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Kelvin-Helmholtz instability

The solar wind flow can trigger boundary surface oscillations along the flanks of the magnetopause, similarly to the wind flow generating surface waves on the free surface of a liquid (Figure 1).
If the solar wind speed exceeds a threshold value, an instability (Kelvin-Helmholtz instability) arises, with an amplification of surface waves. Such waves (with period within the Pc5 range) are strongly attenuated while penetrating deep in the magnetosphere, then they are typically observed just at high latitudes. The solar wind flow around the magnetopause determines the longitudinal propagation of these waves, which occurs in antisunward direction: westward in the morning and eastward in the afternoon, with a reversal of the propagation direction around noon. 

Figure 1 Sketch in the equatorial plane of the excitation of surface waves on the magnetopause flanks by the solar wind flow and of their antisunward propagation.
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