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Figure 3.16. The projections of the spacecraft trajectories in the (x-y) plane(straight lines) and the magnetic field measured by C1 (gray curve). The circles represent the boundaries between different layers of a mirror mode structure. (From Constantinescu et al., 2003).

Horbury et al. (2004) recently performed a statistical survey of anti-correlated magnetic field and density structures observed in the magnetosheath. Their study estimated the motion of 39 isolate magnetic dips and 12 peaks, spread over four days, using the application of four-spacecraft timing analysis. Their results were not therefore dependent on the use of a particular mirror mode model. The important finding is that within timing errors the most probable (average) velocity is very close to zero indicating that the structures are convected with the flow. The scattering of the velocity was — 21 km s^1 much less than the local wave speeds (<5% of the local Alfven speed and <15% of the local slow mode velocity). Hence, observations do not support the view that propagating slow mode waves or solitons are responsible for the formation of such structures in the magnetosheath. Neither are they standing structures, i.e., propagating against the flow. The directions of the normals derived for the mirror modes from timing analysis were consistent with the structures having a cylindrical cross section, inclined by a small angle (< 20°) to the background magnetic field. These results, therefore, suggest a mirror mode geometry more similar to the idealised model presented in this section, rather than a soliton model, where the structures would be expected to be sheet-like.

Figure 3.17. The magnetic field lines derived from the model of a mirror mode structure. The straight lines are the projections of the spacecraft trajectories on the XZ plane. The dots are the points where the spacecraft paths intersect the XZ plane. (From Constantinescu et al., 2003).

Figure 3.17. The magnetic field lines derived from the model of a mirror mode structure. The straight lines are the projections of the spacecraft trajectories on the XZ plane. The dots are the points where the spacecraft paths intersect the XZ plane. (From Constantinescu et al., 2003).

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