Velocity shear instability and plasma billows at the Earth's magnetic boundary

<div class="autor_fcen" id="3984">Gratton, F.T.</div>; <div class="autor_fcen" id="3756">Gnavi, G.</div>; Farrugia, C.J.; <div class="autor_fcen" id="909">Bilbao, L.</div>; Torbert, R.
Publication Year
Published version
The Kelvin-Helmoltz instability (KH) with formation of vortices appears in a wide variety of terrestrial, interplanetary, and astrophysical contexts. We study a series of iterated rolled-up coherent plasma structures (15) that flow in the equatorial Earth's boundary layer (BL), observed on October 24, 2001. The data were recorded during a 1.5 hour-long Wind crossing of the BL at the dawn magnetospheric flank, tailward of the terminator (X≈-13 RE). The interplanetary magnetic field (IMF) was radially directed, almost antiparallel to the magnetosheath (MS) flow. This configuration is expected to be adverse to the KH instability because of the collinearity of field and flow, and the high compressibility of the MS. We analyze the BL stability with compressible MHD theory using continuous profiles for the physical quantities. Upstream, at near Earth sites, we input parameters derived from an exact MHD solution for collinear flows. Further downtail at Wind position we input measured parameters. The BL is found KH unstable in spite of unfavorable features of the external flow. On the experimental side, the passage of vortices is inferred from the presence of low density - hot plasma being accelerated to speeds higher than that of the contiguous MS. It is further supported by the peculiar correlation of relative motions (in the bulk velocity frame): cold-dense plasma drifts sunward, while hot-tenuous plasma moves tailward. This event differs from many other studies that reported BL vortices under strongly northward IMF orientations. This is a case of KH vortices observed under an almost radial IMF, with implicit significance for the more common Parker's spiral fields, and the problem of plasma entry in the magnetosphere.
Fil:Gratton, F.T. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Fil:Gnavi, G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Fil:Bilbao, L. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
J. Phys. Conf. Ser. 2012;370(1)
Bulk velocity
Continuous profile
External flow
Hot plasmas
Input parameter
Interplanetary magnetic fields
Kelvin-Helmoltz instability
Low density
Measured parameters
Northward IMF
Physical quantities
Plasma drifts
Plasma structure
Relative motion
Velocity shear
Plasma diagnostics
Plasma stability
Shear flow
Vortex flow
Access level
Open access
Biblioteca Digital (UBA-FCEN)
Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales
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