SM31C-0569
The Fine Structure of Ion-sound Turbulence Observed in the Terrestrial Bow Shock Transition Layer
High time resolution observations the quasiperpendicular regime of the terrestrial bow shock by Cluster are used to investigate ion sound turbulence in the ramp and foot regions. The four independent probe potential measurements onboard a single satellite are used to distinguish between ion-sound and whistler turbulence. The joint wavevector-frequency spectra are calculated and the waves propagation characteristics are determined. These wave characteristics are use to argue that at least some of observed wave packets have been generated by local currents in the foot region. The amplitude of the ion sound turbulence is used to assess the importance of ion sound based anomalous processes on the energy redistribution at the shock front.
SM31C-0570
A Parametric Study of the Kelvin - Helmholtz Instability of the Supersonic Magnetopause Flanks
Compressibility has a strong influence on the stability of velocity shear layers when the difference of velocity Δ V across the flow becomes supersonic. The flanks of the Earth's magnetopause (MP) are normally supersonic M>1, and super-Alfvenic MA>1, depending on the distance from the dayside terminator (M and MA are the sonic and Alfvén Mach numbers, respectively, of the magnetosheath plasma). The stability of MHD supersonic flows depends, also on several features, such as the finite thickness Δ of the boundary layer, the relative orientation of velocity and magnetic fields, the density jump across the boundary, the magnetic shear angle. We analyze the MHD stability of a set of representative flank sites modeled after data from spacecraft crossings of the MP under a variety of interplanetary conditions, complementing these with extrapolations of likely conditions upstream, and downstream of the crossing site. Under northward IMF conditions, there are solar wind regimes such that the near, but already supersonic, flank of the MP may be locally stable. Stability is possible, e.g., when M becomes larger than ~ 1.2 - 1.4 while MA remains somewhat smaller MA ~eq 1.2, and there is magnetic shear between the geomagnetic and the interplanetary magnetic fields. Solar winds favorable to local stability of the boundary layer are cold, not-too-dense plasmas, with strong magnetic fields, so that MA is smaller, while M is larger, than normal values of the magnetosheath flow. A gap between dayside and tail amplifying regions of KH disturbances over the MP may exist when the above conditions are realized.
SM31C-0571
Slow-Mode Mach-Cone-Like Solitary Waves Found in the Simulation Study of Jet-Flow- Associated Kelvin-Helmholtz Instability in the MHD Plasmas
Jet-flow-associated Kelvin-Helmholtz (K-H) instabilities are studied by means of a two-dimensional magnetohydrodynamic (MHD) simulation. In addition to the vortex structures and the undulant surface waves, we also obtain fast-mode and slow-mode Mach-cone-like solitary waves at the saturation stage of the jet-flow- associated K-H instabilities. The fast-mode Mach-cone-like solitary waves can be found when the fast-mode Mach numbers of the surface waves are greater than one. The slow-mode Mach-cone-like solitary waves are found for the first time in a limited range of MSL0y, θ0, and β0, where MSL0y is the slow-mode Mach number of the surface disturbances observed in the ambient plasma rest frame, θ0 is the angle between the jet-flow direction and the direction of the ambient magnetic field, and β0 is the plasma β of the ambient plasma. A theoretical model is proposed to explain the formation and the characteristics of the slow-mode Mach-cone-like solitary waves. The flaring angles of the slow-mode Mach-cone- like solitary waves measured from the simulation results are in good agreement with the flaring angles predicted by the theoretical model. Applications of our results to the observations in the space plasma are discussed.
SM31C-0572
Nonlinear gyrokinetic simulation of mirror instability
The mirror instability is a low frequency electromagnetic mode destabilized by pressure anisotropy in high- β plasamas. The finite Larmor radius (FLR) effects play an important role in determining the threshold and the growth rate of the mirror instability. A gyrokinetic PIC simulation code has been developed for the simulation of compressible magnetic turbulence driven by the mirror instability. Results of the linear simulation of mirror mode agree well with the analytic dispersion relation. Nonlinear simulations of single mode find that the mirror instability saturates via wave-particle trapping. Nonlinear simulations of multiple modes found important effects of mode coupling.
SM31C-0573
Competition Between the Mirror- and L-mode Electromagnetic Ion Cyclotron Instabilities in the Earthfs magnetosheath: Comparison Between 2-D and 3-D Simulations
Spacecraft observations show that the mirror instability dominates over the L-mode electromagnetic ion cyclotron (EMIC) instability in the magnetosheath, although the theoretical linear growth rate of the L-mode EMIC wave is higher than that of the mirror mode waves. This has been a long-standing puzzle. To analyze the competing processes between the L-mode instability and the mirror instability, we performed both two-D and three-D hybrid simulations, assuming anisotropic energetic ions. In the two-D model, the energy of the L-mode wave is higher at the initial stage because its linear growth rate is larger than that of the mirror mode. However, in the three-D simulation, we find that the mirror mode wave can consume more free energy than the L-mode wave at the initial state of wave growth. To understand this apparent discrepancy, we performed parametric analyses on the nonlinear evolution of the proton temperature anisotropy. We find that the nonlinear evolution of the mirror instability in the three-D model is much different from that in the two-D model. Coalescence of the magnetic field structures of the mirror modes takes place in both models. In the two-D case, the coalescence of the magnetic structures proceeds slowly, while in the three-D case the mirror mode structures changes on a much faster time scale. Through this change of structures, electric fields are induced, and the energy of the electromagnetic fields is converted to the thermal energy of particles.
SM31C-0574
Drift Shell Bifurcation Near the Dayside Magnetopause: IMF By Effects
It is well known that the magnetic field strength along field lines near the dayside magnetopause becomes W- shaped, due to compression of the magnetic field by the solar wind pressure. It is also known that energetic particles drifting across this region may experience chaotic violation of the second adiabatic invariant. Here, we explore an additional mechanism for violation of the invariant due to the east-west asymmetry in the magnetic field caused by non-zero values of the IMF By component. Using several empirical magnetospheric magnetic fields, we present initial results that quantify this previously un-explored effect.
SM31C-0575
Cluster Observations and Kinetic Simulations of Slow Mode Shocks at the Earth's Bow Shock
Slow shocks in association with reconnection are thought to be the main engine of the plasma heating, acceleration and dynamical changes of the magnetosphere. It is therefore very important to study their structure and the related physical processes. Measurements, from CLUSTER spacecraft show clear evidence for slow- mode shocks associated with magnetic reconnection in the near Earth magnetotail in connection with a substorm onset [S. Eriksson et al., 2004]. Most of the knowledge of slow mode shocks was derived from two-fluid theory together with extensive small-scale hybrid simulations of the shock transition. However, it is difficult to perform numerical simulations under realistic plasma conditions. First, it is important to use realistic proton electron mass ratios to study downstream electron heating. Second, the method to initiate a slow mode shock is Might have an impact of the obtained results. Using the piston method the slow mode shock will run into the downstream region of a preceding fast shock wave. Using switch off shock conditions the slow mode shock will run into a quite plasma environment. However, at the Earth's bow shock turbulence will always be around. Finally, dimensional effects in numerical simulations have effects on the results. For instance in 1D simulations all wave vectors are forced in the simulation direction. Furthermore, full particle simulations predict lower downstream ion temperature than predicted by hybrid simulations. This is attributed to electron kinetic processes. We performed a number of 1D full particle simulations with real proton to electron mass ratio and multi-dimensional hybrid simulations to address the above-mentioned topics. As input for the numerical simulations we used plasma- parameters from the established Cluster database. We investigated the dynamics, the structure, and the evolution of the simulated slow mode shocks using the piston and the "switch-off" method for hybrid and the 1D full particle simulations. Furthermore, we investigated the impact of the proton/electron mass ratio and compared these results with Cluster observations.