed in the electron foreshock that is present upstream of planetary bow shocks appear to be a useful diagnostic of the energization processes that occur at the bow shock. First, the waves themselves provide a very clear marker of the tangent field line. As such they can be used for remote sensing of the bow shock /37/, although some care must be exercised in inferring the bow shock location /31/. Some of the more extreme shock shapes in /37/ arise from allowing the focus of the shock to change, rather than the semi-latus rectum. Varying the latter parameter is more physically realistic since it determines the size of the forward part of the shock near the obstacle, which will vary as a function of Mach number, while the focus is mainly fixed by the obstacle.

      Second, the evolution of the waves as a function of both depth and distance provide information on the energization process. The changes that are a function of depth are clearly related to the decrease in reflection energy, and provide confirmation of both the time-of-flight models, and also the validity of Fast Fermi (or alternatively, electron shock-drift) acceleration. The dependence on distance observed within the Venus foreshock may also have important lessons concerning the effects of curved shocks on the electron energization. It appears that the shock radius of curvature is an important controlling factor in limiting the availability of energetic electrons in the foreshock. As such we might expect the plasma emissions to be observed at progressively larger distances as the shock radius of curvature decreases. Studies of the terrestrial foreshock appear to confirm this /32/.

      A comparison with foreshock data from the outer planets and also from interplanetary shocks would be useful. Unfortunately, it is only statistical studies, such as shown in Figures 5 and 6, that allow us to determine the spatial extent of the foreshock emissions. Most wave observations at the outer planets are from flybys, or from relatively close orbiters such as the Phobos spacecraft at Mars /38, 39/.

      The ion foreshock appears to be much more complicated than the electron foreshock. As other papers in this issue indicate /8, 9, 10/, our understanding of the ion foreshock is far from complete. This is not only the case for the VLF waves, but also for the source of the ions in the foreshock, and the role that ULF waves have to play in modifying the ion distributions. From our studies of the ion foreshock at Venus we are drawn to the conclusion that the ULF and ion acoustic waves are not necessarily observed at the same time, especially further away from the shock. Figure 5 indicates that the VLF waves in the ion foreshock are generated well downstream of where ion beams are expected to be present, rather they are observed where we might expect diffuse ions to be present. Unfortunately, the solar wind instrument on PVO was not able to provide detailed distributions within the foreshock. Also, since similar studies have not been carried out at the Earth, we do not know if this separation of ULF and VLF signatures is specific to the Venus foreshock, or if such features are also present at the Earth.

      In conclusion, VLF waves observed in planetary foreshocks can provide information about the underlying plasma that generates the waves. The waves of course indicate the presence of free energy within the plasma. However, through maps of the wave emissions we may be able to infer more about the mechanisms that are responsible for the free energy. This appears to be the case for the electron foreshock, where maps of the VLF emissions suggest that shock curvature limits the energization of the electrons. For the ion foreshock, on the other hand, it may be necessary to reconsider the source of the VLF waves. Rather than being generated by ion beams /40/, the waves may be generated by the more diffuse distributions, implying that the waves are generated through pitch angle anisotropy within the ion distribution. Maps of the ion foreshock emissions at planets other than Venus would probably help in determining the validity of this speculation.

Acknowledgements

This work was supported by NASA grants NAG2-485 and NAGW-3497.


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