Conditions for a Kelvin-Helmholtz stable gap at the supersonic near equatorial magnetopause flanks for northward IMF

GNAVI, G.; FARRUGIA, C. J.; GRATTON, F. T.

Perugia, Italia

Congreso; IUGG XXIV General Assembly; 2007

IAGA (International Association of Geomagnetism and Aeronomy)

There are physical regimes of the solar wind under northward IMF conditions such that the near, but already supersonic, flank of the magnetopause (MP) is stable to Kelvin-Helmholtz (KH) perturbation modes. We analyze the stability with an equilibrium model of the low latitude boundary layer (LLBL), based on hyperbolic tangent profiles for the main scalar and vector fields, and ideal, compressible, MHD equations. When the sonic Mach number becomes larger than ~1.2-1.4 while the Alfven Mach number is somewhat smaller (MA < or ~ 1.2), negligible growth rates of the perturbations in the LLBL are possible. Local stability is obtained when, at the same time, there is magnetic shear between the geomagnetic and the interplanetary magnetic field. This outcome is due to the stabilizing effect of compressibility on the supersonic side of the boundary layer, combined with the effect of magnetic tensions in the current sheath part of the LLBL. Solar winds favourable to stability are cold, not too dense, plasmas, with strong magnetic fields, so that MA increases, while M decreases, compared to average values. We report also theoretical results where the input parameters for the boundary layer model are suggested by observations of spacecrafts crossings at the equatorial flanks, complemented with judicious extrapolations of the conditions upstream, and downstream, of the crossing locale. We conclude that a gap in the amplification regions for KH disturbances of the near equatorial flank should be expected when the above mentioned features are realized. When large scale KH vortices are observed in the flank LLBL the question arises as how far upstream they have been generated. Under the conditions discussed, it may be the case that the source of the LLBL turbulence is not a locale only a number of amplification lengths upstream, but a far-away site on the dayside MP.