Upstream Waves and Particles

It has been mentioned earlier that the main field signature of the ion foreshock is the presence of low frequency fluctuations. Identification of the properties of these waves, their nature, dispersion, diffusion and free energy source as well as their effect on the solar wind are therefore of prime interest. In this section we review Cluster observations and the Cluster-based analysis of these waves.

Figure 2.9. Magnetic field observed by Cluster 1 between 00:00 and 12:00UT, January 16, 2001. The data are expressed in GSE polar coordinates at spin resolution. The dashed red line in the middle panel represents the azimuth corresponding to a sunward-pointing Parker spiral. A short interval of foreshock wave activity was observed between 04:10UT and 04:30UT, when the azimuthal angle was at its largest. It is thought that the rotation of the magnetic field in the ecliptic plane caused the spacecraft to enter and exit the foreshock. (Figure provided by J. P. Eastwood).

Figure 2.9. Magnetic field observed by Cluster 1 between 00:00 and 12:00UT, January 16, 2001. The data are expressed in GSE polar coordinates at spin resolution. The dashed red line in the middle panel represents the azimuth corresponding to a sunward-pointing Parker spiral. A short interval of foreshock wave activity was observed between 04:10UT and 04:30UT, when the azimuthal angle was at its largest. It is thought that the rotation of the magnetic field in the ecliptic plane caused the spacecraft to enter and exit the foreshock. (Figure provided by J. P. Eastwood).

2.3.1 Low-frequency waves

2.3.1.1 Monochromatic fast magnetosonic waves

A particular type of ULF waves are the so-called 30 s (period) waves, a term which corresponds to quasi-monochromatic ultra-low-frequency waves with characteristic periods of approximately 30 s in the magnetic field (e.g., Le and Russell, 1994). These waves are usually observed to be left handed in the spacecraft reference frame (Fairfield, 1969). Minimum variance analysis can be applied to single spacecraft observations and used to compute the direction of wave propagation, with a 180° ambiguity (e.g., Song and Russell, 1999), but it is not possible to identify the wave speed, the exact direction of propagation or the wavelength; for this, multi-point observations are required. Analyses of ISEE dual spacecraft data have shown such waves to in fact be intrinsically right handed, propagating against the solar wind flow (Hoppe et al., 1981; Hoppe and Russell, 1983). Their polarisation identifies them as kinetic fast magnetosonic waves (Krauss-Varban et al., 1994), generated via the ion-ion right-hand resonant beam instability (e.g., Brinca, 1991;

Figure 2.10. The GSE latitude, 9b, of the magnetic field observed by each of the four Cluster spacecraft between 04:05UT and 04:35UT on January 16, 2001. The onset of wave activity is a function of spacecraft location. The signatures are nested, rather than convected; it is therefore likely that the spacecraft crossed into and out of the ULF wave field. Calculations (not shown) indicate that the rotation of the average IMF orientation is consistent with this interpretation. (Figure provided by J. P. Eastwood).

Figure 2.10. The GSE latitude, 9b, of the magnetic field observed by each of the four Cluster spacecraft between 04:05UT and 04:35UT on January 16, 2001. The onset of wave activity is a function of spacecraft location. The signatures are nested, rather than convected; it is therefore likely that the spacecraft crossed into and out of the ULF wave field. Calculations (not shown) indicate that the rotation of the average IMF orientation is consistent with this interpretation. (Figure provided by J. P. Eastwood).

Gary, 1993) and transferring beam energy and momentum via the resonantly excited mode to the main plasma component.

Previous single and dual spacecraft analysis has been based on MVA of the magnetic field. Cluster can be used to test the performance of MVA and also provides the possibility of routinely determining the correlations between the plasma and field perturbations (Fazakerley et al., 1995). Figure 2.11 shows the spin resolution Cluster 1 FGM magnetic field between 00:00 UT and 15:00 UT on April 23, 2001, in GSE polar coordinates, as observed by Eastwood et al. (2002). The upper panel shows the magnetic field strength, the central panel shows Ob and the lower panel shows 9b, the elevation of the magnetic field vector out of the ecliptic plane. From the start of this interval until 08:12 UT, Cluster was located in the solar wind. At this time, Cluster was moving Earthward from apogee in the solar wind towards a southern cusp entry. The spacecraft separation was ^600 km.

Initially, the magnetic field was strongly southward, changing its orientation to lie in the ecliptic plane at 04:13 UT with the Cluster spacecraft becoming magnetically connected to the bow shock at this time. The magnetic field remained in this orientation until about 07:30 UT, when the field direction began to rotate such that

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Solar Panel Basics

Solar Panel Basics

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