Abstract
Abstract. In this work we use ion and magnetic field data from the AMPTE-UKS mission to study the characteristics of low frequency (ωr « Ωp) waves observed upstream of the Earth's bow shock. We test the application of various plasma-field correlations and magnetic ratios derived from linear Vlasov theory to identify the modes in this region. We evaluate (for a parameter space consistent with the ion foreshock) the Alfvén ratio, the parallel compressibility, the cross-helicity, the noncoplanar ratio, the magnetic compression and the polarization for the two kinetic instabilities that can be generated in the foreshock by the interaction of hot diffuse ions with the solar wind: the left-hand resonant and the right-hand resonant ion beam instabilities. Comparison of these quantities with the observed plasma-field correlations and various magnetic properties of the waves observed during 10 intervals on 30 October 1984, where the waves are associated with diffuse ions, allows us to identify regions with Alfvénic waves and regions where the predominant mode is the right-hand resonant instability. In all the cases the waves are transverse, propagating at angles ≤ 33° and are elliptically polarized. Our results suggest that while the observed Alfvén waves are generated locally by hot diffuse ions, the right-handed waves may result from the superposition of waves generated by two different types of beam distribution (i.e. cold beam and diffuse ions). Even when there was good agreement between the values of observed transport ratios and the values given by the theory, some discrepancies were found. This shows that the observed waves are different from the theoretical modes and that mode identification based only on polarization quantities does not give a complete picture of the waves' characteristics and can lead to mode identification of waves whose polarization may agree with theoretical predictions even when other properties can diverge from those of the theoretical modes.
Highlights
The terrestrial ion foreshock is permated by a variety of particle distributions and wave modes (Greenstadt, 1985; Thomsen, 1985; Hoppe and Russell, 1983; Winske et al, 1990)
For both instabilities the ions in the beam are cyclotron resonant with the wave providing the free energy necessary for the wave to grow. The first of these modes evolves out of the Alfven wave, while the second evolves from the magnetosonic branch. When these two modes propagate at small angles to the magnetic field, the values of the transport ratios can be very similar for both modes (Gary and Winske, 1992), so we evaluate the sense of polarization of the waves
As an example case to describe how the different wave parameters are used to identify a wave mode, we selected the interval 11:14:45–11:17:20. This interval corresponds to a location deep in the foreshock, where large amplitude waves are observed in the presence of diffuse ions
Summary
The terrestrial ion foreshock is permated by a variety of particle distributions and wave modes (Greenstadt, 1985; Thomsen, 1985; Hoppe and Russell, 1983; Winske et al, 1990). Most of them have relied only on the magnetic field fluctuations and have based their mode identification on MHD dispersion relations These works have shown that the low frequency (10ÿ2 Hz), transverse electromagnetic waves associated with diffuse ions have large amplitudes (5 nT peak to peak) and often steepen and show the formation of shocklets at their upstream edge. These fluctuations have been observed with left-hand and right-hand polarizations, that due to Doppler shift correspond respectively to right-hand and left-hand polarizations in the plasma frame. Because the plasma in the ion foreshock is hot, with a plasma beta 1, and because kinetic effects such as the generation of ion instabilities by back-
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