Velocity Shear Instability and Plasma Billows at the Earth?s Magnetic Boundary

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

Mar del Plata

Conferencia; XIV LAWPP; 2011

INFIP, CONICET y Universidad de Mar del Plata

The Kelvin-Helmoltz (KH) instability with its concomitant formation of vortices in the nonlinear stage appears in a wide variety of natural theaters. The instability of layers in relative motion is indeed ubiquitous, and its importance relies in the enhancement of momentum, energy, and mass transport that it causes. In plasmas, the magnetic field affects the onset of this process, and adds to the complexity of its evolution. A short overview of some aspects of KH magnetohidrodynamic theory is given to introduce a specific application. We study rolled-up coherent structures in the plasma flow of the equatorial Earth?s boundary layer (BL) observed on October 24, 2001. The data were recorded by a Wind crossing of the BL at the dawn magnetospheric flank, tailward of the terminator (X = -12 RE). A series of iterated billows (15) was observed during the 1.5 hour-long traverse. The interplanetary magnetic field (IMF) was radially directed, almost antiparallel to the magnetosheath (MS) flow. In general, this configuration is expected to be adverse to a velocity gradient instability because of the collinearity of field and flow, and the high compressibility of the motion, at least in the outermost part of the BL (close to the MS). However, the sonic Ms and also the Alfvén MA Mach numbers of the MS are large, due to particular plasma properties of the tail of a coronal mass ejection that reached Earth a few days before the BL event. To investigate the origin of the billows, we analyze the stability of the BL with compressible MHD Kelvin - Helmholtz models, built with continuous profiles of the physical quantities. We input parameters derived from an exact MHD solution for collinear flows (Spreiter and Rizzi, 1974) for upstream, near Earth positions. Further downtail, at Wind?s orbit locale we input measured data. We find that the configuration is indeed KH unstable in spite of some adverse features of the external flow. On the experimental side, the rolling-over is inferred from the presence of low density and hot plasma being accelerated to speeds higher than that in the adjoining MS. It is also supported by the peculiar correlation of the billows? velocity components: the sunward - inward, and the tailward - outward motions of cold-dense and hot-tenuous plasma elements, respectively. This is a case of KH vortices observed under an almost radial IMF. The event differs from many other studies that reported BL vortices under strongly northward IMF orientations. FTG, GG, and LB acknowledge the help of CONICET, PIP 2010-2012 n.11220090100608; CJF and RT were supported by NASA, grant NNX08AD11G.