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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Introduction

      Collisionless shocks provide a myriad of phenomena that occur on many different scales, from ion inertial lengths, through ion Larmor radii, to scales of many planetary radii. At the same time, the physics involved may be expressed by simple magnetohydrodynamics, or particle trajectory analysis, or fully kinetic plasma theory. The tools employed include analytic theory, simulations, and data analysis. In this paper we will discuss just one aspect of collisionless shocks, that is the VLF waves that are observed upstream of a planetary bow shock in the region known as the foreshock. Although primarily based on data analysis, we will also discuss some wave instability theory, and the mechanisms by which particles gain energy at a shock.

      VLF wave generation is a microscale phenomenon, but the morphology of the waves allows us to make inferences concerning the energization processes that occur at a shock. Moreover, we will show that while the energization occurs on mesoscales, the macroscale of the shock itself appears to be a limiting factor on this process. This will be most clear through analysis of the waves observed in the electron foreshock, and we will devote much of this paper to the electron foreshock, although we will also discuss ion foreshock waves. Our understanding of the electron dynamics within the shock and foreshock appears to be much firmer /1, 2, 3/. While there has been significant progress in our understanding of the ion dynamics /4, 5, 6, 7/, how the ion distributions within the foreshock interact, how they evolve, and nature of their relationship with both ULF and VLF waves is still a topic of some debate /8, 9, 10/.

      Before discussing the VLF observations we will briefly review some of the basic theory on electron reflection and energization at a shock. This will not be a comprehensive review of particle dynamics and energization. In particular we will not discuss in detail why electron dynamics are best investigated in the de Hoffman-Teller (HT) frame /11/, while ion dynamics are more readily understood in the normal incidence frame (NIF). Instead we refer the reader to the excellent articles that discuss why the presence of a non-coplanar magnetic field allows the magnetized electrons to be only affected by the HT frame cross-shock potential, while the unmagnetized ions respond to the full potential in the NIF /12, 13, 14/. Having reviewed the electron reflection process at the shock, we will present some examples of VLF wave data, obtained by the Pioneer Venus Orbiter (PVO). We will then present images of the VLF emissions in the ion and electron foreshock generated from statistical studies of the foreshock at Venus. The most significant result of the imaging is the observation of a finite extent to the electron foreshock emissions, which we attribute to the scale size of the shock acting to restrict the availability of energetic electrons far from the shock. We will then discuss why the limitations of the PVO wave instrument do not allow us to observe down-shifted plasma oscillations in the foreshock. Lastly, we will summarize the results presented in the paper, including some comments on the apparent disparity between the ULF and VLF waves within the ion foreshock.


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