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Figure 2.16. Results of MVA applied to Cluster magnetic field data for wave activity observed on February 3, 2002. The lower two panels show the projections of the field (dotted line) and minimum variance directions (solid line) into the x — y and x — z GSE planes. (From Eastwood et al., 2003).

Applying the same technique as before, Eastwood et al. (2003) found that in the solar wind rest frame the waves propagate away from the shock at speeds of the order of the local Alfven speed. The ^-filtering technique (Pincon and Motschmann, 1998) was also applied to the data. The frequency at which the maximum power was carried was chosen, and the distribution of field energy density in k-space at this frequency was estimated. A single maximum was found in the distribution. Applying the solar wind Doppler shift, the phase speed in the plasma frame was again found to be of the order of the Alfven speed, directed back into the upstream region.

Both techniques result in the same conclusion: The waves are intrinsically left handed attempting to propagate upstream but are blown anti-sunward, causing the observed polarisation to be reversed. These waves are therefore presumably generated by hot back-streaming distributions.

Figure 2.17. Ion energy spectrograms for the interval 04:00-04:10 UT on February 3, 2002. Counting from the top, the panels show the ion fluxes in the sunward, duskward, earthward and dawnward look directions, as obtained from the CIS/HIA sensor on Cluster 3. In the sunward direction, the solar wind is manifested as a continuous signal just below 1 keV. (Adapted from Eastwood etal., 2003).

Figure 2.17. Ion energy spectrograms for the interval 04:00-04:10 UT on February 3, 2002. Counting from the top, the panels show the ion fluxes in the sunward, duskward, earthward and dawnward look directions, as obtained from the CIS/HIA sensor on Cluster 3. In the sunward direction, the solar wind is manifested as a continuous signal just below 1 keV. (Adapted from Eastwood etal., 2003).

Figure 2.17 shows energy spectrograms from the CIS on Cluster 3 for the interval 04:00-04:10 UT. After 04:02:30UT, at the onset of the waves, there are significant fluxes of energetic ions in all look directions at energies extending to above 10 keV. Figure 2.18 shows the ion distribution from CIS/CODIF averaged over the interval 04:0304:10UT, where the hot backstreaming ('diffuse') ion distribution can clearly be seen (the backstreaming ions are to the left; the red region to the right corresponds to the unresolved solar wind). The ion distribution is qualitatively consistent with the wave properties. Before the onset of the waves, the ions are not diffuse; instead a dense backstreaming beam was observed. Further work quantifying this association is ongoing.

This result confirms the existence of waves in the foreshock that lie on the Alfven/Ion-cyclotron branch of the kinetic dispersion relations. The waves are intrinsically left handed, but appear as right handed in the spacecraft time series. This result does not preclude the existence of waves driven by firehose instabil-

Sat 1 CLUSTER CQOir H+ (Product 13) 2002—02-$(3/04:0 5:09—>04:10$7

Sat 1 CLUSTER CQOir H+ (Product 13) 2002—02-$(3/04:0 5:09—>04:10$7

-1500 1000 ■■ SOO 0 500 1000 1500 V Para (km/sec)

Figure 2.18. Ion distribution from CIS/CODIF on Cluster 1 on February 3, 2002, averaged over the interval 04:03 - 04:10UT, indicating the existence of a hot, diffuse, ion population. The backstreaming ions fill the plot; the red region to the right corresponds to the unresolved solar wind. The existence of a hot ion distribution is necessary to produce the observed waves. (Figure provided by M. Scholer).

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