[P-EM17] Space Plasma Science

[PEM17-10] Inverse and Forward Karman Vortex Systems in Magnetosphere in a Northward IMF

*DongSheng CAI¹, Shigeru Fujita², Masakazu Watanabe³ (1.University of Tsukuba, 2.The Institute of Statistical Mathematics, 3.Kyushu University)

Keywords:Inverse and Forward Karman Vortex Street, Coherent Turbulent Dynamics , Lobe reconnection

The Kelvin-Helmholtz instability (referred to as K-H instability) at an initial stage is a linear shear instability and can occur in a fluid or a plasma flow where a velocity shear is generated in a single continuous fluid or plasma, or where a velocity difference is generated across two fluids or plasmas (Chandrasekhar 1968). The K-H instability can also generate linear “transverse” vortexes, whose vortex-core lines are almost perpendicular to the shear flow direction. These linear transverse vortexes grow inside the shear layers, and soon the growths of linearly unstable modes saturate, the vortexes shed-off from the shear layers, and become the so-called free vortexes. Recently, several papers have discussed the Kelvin-Helmholtz (K-H) instability and its related waves and vortex structures by using global 2D/3D MHD simulations(Guo, Wang, & Hu 2010; Li, Guo, & Wang 2012; Li et al. 2013; Merkin, Lyon, & Claudepierre 2013). Merkin et al.(2013) found a double-vortex sheet in which a vortex train propagates along the inner and outer edges of the magnetopause. Two vortex sheets composed with paired vortices rotating in opposite direction one each other are formed to be the so-called Karman vortex street. This structure suggests a double vortex sheet of velocity perturbations and is most apparent behind the dawn-dusk “terminator”. The vortexes core lines of these Karman vortexes extend along the most outer magnetic field lines. These vortexes extended both from the dusk and dawn side of the magnetosphere and face each other in both the polar regions. However, these vortexes facing each other do not rotate in opposite direction and rotate in the same direction to form the so-called “inverse Karman vortex street.” The inverse Karman vortex streets are maintained by the propulsive flow related to the polar lobe reconnections. Thus, the vortex shedding and the polar lobe reconnections can be related near the magnetospheric polar regions.