SM51B-0359 0800h
Gyrokinetic Stability of Z-pinch and Dipole-like Configurations
In this work we explore the kinetic stability of Z-pinch and dipole-like plasma geometries. In an effort to model configurations ranging from the Earth's magnetosphere to the MIT Levitated Dipole Experiment (LDX), we consider a variety of collisionality regimes, plasma beta values, and along-the-field-line boundary conditions. Our study uses the nonlinear gyrokinetic code GS2 as well as various analytic methods. GS2 is an Eulerian, electromagnetic, kinetic plasma turbulence code that is widely used in the magnetic confinement fusion program. It simulates the nonlinear Frieman-Chen gyrokinetic equations [E. Frieman and L. Chen, ``Nonlinear Gyrokinetic Equations for Low-Frequency Electromagnetic Waves in General Plasma Equilibria", Phys. Fluids, Vol. 25, 502 (1982)] , which describe the evolution of 5D distribution functions for each plasma species (including self-consistent Maxwell's equations) for processes slow compared to the inverse ion cyclotron frequency. The two velocity space coordinates are energy and magnetic moment, which are conserved quantities at these frequencies. Trapping of the particles in magnetic wells is also included.
SM51B-0360 0800h
Finite Larmor Radius Effects on Mirror waves
A fully kinetic theory of magnetic mirror instability is used to deduce the finite ion Larmor radius effects on the growth rate and polarisation mirror waves. The incorporation of this effect results in a substantial modification of both the instability growth rate and the instability threshold. The modification is due to the fact that for wavelengths of the order of the ion Larmor radius the effective elasticity of the magnetic field lines is substantially enhanced. Theoretical results are compared with experimental finding.
SM51B-0361 0800h
Dependence of Frequency of Nonlinear Cold Plasma Cylindrical Oscillations on Electron Density: Significance to Space-Plasma Diagnostics
In a cold plasma, the frequency of small-amplitude oscillations fpe along one Cartesian coordinate is proportional to the square root of the electron density. We found that for cold-plasma nonlinear cylindrical oscillations this dependence is logarithmic with an excess of negative charge near the axis of symmetry. The electron plasma frequency increases monotonically with increasing electron density with an upper limit equal to the square root of 2 times fpe [Osherovich and Fainberg, 2004; Osherovich et al., 2004]. These results are of importance for the diagnostics of large-amplitude plasma waves in the solar wind and in the magnetosphere. Osherovich, V. A., and J. Fainberg, Phys. Plasmas, 11, 2314, 2004. Osherovich, V. A., J. Fainberg, and C. J. Farrugia, Phys. Rev. Lett., submitted, 2004.
SM51B-0362 0800h
Bump on the tail instability in mirror stable space plasmas
A fully kinetic theory of mirror type modes in non-Maxwellian space plasmas accounting for the arbitrary ion Larmor radius effect (FLR) is developed. A general mirror mode dispersion relation for the arbitrary velocity distribution in a fully kinetic limit is obtained. The analyses of this dispersion relation shows that in a fully kinetic limit it describes two different instabilities. The first one corresponds to the classical mirror instability which growth rate attains the maximum value at the wavelengths of the order of the ion Larmor radius. In addition, we found that if plasma is mirror stable, i.e. when the condition for the mirror instability is not satisfied, the plasma can still be unstable. The instability in this case arises in the presence of the bump on the tail of the ion distribution function and FLR effects. This new instability which we term as bump on the tail instability occurs for the wavelengths shorter then the ion Larmor radius. A compact expression for the instability growth rate is obtained. The results of the theory are compared with the existing satellite observations.
SM51B-0363 0800h
Geomagnetic Effects From the Plasma Cloud Extending in the Ionosphere
Geomagnetic variations on the Earth surfaces created by the magnetohydrodynamics stage of the expansion of the plasma cloud of the high-altitude nuclear explosions are considered. It is shown that the distribution of the amplitudes of these variations on the different distances from the cloud submits to the specific regularity. The analysis of the experimental data and magnetohydrodynamics numerical simulation have confirmed that the temporal form and phases of the electromagnetic field on the Earth surface are determined by its propagation through three channels: through the atmosphere, through the ionosphere and through the lithosphere. The good agreement between the results of the numerical simulations and the measurements is shown. Comparison of the disturbances caused by the explosions and substorms show the difficulties of its separation on the distant zone from the explosions.
SM51B-0364 0800h
Auroral Electric Field Generation by Earthward Motion of Hot Plasma
A neutral plasma beam streaming toward Earth along magnetic field lines has no opposing electric force, and yet electric tensions may arise internal to the beam in association with outer edges and other density gradients, especially when gradient scale lengths are comparable to the ion gyro radii. This is due to differential mirroring of electrons and ions, an effect that occurs even if the electrons and ions both have isotropic velocity distributions about their gyro centers, assuming that the typical electron gyro radii are significantly smaller than those of the ions. The effect has many important implications, one being the generation of auroras. It is easy to imagine that as the beam enters the far topside ionosphere, its positively charged filaments will discharge into the lower ionosphere by releasing local ionospheric electrons from the grip of an ambipolar electric field, thus setting up the normal conditions for accelerating magnetospheric electrons downward. This and other implications are discussed in this paper in the context of simple modeling, using plasma parameters inferred from Polar satellite data.
SM51B-0365 0800h
The Cusp Aurora in the Conjugate Hemispheres of the Earth
In this paper we examine the Earth's cusp aurora associated with high latitude lobe reconnection. The cusp aurora is observed simultanously in both the southern and northern hemispheres in similar wavelengths by the cameras on IMAGE and Polar satellites. Due to the high solar wind pressure both the proton and electron aurorae are fairly bright, dominated by ~8 keV protons and 0.5-1 keV electrons. As predicted by theory, the longitudinal asymmetric location of the cusp aurora is strongly controlled by the IMF By component. The southern cusp aurora is observed several degrees poleward of the northern cusp aurora and can be attributed to either a tilt effect or an IMF Bx effect. These rare observations also confirm our earlier findings that theta aurora can be a non-conjugate phenomenon controlled by the IMF Bx component.
SM51B-0366 0800h
Time-dependent Dispersive Whistlers Close to the Magnetopause
Whistler waves inside the magnetopause are found to be generated in thin sheets moving with the plasma drift velocity. In the same region we also observe time-dependent dispersive whistler emissions. The duration of each emission is about 1 second and the frequency ranges from 50 to 150 Hz. Using electric and magnetic wave field observations from the four Cluster satellites we estimate the wave length, the direction of propagation and the size and location of the generation region. We also discuss possible generation mechanisms and their relation to the magnetopause.
SM51B-0367 0800h
Why Does the Plasma Depletion Layer Exist Near the Magnetopause?
In the late seventies of the last century, it has been found that a layer with increased modulus of the magnetic field and decreased density (called in different sources both the Plasma Depletion Layer or the Magnetic Barrier) does exist in the inner magnetosheath close to the Earth's magnetopause. Nevertheless, the paper of Zwan and Wolf (1976) where the physical explanation of this phenomenon was given seems to be controversial and was criticized, for example, by Southwood and Kivelson (1992,1995). While later MHD models have confirmed the existence of the PDL (plasma depletion layer), there has been given no clear answer on the question of what is the physical mechanism of the plasma depletion. In the present work with the help of our 3-D MHD model of the magnetosheath, we study in details which forces accelerate the plasma along the magnetopause. We clearly show the difference in the acceleration mechanism for hydrodynamic and MHD flows. The enhanced magnetic field in the PDL accelerates the plasma in the direction perpendicular to the magnetic field, while in the direction along the magnetic field the plasma flow is due to the plasma pressure gradient.
SM51B-0368 0800h
Large Amplitude, Extremely Rapid, Predominantly Perpendicular Electric Field Structures at the Magnetopause
Electric fields with amplitudes to 150 mV/m and average durations of 7.5 milliseconds have been observed frequently at the magnetospheric side of the dayside magnetopause and in the near-earth tail. These fields are predominantly perpendicular to B, they are electrostatic, and they occur inside local minima in the plasma density. Some aspects of their occurrence and statistics are described.
SM51B-0369 0800h
FTE Structures: Multi-Spacecraft Observation and Global Modeling Combined
The solar wind is a crucial driver for the dynamics of the Earth's magnetosphere. One of the most effective ways to transfer mass, momentum and energy from the solar wind into the Earth's magnetosphere is through magnetic reconnection at the magnetopause. An important mode of reconnection produces patchy, transient flux transfer events (FTEs). The structure of FTEs carries important information about the Sun-Earth coupling. However, conventional observations with single satellites and simulations with restricted spatial range have significant limitations for revealing the real nature of FTEs. Recent progress in both FTE observations with multi-spacecraft and FTE simulations with global MHD models greatly advances our understanding of FTEs. However, even multi-spacecraft observations still fall short of obtaining the global structure and evolution of FTEs, e.g., the determination of FTEs on some typical characteristics of observations may be misleading because of the complexity of FTE structures and different paths of the spacecraft across FTEs. In this study, we use global FTE simulation results to investigate the influence of different factors, including the path of the spacecraft through an FTE flux tube, on FTE observational signatures by running pseudo-spacecraft through a simulated FTE. We find significant dependence of FTE observational signatures on those factors. The results of this study are further used to determine a better observational FTE definition and expected signatures. We show that the combination of state-of-the-art FTE observations and simulations can significantly enhance our ability to interpret multi-spacecraft FTE observations. Meanwhile, it helps us to better understand model results for further model improvements.
SM51B-0370 0800h
Particle Diffusion in Heliospheric Vortex Flows
Vortex steets were revealed in satellite measurements in the helisophere. They can efficiently affect energetic particle transport. Numerical study of cosmic ray diffusion in a presence of a layer of vortex flows is performed. Random phase linear waves with a power-law spectrum are introduced to the system. This model represents a mixture of coherent structures which belong to strong turbulence and weak turbulence effects in the medium. Different diffusion regimes, i.e. classical diffusion, subdiffusion and superdiffusion, are analyzed. Importance of regular and stochastic components is studied. Comparison with theoretical estimates for radial transport coefficients is made.
SM51B-0371 0800h
Dispersion Characteristics for Plasma Resonances of Maxwellian and Kappa Distribution Plasmas and their Comparisons to the IMAGE/RPI Observations
The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite stimulates short-range plasma wave echoes and plasma emissions, known as plasma resonances, detected on plasmagrams. These resonances are used to provide measurements of the local electron density n$_{e}$ and magnetic field strength $|B|$. The RPI-stimulated resonances are the magnetospheric analog of plasma resonances stimulated by topside ionospheric sounders. These resonances are stimulated at the harmonic of the electron cyclotron frequency f$_{ce}$, the electron plasma frequency f$_{pe}$ and the upper-hybrid frequency f$_{uh}$ (where f$_{uh}$$^{2}$ = f$_{pe}$$^{2}$ + f$_{ce}$$^{2}$). They are also observed between the harmonics of f$_{ce}$ (i.e., n f$_{ce}$) both above and below f$_{pe}$ and are known as the Qn and Dn resonances, respectively. In this work we solely focus on the Qn resonances. Calculations of these resonances in the ionospheric environment, based upon a thermal Maxwellian plasma model provided confidence in the resonance identification. There is an apparent difference however, between the ob-served Qn resonances and calculations based on a Maxwellian plasma model in the magnetosphere environment. For example, the Qn's are often (and perhaps consistently) observed at frequencies slightly lower than expected for a Maxwellian plasma. We present a new set of resonance calculations based upon the non-thermal kappa distribution that provides dispersion characteristics of these resonances. Comparisons of these calculations to the IMAGE/RPI observations seems to resolve the frequency discrepancy. The results also provide insight into the nature of the electron distribution function in the magnetosphere.
SM51B-0372 0800h
Electromagnetic Particle Code With Adaptive Mesh Refinement Technique: Application to the Plasma Sheet
It is widely believed that magnetic reconnection plays an important role in the magnetospheric substorm and solar flares. However, physical processes around the diffusion region are not well understood. Recently, it has been suggested that multi-scale coupling process should be important in the reconnection triggering and the anomalous plasma heating and acceleration around the diffusion region. It is necessary to conduct a self-consistent large-scale simulation including phenomena with various scales to describe multi-scale coupling. However, a realization of such a simulation with an ordinary PIC technique is still difficult because electron-scale phenomena are very localized and embedded in an ion-scale or MHD-scale system. To overcome this difficulty, we made a new 2-1/2 dimensional electromagnetic particle code with adaptive mesh refinement (AMR) technique. The AMR technique dynamically subdivides the cells that satisfy a refinement criterion and effectively achieves high-resolution simulations. In fact, it is only in the vicinity of the central current sheet where high-resolution simulations are required and we can reduce the number of cells in the lobe region where plasma density is low so that both the electron Debye length and a characteristic scale length are large. Though conventional AMR algorithm has been unsuitable to the parallel computation, our code is improved by adopting the fully threaded tree (FTT) algorithm developed by Khokhlov (1998), which facilitates to modify the cell structure in parallel. The AMR technique is expected to be one of the promising methods to realize self-consistent multi-scale simulations. In this paper, we report the results of some test simulations and discuss the limitations of our code, especially the reflections at the boundaries of the refined regions. We also report the results on the time developing Harris plasma sheet and show that the hierarchical cell structure is automatically reconstructed and fine cells are produced in the central plasma sheet, especially around the X-type neutral line.
SM51B-0373 0800h
Equation Free Projective Integration and its Applicability for Simulating Plasma
We examine a novel simulation scheme called equation free projective integration$^1$ which has the potential to allow global simulations of plasmas while still including the global effects of microscale physics. These simulation codes would be ideal for such multiscale problems as the Earth's magnetosphere, tokamaks, and the solar corona. In this method, the global plasma variables stepped forward in time are not time-integrated directly using dynamical differential equations, hence the name "equation free." Instead, these variables are represented on a microgrid using a kinetic simulation. This microsimulation is integrated forward long enough to determine the time derivatives of the global plasma variables, which are then used to integrate forward the global variables with much larger time steps. We are exploring the feasibility of applying this scheme to simulate plasma, and we will present the results of exploratory test problems including the development of 1-D shocks and magnetic reconnection. $^1$ I. G. Kevrekidis et. al., ``Equation-free multiscale computation: Enabling microscopic simulators to perform system-level tasks,'' {\it arXiv}:physics/0209043.
SM51B-0374 0800h
Parallel, time asynchronous modeling of plasmas: Overcoming the computational challenges in traditional (MHD, Hall MHD, Full Particle, Hybrid, Vlasov) codes
Computer simulation of many important complex physical systems such as the Earth's magnetosphere has reached a plateau because most conventional techniques are ill equipped to deal with the multi-scale nature of such systems. The traditional approach to modeling spatially distributed physical systems has been based on time-driven (or time-stepped) simulations (TDS) where the whole state of the system is updated synchronously at discrete time intervals. This method has two inherent inefficiencies with severe consequences: (i) the well-known time step restriction imposed by a global CFL (Courant-Friedrichs-Levy) condition, (ii) uniform (and unnecessary system update) computational work independent of the level of activity in a given region. We have been working on an entirely different (asynchronous) simulation methodology based on a discrete event-driven (as opposed to time-driven) approach. Our ultimate goal is to develop a 3D global multi-physics code for application to Earth's magnetosphere. Here we report on our progress where we have developed a general parallel infrastructure based on this new technique. We demonstrate the power of this technique through a 1D parallel hybrid simulation of a fast magnetosonic shock. We find that the code is over a factor of 30 faster than the traditional hybrid codes. In our technique, individual parts of the global simulation state are updated on a "need-to-be-done-only" basis and all simulation entities (individual particles/phase space elements/fluid elements, local fields) evolve on their own physically determined time scales. This has immediate implications for all types of plasma simulations. For example, one of the obstacles to the use of Vlasov codes in 2D and 3D is the fact that most of phase space is inactive but still has to be carried in the computation using standard techniques. This inefficiency makes the Vlasov codes almost unusable in 3D where the phase space (consisting of three spatial coordinates and three components of velocity) is very large. In our technique, only the "active" regions of phase space are updated. Another example is in regards to simulation of interplanetary shocks and physics of particle acceleration which remains beyond the scope of existing hybrid codes due to the multi-time scale nature of the problem.
SM51B-0375 0800h
Initial Expansion of a Magnetic Bubble
The mini-magnetosphere plasma propulsion scheme (plasma sail) involves the inflation of a magnetized plasma bubble attached to a satellite in space that is large enough to deflect the solar wind and consequently achieve velocities on the order of 50 km/s [Winglee et al., JGR, 2000]. Here we present simulations of the initial expansion of the magnetic bubble in a simplified geometry. We consider plasma injected into a stationary magnetized background plasma in the presence of a dipole magnetic field with a 2-D kinetic ion, massless fluid electron (hybrid) electromagnetic code. For small values of the dipole field, the injected ions exclude the ambient magnetic field and plasma to form a diamagnetic cavity. For large magnetic dipoles, the injected ions remain magnetized and produce additional plasma currents that expand the dipole field, leading to a magnetic bubble. A criterion to distinguish between these two regimes is derived. Issues associated with the sensitivity of symmetry of the bubble that is formed to the plasma injection scheme are also discussed.
SM51B-0376 0800h
Stability of Magnetic Bubbles
We use electromagnetic hybrid (fluid electrons, kinetic ions) simulations to investigate the expansion of a dipolar magnetic field through plasma injection and the stability of the inflated bubble in the presence of plasma loss and other processes. The simulations consist of a stationary, magnetized, background plasma and a magnetic dipole. Another population of plasma is then injected at the center of the dipole and along its axis. The initial expansion of the bubble is described in the adjoining poster. When the dipole is weak, plasma injection results in the formation of a diamagnetic cavity without any appreciable change in the dipolar field. On the other hand, when the dipole field is strong enough to trap the injected plasma, the initial field is expanded and a magnetic bubble is formed. By stopping plasma injection after the bubble formation, we show that the bubble is quite stable and can survive for times much longer than the initial expansion phase. This stability is tied to the trapping of the injected plasma which provides the necessary currents to keep the bubble expanded. It is also found that plasma loss through magnetic reconnection results in depletion of the trapped population requiring periodic injection of plasma to keep the bubble inflated. Relevance of these results to radiation shielding of energetic ions will be discussed.
SM51B-0377 0800h
Automated Technique to Determine the Magnetospheric Electron Density From Passive Dynamic Spectra
The magnetospheric electron density (Ne) is a fundamental space-physics parameter. It is often difficult to make Ne measurements to an accuracy better than a factor of two in low-density (Ne apprximately 1 cm-3 or less) space plasmas due to problems associated with spacecraft/plasma interactions which become enhanced in such an environment. Passive or active plasma-wave techniques are the experiments of choice under such conditions because they are based on the reception of plasma waves that sample a region of space around the spacecraft that is large compared to the perturbed spacecraft/plasma interaction region. Also, they are not restricted to energetic electrons as are particle detectors. The frequency of the amplitude-modulated signal received during passive observations is usually displayed as a function of time to form a dynamic spectrum. An automated fitting technique has been developed to extract Ne from dynamic spectra obtained from the Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. This technique is often able to provide in-situ Ne measurements along extended portions of the orbit of IMAGE. While this automatic fitting technique will be demonstrated with RPI data, it promises to have valuable applications to other data sets from planetary magnetospheres.
SM51B-0378 0800h
Characteristics of wire antennas onboard Akebono satellite
Characteristics of a wire antenna used for the electric field observation in the magnetized space plasma could be different from those in free space because of the plasma sheath surrounding the antenna. We need to know the antenna characteristics (antenna impedance and effective length) in the magnetized plasma to calibrate the observed data and obtain the absolute intensity of the electric field. In our previous antenna study, we have estimated the effective length from the ratio of the open voltage induced at the antenna terminal and the electric field theoretically calculated from Maxwell's equations using the observed plasma density and geomagnetic field. We have estimated the effective lengths of antennas onboard the GEOTAIL and Akebono satellites. The two types of wire antennas onboard the GEOTAIL are nearly equal to the effective length in free space. However, we see their fluctuations depending on the angle between the antenna and the sunward direction which varies with the satellite spin. The effective lengths of the two wire antennas onboard the Akebono satellite are also nearly equal to those in free space. Again, we see a small variation depending on the angle between the antenna and the geomagnetic field. The impedance of the antennas onboard GEOTAIL has been measured by Tsutsui et al.[1997] by using the calibration function onboard GEOTAIL. They have found that the impedance depends mainly on the ambient electron density and the satellite spin. The impedance onboard Akebono has been measured by Hashimoto et al.[1991], where the results also depends on the electron density and the satellite spin. For the GEOTAIL case, the dependence of the effective length and the impedance measurement on the satellite spin is caused by the fluctuation of the photoelectron emissions from the satellite body and the antenna surfaces. On the other hand, for the Akebono case, the photoelectrons do not play an important role because the spin axis of Akebono always points to the sun. In this study, we estimate capacitance component of the impedance of the two wire antennas onboard Akebono with assuming that the effective length is constant, and discuss the dependence of estimated impedance on the angle between the antenna and the geomagnetic field are found. The fluctuation range of the estimated impedance is in good agreement with the measurement. The theoretical capacitance component would not be found by using a co-axial capacitor model, because the fluctuation depending on the geomagnetic field cannot be explained by using this model. The thickness and/or the dielectric constant of the plasma sheath would change by the effect of the geomagnetic field. So we will need another model to know the accurate characteristics of the antennas in the magnetized plasma.