Kelvin – Helmholtz modes in transition: from the dayside active strips to the equatorial flanks of the magnetopause

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

São Pedro, SP, Brasil

Congreso; IX Brazilian Meeting on Plasma Physics; 2007

Sociedade Brasileira de Fisica

When large-scale, vortex-like, perturbations are observed by spacecraft crossings of the velocity gradient layer of the magnetopause (MP) equatorial flanks, the question arises as how far they have been generated, since the Kelvin-Helmholtz (KH) process is a convective instability. Under conditions discussed here it may happen that the source of the low latitude boundary layer (LLBL) turbulence is not a region distant upstream only a small number of amplification lengths of the instability, but a faraway upstream site on the dayside MP. During periods of northward interplanetary magnetic field (IMF) the KH instability develops in special strips of the dayside MP (as was shown by Farrugia et al., J. Geoph. Res., 103, 6703,1998), and then advected by the flow it may traverse a neutral amplification region, before it reaches the unstable far flank downstream. We show in fact that under northward IMF there are solar wind regimes such that the near, but already supersonic, flank of the MP is stable for KH modes. When the sonic Mach number M = U1=cs becomes larger than ¼ 1.2 - 1.4 (U1, flow velocity, and cs, sound speed of the magnetosheath, respectively), while the Alfvénic Mach number MA = U1=VA (VA Alfvén velocity in the magnetosheath) remains somewhat smaller, MA < or ¼ 1.2, stability or marginal stability of the LLBL is possible. The LLBL is locally stable when, together with the above values of M, MA, there is also magnetic shear between the geomagnetic and the interplanetary magnetic fields. The outcome is due to the stabilizing effect of compressibility on the supersonic side of the boundary layer, combined with the restraining action of the magnetic tensions in the current sheath part of the LLBL. Solar winds favorable to the local stability of the LLBL are cold, not too dense plasmas, with strong magnetic fields, so that MA is smaller, while M is larger, than the average values of the magnetosheath flow. These conditions often occur during the passage of the tail of CMEs by Earth. We take input parameters for the boundary layer model from spacecraft crossings at the equatorial flanks, complemented with judicious extrapolations of conditions upstream, and downstream, of the crossing site. We conclude that a gap between amplifying regions of KH disturbances over the MP may exist when the above conditions materialize. Acknowledgements: FTG, LB and GG are supported by CONICET grants PIP5291/05, and UBACYT-X291/04, CJF by NASA Grant 143139.