A Parametric Study of the Kelvin - Helmholtz Instability of the Supersonic Magnetopause Flanks

GNAVI, G.; FARRUGIA, C. J.; GRATTON, F. T.; BILBAO, L.; TORBERT, R.

San Francisco, USA

Congreso; 2007 AGU Fall Meeting; 2007

AGU - American Geophysical Union

Compressibility has a strong influence on the stability of velocity shear layers when the difference of velocity ∆ V across the flow becomes supersonic. The flanks of the Earth´s magnetopause (MP) are normally supersonic M>1, and super-Alfvenic M>1, depending on the distance from the dayside terminator (M and M are the sonic and Alfvén Mach numbers, respectively, of the magnetosheath plasma). The stability of MHD supersonic flows depends, also on several features, such as the finite thickness ∆ of the boundary layer, the relative orientation of velocity and magnetic fields, the density jump across the boundary, the magnetic shear angle. We analyze the MHD stability of a set of representative flank sites modeled after data from spacecraft crossings of the MP under a variety of interplanetary conditions, complementing these with extrapolations of likely conditions upstream, and downstream of the crossing site. Under northward IMF conditions, there are solar wind regimes such that the near, but already supersonic, flank of the MP may be locally stable. Stability is possible, e.g., when M becomes larger than ~ 1.2 - 1.4 while M remains somewhat smaller M ~ 1.2, and there is magnetic shear between the geomagnetic and the interplanetary magnetic fields. Solar winds favorable to local stability of the boundary layer are cold, not-too-dense plasmas, with strong magnetic fields, so that M is smaller, while M is larger, than normal values of the magnetosheath flow. A gap between dayside and tail amplifying regions of KH disturbances over the MP may exist when the above conditions are realized.