Figure 1.4. The percentage of slab (planar, parallel propagating) Alfven waves observed in the solar wind. Square symbols represent averages over the time intervals represented by horizontal lines. Vertical lines give the variance of the underlying subset. Adapted from Smith (2002).

netic fluctuations was carried out using the statistical ensemble mentioned above by Matthaeus et al. (1990). They showed that solar wind fluctuations consisted of two components. The first component was comprised of slab (planar, parallel propagating) Alfven waves; the second component consisted of fluctuations with wave vectors nearly transverse to the mean magnetic field. The two-dimensional correlation function from that analysis is shown in Figure 1.3.

Subsequent single spacecraft analyses of Bieber et al. (1996) and more recently from Ulysses data Smith (2002) found that typically 50% of the magnetic fluctuation energy resides in the quasi-two-dimensional component and 50% in the slab component (see Figure 1.4). There was no obvious way to tell, however, whether the quasi-2D component reflected a nearly-incompressible population of fluctuations or merely one that contained nearly perpendicular k-vectors.

1.1.2 Studies using Cluster

The apogee of the Cluster orbit precesses so that the spacecraft are outside the magnetosphere and magnetosheath for at most 12 — 24 hours/orbit during a significant part of the year - from late fall to early spring. To date, the separation of the four spacecraft when in the solar wind has ranged from about 100 — 5000 km. At those relatively small separations, most of the studies using Cluster solar wind data have focused on sharp gradients, structures, and discontinuities, i.e., as mentioned above, the microphysics or kinetic physics, of the wind. The studies to date include:

■ The properties of the heliospheric current sheet, Section 1.2.1. Both timing and minimum variance analyses have been used to determine the orientation and speed of the current sheet crossing. A change in the sign of the electron heat flux, seen by the PEACE instrument, is consistent with ACE observations of the same HCS crossing. Using the curlometer technique (Dunlop et al., 1988, 2002b) to analyse the crossing, the main component of the current was independently determined to be confined to the plane of the discontinuity. This current sheet crossing also provided an opportunity to look for magnetic holes, which are often associated with crossings of the HCS (see discussion below in Section 1.2.1).

■ The signatures of magnetic clouds and interplanetary coronal mass ejections, Section 1.2.2. The orientations of discontinuities and shocks associated with interplanetary coronal mass ejections (ICMEs) provide information as to the large-scale morphology of the ICME (see Jones et al., 2002, among others). Small-scale flux ropes are associated with the HCS (Moldwin et al., 1995). In particular, using minimum variance analysis, Cluster data has been used to determine the change in the orientation of a small flux rope as it passed through Earth's bow shock.

■ Properties of discontinuities, Section 1.2.3. For the first time, with four spacecraft, it has become possible to remove the ambiguities in determining the normals of interplanetary discontinuities and thus better ascertain the distribution between rotational (RDs) and tangential discontinuities (TDs). Such analyses are important in understanding the general morphology and dynamics of the solar wind in that TDs represent the boundaries between separate flow regions or flux tubes while RDs may be related to the dissipation of Alfvenic fluctuations (Tsurutani et al., 2005). Thus far, it has been difficult to find unambiguous examples of RDs (Tsurutani and Ho, 1999). For very well defined directions of minimum variance (i.e., with the ratio of intermediate to minimum eigenvalues exceeding 10), the discontinuities analysed were found to be planar on the scale of the Cluster tetrahedron (600 - 900 km).

■ Isolated magnetic holes, Section 1.3.1. Isolated magnetic drop outs have been found to convect with the solar wind, but sufficient uncertainty remains so as to allow for propagation at ~ VA. The observed holes have scale sizes consistent with excitation via a mirror mode instability in which cold electrons are important (Franz et al., 2003; Treumann et al., 2004).

■ Weak double layers, Section 1.3.2. The Wide Band Data experiment on Cluster has demonstrated sensitivity sufficient to detect both bipolar and tripolar electrostatic structures in the solar wind. While the bipolar structures exhibit small potential jumps, the tripolar structures show virtually no change in potential. The solar wind observations have been used in a statistical study that shows that the amplitudes of the bi- and tripolar structures are related to the strength of the background magnetic field.

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