Physics – Plasma Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm13a1595n&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SM13A-1595
Physics
Plasma Physics
[2752] Magnetospheric Physics / Mhd Waves And Instabilities, [6235] Planetary Sciences: Solar System Objects / Mercury, [7836] Space Plasma Physics / Mhd Waves And Instabilities, [7859] Space Plasma Physics / Transport Processes
Scientific paper
Recent in-situ observations have revealed that Kelvin-Helmholtz (KH) vortices can roll-up not only at the Earth's magnetopause but also at the Mercury's magnetopause. Since kinetic effects cannot be neglected in Mercury-like small-scale situations, to universally understand the structure of the KH vortex the kinetic effects should be considered. Thus, in this study, we have performed 2D full particle (EM-PIC) simulations of KH vortices arising from kinetic and non-kinetic scale velocity shear layers. In this study, we focus on the basic situation in which the initial density, temperature and magnetic field are uniform and the magnetic field is perpendicular to the k-vector of KH instability. First, we investigated the kinetic equilibrium of velocity shear layers. In our simulation settings, particles are initialized with shifted Maxwellian velocity distributions having a bulk flow Vx0=±V0*tanh(Y/D0), where D0 is the initial half thickness of the velocity shear layer and V0 is the initial velocity jump across the shear layer. The +V0 (-V0) case corresponds to the dawn (dusk) case of the Earth’s and Mercury's situations. The Maxwellian loading of the particles, however, is only an approximation of equilibrium conditions, and past kinetic studies have shown that the true equilibrium condition is affected by the ion gyro-motion especially when D0<ρi, where ρi is the ion gyro radius. In this study, to exactly understand ion kinetic effects to the true equilibrium of various-scale velocity shear layer, we performed a parameter survey of D0 and V0. As a result, we found that in all cases until about 10 ion gyro-cycles the shear layer reaches the kinetic equilibrium, and further that when D0<ρi the thickness of the shear layer in the kinetic equilibrium always becomes 2ρi. It means there is a low threshold of the velocity shear layer which is determined by ρi. Moreover, we also found that the low threshold of the thickness in the dawn (dusk) case becomes thicker (thinner) as V0/Vthi increases, where Vthi is the ion thermal speed. This is because gyro-radii of ions which cross the boundary become larger (smaller) by the outward (inward) convection electric field in the dawn (dusk) case. Next, we investigated the evolution process of the KHI arising from kinetic and non-kinetic scale velocity shear layers. We first found that the linear growth rates of KHI are not affected by kinetic effects even when D0<ρi. This is because before the KHI onset, the velocity shear layer reaches the true kinetic equilibrium and is flattened to 2ρi. We next found that the ion rotation speed of the KH vortex flow in the dawn (dusk) case is larger (smaller) than the electron rotation speed. This result can be explained by the centrifugal drift for ions; since the directions of the centrifugal force in both cases are outward from the vortex centers, the directions of the centrifugal drift are different according to the rotation directions of vortices. In the dawn (dusk) case, the ion centrifugal drift strengthens (weakens) the ion rotation speed. Note that this centrifugal drift effect becomes larger as the vortex size becomes smaller. In our presentation, we will discuss the application of these results to the Earth's and Mercury's magnetopause.
Hasegawa Hidenao
Nakamura Takashi
Shinohara Iku
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