SM23D-01
Solar-Wind Ion Motion in an Analytical Field-Line Model of the Magnetosheath
We investigate the transport of representative solar-wind ions through the Earth's magnetosheath by tracing their trajectories in an analytical model of the magnetosheath, based on the draping of plasma streamlines and magnetic field lines around a conducting magnetopause that consists of a prolate ellipsoid, extending ~ 11 RE upstream and ~ 65 RE downstream from Earth, matched to a cylinder of radius ~ 28.4 RE. The bow shock is well represented in our model by an axisymmetric paraboloid that intersects the Sun-Earth line about 26% farther upstream from the point dipole than the nose of the magnetopause does. For a uniform but arbitrarily directed interplanetary magnetic field (IMF), the magnetosheath's magnetic field can be obtained by superposition of results for special cases in which the IMF is respectively parallel to and perpendicular to the solar-wind velocity. For the case in which the IMF is parallel to the solar-wind velocity, the magnetosheath's magnetic field is derivable from Euler potentials that can be expressed analytically as a function of ellipsoidal or cylindrical coordinates that scale inversely with the sixth root of solar-wind pressure. When the IMF is perpendicular to the solar-wind velocity, we can calculate the magnetosheath flow velocity from Bernoulli's equation and compute the time delay for the magnetosheath plasma to have traveled to any point of interest from the bow shock. Points along a magnetosheath field line are points of equal time delay from an upstream field line. We compare the resulting field lines and streamlines with those obtained by Spreiter and Alksne [ Rev. Geophys., 7, 11-50, 1969] from their hydrodynamic calculation. As an application of our analytical model magnetosheath, we trace the full motion of representative solar-wind ions through it for different IMF configurations.
SM23D-02
Simulation of Solar-Wind Ion Entry into the Magnetosphere with the Plasma Transport Numerical Magnetosphere Model (PlATNUMM)
We present a new global simulation model of plasma transport from the solar wind through the magnetosheath and into the magnetosphere that will be used to investigate the entry and transport of particles in the magnetosphere. On interplanetary magnetic field (IMF) lines and on open magnetospheric magnetic field lines, the model computes the full particle drift of ions and electrons using a Lorentz force solver. The particle tracing model will be coupled with the Rice Convection Model in order to compute the bounce-averaged gradient/curvature drift transport in the closed field line region of the inner and middle magnetosphere. The 3D magnetic field model includes an analytic magnetosheath magnetic field combined with a Tsyganenko magnetospheric magnetic field, while the electric field model is specified on a 3D grid by tracing magnetic field lines to the ionosphere or the unshocked solar wind where the electric field is known. The Lorentz force solver uses an adaptive Runge-Kutta time-stepper that calculates the drift path of large numbers of particles in parallel. The equivalent phase space density of particles is computed along the RCM outer boundary in order to provide the plasma boundary condition for the RCM. The magnetic and electric fields inside the magnetosphere are computed to evolve self-consistently with the plasma distribution given by the RCM and the particle tracing code. Specifying simple configurations of the IMF, we demonstrate the computation of drift path trajectories for a large number of particles launched upstream of the bow shock.
SM23D-03
Global hybrid simulations: ULF waves in the magnetosheath
Solar wind interaction with Earth's magnetosphere has been found to be more complex than originally thought. This complexity arises because the interaction region is dominated by kinetic effects that affect the large-scale dynamics of the sub-regions that form, namely the foreshock, bow shock and magnetosheath. In this work we use global hybrid simulations to study solar wind coupling with the magnetosphere for oblique (45°) and radial IMF geometries. Global hybrid simulations give a collective picture of processes taking place in the foreshock, bow shock and magnetosheath. Because ions are treated as particles, these codes also give information on ion-scale microphysics allowing us to understand how kinetic phenomena modulate global characteristics. We concentrate on the origin and characteristics of low frequency waves in the magnetosheath. This region is extremely inhomogeneous characterized by a variety of waves, which result from the convection of upstream waves, modes generated at the shock and also from local instabilities. When the IMF is oblique, our simulations show the existence of mirror mode and ion cyclotron waves. We study ion association with the waves to determine instability thresholds. For a radial IMF geometry large structures with density and magnetic field anticorrelated form in the dayside magnetosheath.
SM23D-04
Lower Hybrid Activity in Shock compression
Current sheet formation and intensification can be studied by compressing a initially uniform or weakly stratified plasma. Recently, the community has been challenged to conduct a common test, the so-called Newton challenge, where a current sheet is slowly compressed by a localized boundary push, eventually leading to magnetic reconnection in the sheet. We revisit the effect of a compression in a kinetic framework, considering also stronger and more sudden pushes leading to shock compression. We consider specifically the additional physics brought about by a full kinetic model. The primary conclusion is that two competing effects are present during compression. First, the heating due to shock compression is not isotropic and tends instead to create strongly anisotropic and non-gyrotropic distributions responsible for bifurcating the current channel. Second, current aligned instabilities, developing in the dawn-dusk direction (often neglected in previous compression studies) create small scale ripples on the shock front, leading to a more isotropic heating and creating a non-monotonic final current profile with still a centrally peaked structure but with significant side currents at the edges.
SM23D-05
Mirror instability near the threshold: Hybrid simulations
Nonlinear behavior of the mirror instability near the threshold is investigated using 1-D hybrid simulations. The simulations demonstrate the presence of an early phase where quasi-linear effects dominate [ Shapiro and Shevchenko, 1964]. The quasi-linear diffusion is however not the main saturation mechanism. A second phase is observed where the mirror mode is linearly stable (the stability is evaluated using the instantaneous ion distribution function) but where the instability nevertheless continues to develop, leading to nonlinear coherent structures in the form of magnetic humps. This regime is well modeled by a nonlinear equation for the magnetic field evolution, derived from a reductive perturbative expansion of the Vlasov-Maxwell equations [ Kuznetsov et al., 2007] with a phenomenological term which represents local variations of the ion Larmor radius. In contrast with previous models where saturation is due to the cooling of a population of trapped particles, the resulting equation correctly reproduces the development of magnetic humps from an initial noise. References Kuznetsov, E., T. Passot and P. L. Sulem (2007), Dynamical model for nonlinear mirror modes near threshold, Phys. Rev. Lett., 98, 235003. Shapiro, V. D., and V. I. Shevchenko (1964), Sov. JETP, 18, 1109.
SM23D-06
Full particle simulation of perpendicular collisionless shocks with a shock-rest-frame model
The full kinetic dynamics of a perpendicular collisionless shock is studied by means of a one-dimensional electromagnetic full particle simulation. The present simulation domain is taken in the shock rest frame in contrast to the previous full particle simulations of shocks. Preliminary results show that the downstream state falls into a unique cyclic reformation state for a given set of upstream parameters through the self-consistent kinetic processes.
SM23D-07
Magnetopause Shape and Polar Cap Potential Under Large Solar Wind Dynamic Pressure
We use three-dimensional MHD (magnetohydrodynamics) simulations to study the shape of the day side magnetopause under conditions of relatively large solar wind dynamic pressure (around or above 10nPa). Our MHD code is a finite-difference code accurate to second order in space and time and solves the MHD equations on a spherical coordinate grid. We produce a simulated magnetosphere by running the code in constant solar wind conditions and allow the solution to asymptotically approach a near-constant solution. The magnetopause shape is determined by locating the last closed field lines in the GSE x-y plane at various azimuthal angles. The subsolar radius of the magnetopause agrees well with published models. More extensive details on the shape of the magnetopause off the subsolar line will be reported. From the extent of the magnetopause on the equatorial flanks, we also estimate the dawn-to-dusk electric field potential across the magnetopause.
SM23D-08
Oblique Propagation, Wave-Particle Interaction and Particle Distribution Functions in the Quasi-Perpendicular Bow Shock.
Recent results from the Cluster mission have stimulated theoretical investigations and simulations to explain ion distribution functions observed in the quasi-perp bow shock. [Meziane et al., 2001] High-time resolution observations have revealed distributions of gyrating ions that are gyrophase-bunched. When not produced at the shock, such distributions are believed to be resulting from interactions between field-aligned beams and low frequency beam-driven waves (ω <Ømega) [Hamza et al., 2005] The Conventional models used to account for such distributions assume that the waves are purely transverse, and that they propagate parallel to the ambient magnetic field. However observations indicate that these waves are propagating obliquely (θ <e; 30 ° ) with respect to the ambient magnetic field [Meziane et al., 2001]. A theoretical investigation of the non-relativistic wave-particle interaction in a background magnetic field with the electromagnetic wave propagating obliquely has been addressed previously, resulting in a dynamical system describing the wave interaction with a single ion in the absence of dissipation mechanisms. [Hamza et al., 2005] This dynamical system can be numerically integrated to construct the ion distribution functions by seeding the particles with different initial conditions. We compute the particle orbits and simulate the time evolution of the distribution functions based on Liouville's theorem of phase space density conservation. This exercise can then provide insights on the particle dynamics away from the shock. Meziane, K., C. Mazelle, R.P. Lin, D. LeQueau, D.E. Larson, G.K. Parks, R.P. Lepping (2001), Three dimensional observations of gyrating ions distributions far upstream from the Earth's bow shock and their association with low-frequency waves, J. Geophys. Res. 106, 5731 Hamza, A. M., K. Meziane, and C. Mazelle (2006), Oblique propagation and nonlinear wave particle processes, J. Geophys. Res., 111, A04104