Vortical Structures in the Near-tail Boundary Layer Under Collinear Magnetic and Velocity Fields in the Solar Wind

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

San Francisco

Congreso; 2011 AGU Fall Meeting; 2011

AGU

A data example is presented of rolled - up vortical structures in the near-equatorial plasma sheet-boundary layer tailward of the terminator (X = -12 RE) under an almost radially-directed IMF vector. The observations were made by the Wind spacecraft moving across the region. The rolling up is inferred from the presence of tenuous-hot plasma being accelerated to speeds higher than that in the adjoining magnetosheath. The low-density plasma phase alternates quasi-periodically with dense cold matter, that moves sunward with significant velocities with respect to the average tailward speed. These features form a good criterion to decide the presence of whirling plasma in the BL using records of one spacecraft alone. A repetitive sequence of about 15 vortices are observed over the entire 1.5 hour-long crossing. A blob of cold, dense plasma sheet was entrained in each vortex, an instance of CDPS. On the opposite side of noon tailward of the terminator spacecraft Polar was crossing the equatorial current sheet. It recorded several instances of compressive magnetic field oscillations of period ~4.1 min, similar to the periodicity of the dynamic pressure changes occasioned by the vortices. Inquiring into the generation mechanism of the vortical structures, we analyze the plasma sheet using compressible MHD Kelvin - Helmholtz stability theory, using continuous profiles for the physical quantities. We input parameters resulting from the exact theory of magnetosheath flow under aligned solar wind field and flow vectors (Spreiter and Rizzi, 1974) near the terminator, and derived from Wind data at the spacecraf´s locale. Theory shows that the configuration is indeed KH unstable, all the way from near Earth to the Wind position. Therefore, the analysis of BL data proves the presence of vortical motions with plasma mixing, and theory suggests a KH origin of the structures, under an almost radial IMF and supersonic magnetosheath flow. Here we depart substantially from previous studies which showed vortices to occur under a strongly northward (or, more rarely, southward) IMF orientation.