SH23A-1136
Density fluctuations in the solar wind : effects of nearly incompressible theory
Previously derived nearly incompressible theory with the inclusion of a simple large-scale inhomogeneous background (static and spherically symmetric) is examined by direct numerical simulations of non-stationary two dimensional turbulence. According to the theory, leading order density fluctuations satisfy a passive scalar evolution equation with additional source term induced by coupling to the large-scale inhomogeneous background. In this poster we investigate the decay rates and spectral shapes of density fluctuations together with implications for solar wind data.
SH23A-1137
Four fluid model and numerical simulations of magnetic structures in the heliosheath
A magnetic hole/hump is a stable structure with small scale minima/maxima of the mean magnetic field in the centre. Such structure has been observed in inter-planetary magnetic field, planetary magneto sheath, cometary's plasma, and very recently in the heliosheath, as revealed by Voyager I observations. Recently, we have proposed a realistic three fluid model that comprises of three fluids in the model are electrons, heliosheath ions, and neutrals. Stationary, time independent solutions of this model consisting of holes, humps, trains of holes and humps etc. were found to be consistent with Voyager observations e.g. a few tens of ion gyro-radii width, large magnetic maxima/minima, oblique angles of propagation and well approximated by Gaussians. In the first part of the present work, we extend the three fluid model to a four fluid model consisting of electrons, pick up ions (PUI), solar wind ions (SWI), and neutrals. The PUIs are generated by neutrals via charge exchange with SWI. The kinetic pressure of PUI is nearly three to four times the pressure of SWI. Hence these are more suited to mediate small scale structures in heliosheath like shocks, magnetic holes/humps etc. We show that the constant energy exchange between these two fluid drives them non adiabatic. The modified adiabatic index, is calculated by solving the corresponding enthalpy equation. The PUI are found to be isothermal ( = 1) while SWI have 1.25. In the four fluid model, these effects are captured by including a modified equation of state for PUIs and SWIs. The phase space of time independent solutions in terms of the Mach numbers of PUI and SWI is constructed to delineate the parameter space which allows structure formation. In the second part of the present work, we examine the stability of our time independent solution by evolving them via a full set of modified Hall-MHD equations. The evolutions are examined using two codes e.g. a pseudo spectral code and a code based on finite difference scheme and the results are cross checked. Our results show that (a) single structures, a hole or a hump, are robustly stable. They propagate without change of shape in homogenous plasma for long time (b) in collision of two of these structures (hole-hole, hump-hump, hole-hump), the structures are not quite stable. There seems to be a significant energy exchange between them, consequently structures which go in are different from the ones which come out of collision event. Besides a portion of energy is also transferred to high frequency noise and other type oscillations.
SH23A-1138
Perpendicular Transport in the Inner Heliosphere: A Quick and Dirty Approach
In previous studies, particle transport in the inner heliosphere is regarded as one dimensional along the archimedian spiral; any perpendicular transport is neglected. We extend Roelof's equation of focused transport to accommodate perpendicular transport in the plane of ecliptic. Numerically, this additional term is solved with the implicit Laasonen scheme for azimuthal instead of perpendicular transport - which is identical in the inner heliosphere where the archimedian field is almost radial. For typical ratios λ\|/λ\perp perpendicular transport close to the Sun is unreasonably strong. For a ratio scaled with a spiral angle dependent function we find that (a) azimuthal spread over some ten degrees occurs within a few hours, (b) the variation of maximum intensities with longitude is comparable to the ones inferred from multi-spacecraft observations, and (c) on a given field line intensity- and anisotropy-time profiles are modified such that fits with the 2D transport model give different combinations of injection profiles and mean free paths.
SH23A-1139
Whistler Turbulence: Particle-in-Cell Simulations
The broadband, incoherent magnetic fluctuations in the solar wind measured by in situ spacecraft instrumentation typically show two distinct frequency ranges: The inertial range at observed frequencies f less than the proton cyclotron frequency with power spectra which scale as ~ f-5/3 and the so-called "dissipation range" at higher frequencies with steeper power-law dependences. There are two competing hypotheses as to which basic mode populates the latter, short wavelength turbulence domain: whistler fluctuations which propagate approximately parallel to the background magnetic field and at frequencies above the proton cyclotron frequency, Ømegap, and kinetic Alfvén fluctuations which propagate at steeply oblique angles to the background field and at frequencies below Ømegap. We have carried out one-dimensional particle-in-cell (PIC) simulations in which enhanced narrowband whistler spectra are introduced; the computations show no evidence of fluctuation energy migration in wavenumber. We have also done PIC simulations in two spatial dimensions. The whistler anisotropy instability driven by an electron anisotropy produces enhanced fluctuations near kc/ωpe ~ 1; the simulations show a clear cascade of fluctuation energy to much longer wavelengths. Furthermore, our PIC simulations in which a two-dimensional spectrum of enhanced whistlers is initially introduced at kc/ωpe < 0.4 show evidence of a cascade of magnetic fluctutaion energy to kc/ωpe ~eq 1. Implications for solar wind observations will be discussed.
SH23A-1140
CIR Structures and Their Shocks at ~5AU: Ulysses Fast Latitude Scans
Corotating interaction regions (CIRs) are caused by fast stream-slow stream interactions. At large heliocentric distances, they are typically bounded by a pair of fast forward and reverse shocks Ulysses magnetic field and plasma data during fast latitude scans are used to study the structure of CIRs and their shocks as a function of heliolatitude. The fast forward and reverse shock properties (shock normal angles, Mach numbers) bounding CIRs are presented and discussed. What is perhaps most surprising is the complexity of the CIRs at middle latitudes. CIRs do not have the simple "boxcar" shape bounded by fast shocks as previously noted for events detected in the ecliptic plane. The causes of this complexity will be discussed.
SH23A-1141
Energy Dependence of Perpendicular Scattering of Electron Beams by Electron/Electron Instabilities in Solar Bursts
Solar activity such as type III bursts can produce energetic electron beams moving along the background magnetic field in the solar wind. At 1 AU observations have detected electron bursts with broader electron pitch angle distributions than predicted by magnetic focusing. Observations also have found a maximum of pitch angle width as a function of energy. If the broadening were associated with an instability, one plausible candidate would be the electron/electron instability. We have done Particle-In-Cell (PIC) simulations to demonstrate the energy dependence of perpendicular heating of the electron beams with variations of both density and beam speed. Our results suggest that relatively intense solar bursts can lead to a maximum in the pitch angle width-energy profile.
SH23A-1142
What Isolated Foreshock Density Holes Tell Us
Short-term (~20~s), order of magnitude depletions in foreshock density and magnetic field intensity have been reported recently. These density holes typically exhibit a greatly reduced solar wind beam, and diffuse-like energetic ions. There is increased wave activity, including enhanced ion cyclotron waves and whistler mode waves in their interiors, along with strong electrostatic waves near density minima. These are often, but not exclusively, observed in the presence of large amplitude ULF waves, or at the interface between quasi-parallel and quasi-perpendicular foreshock regions resulting from abrupt interplanetary magnetic field (IMF) rotations. While they are commonly seen in regions containing large-amplitude magnetic pulsations, the relationship between density holes and those SLAMS has been uncertain. A recently identified statistical sample of density holes occurring without associated SLAMS provides insight into their character and development. Here we present a few representative case studies, including events from 2002, for which short Cluster spacecraft separations permit accurate measurement of boundary normals and speeds, and from 2003 when the inter- spacecraft distances allowed for spatial and temporal variation. The occurrence of events isolated from SLAMS immediately suggests that they cannot be accounted for merely as `wake effects' downstream of flow-diverting magnetic structures. Some cases appear to show structure growth in time, while others indicate collapse. In contrast to original reports for a broader sample of density holes, these isolated cases exhibit moderate-to-strong IMF shear. This may indicate a link to hot flow anomalies that have been examined in detail previously, and which are thought to arise from interactions of solar wind current sheets with the bow shock. Published simulation studies of such mechanisms have not considered small-scale or early development phases, but such results might provide greater insight into the processes leading to density holes.
SH23A-1143
A comprehensive statistical study of foreshock density holes
Recently, Parks et al [2006]1 reported on the occurrence of sub-minute duration, order-of-magnitude density and magnetic field reductions within regions of Earth's foreshock. That paper included results from a preliminary survey of ~145 events seen by Cluster during a small number of foreshock transits. Here we present a first comprehensive survey of foreshock density holes, including events from many distinct spacecraft passes, and a significantly more robust description of the events. Characterizations include the solar wind plasma and interplanetary magnetic field (IMF) conditions in which they arise, flow and IMF shears across the structures, and values for the normal components of the convection electric fields. Where possible, boundary normals and velocities, scale sizes, local geometries and Mach numbers, as well as downstream shock geometries are determined. Comparison will be made with published characterizations of other foreshock structures, including large- amplitude magnetic pulsations, hot flow anomalies and foreshock cavities. 1Parks et al 2006, Parks et al 2007.
SH23A-1144
Statistical Analysis of the High-Frequency Spectral Break of the Solar Wind Turbulence at 1 AU
The physical mechanism responsible for the dissipation of the solar wind turbulence and the resulting plasma heating is not completely understood. To be a viable means of dissipation, any mechanism has to reproduce several observational features of the turbulence spectra. One of the important characteristics of the spectrum is its high-frequency break where the spectral slope becomes considerably steeper than the Kolmogorov-like scaling law observed in the inertial range. The onset of the spectral steepening can be inferred from the observations fairly accurately and it is a good benchmark to test various theories of the turbulence dissipation. We use a large database of magnetic field spectra and plasma parameters at 1 AU measured by the ACE spacecraft to determine the spectral break. The statistical correlation of the data points calculated according to various theoretical formulas for the break is analyzed and the least squares fits to the data are compared with the theoretically predicted scalings. We conclude that the position of the spectral break is not determined just by a scale of the turbulent fluctuations but by a combination of their scale and the amplitude at that scale. This means that the dissipation of the solar wind turbulence is an essentially nonlinear process.
SH23A-1145
Asymmetry of Power Spectra for Interplanetary Fluctuations at 1 AU
The mean interplanetary magnetic field (IMF) is thought to provide a direction that breaks the isotropic symmetry of turbulence by providing a preferred direction. Other authors have explored how the resulting axisymmetric turbulence may be described and evolves, but to date only a single form of axisymmetry has been discussed in detail: The preferential orientation of the wave vector perpendicular to the mean IMF. With this observation there comes a hybrid model for interplanetary turbulence where field-aligned wave vectors are coupled to perpendicular wave vectors and the resultant system evolves as a coupled two-component turbulent geometry. However, asymmetry can take on many forms and there are numerous predictions for the spectral index of MHD turbulence. The spectral index of the fluctuation spectrum has significant implications for the underlying dynamics and the rate of energy trasport through the spectrum. We examine ACE observations of magnetic field, velocity, density, and temperature measurements at 1 AU and ask if the spectral form of field-aligned wave vectors is similar to that of the perpendicular component. The measurements we use are at scales corresponding to the inertial range, but a better understanding of these scales will illuminate the manner in which the dissipation range is driven.
SH23A-1146
Stochastic features of small scale magnetic field fluctuations in solar wind turbulence
Since early 90's it was shown that Probability Distribution Functions (PDFs) of small scale fluctuations of solar wind parameters (magnetic field, plasma velocity, etc.) display significant departures from Gaussianity. The non- Gaussian shape of PDFs was ascribed to intermittency and discussed in the framework of intermittent MHD turbulence. Here, we investigate the stochastic nature of the small scale fluctuations of magnetic field, as observed by Ulysses, by comparing the shape of PDFs (and cumulative distribution) to the prediction of a simple theoretical probabilistic model based on the subordination principle. The results suggest that small scale magnetic field fluctuations results from the superposition of a comprise of multiscale coherent magnetic structures.
SH23A-1147
Low Frequency Wave Aspect of Density Holes
The formation of density holes involves non-linear steepening of magnetic field and plasma density, which results in shock-like boundaries followed by a drop in both density and magnetic field. In this study we present the low frequency wave properties of density holes. The propagation properties of the waves, especially the upstream edges of the structure, have been studied using Cluster's four point observations. They include the phase velocities in the spacecraft and plasma rest frames, propagation directions and polarization. We have analyzed two events in great detail. They show that for these events, the structures are convected with the solar wind, and the phase speed in the plasma frame is much larger than the Alfven velocity. The waves are mostly circularly polarized in the left hand sense. The phase velocities calculated from four spacecraft timing analysis are compared with the velocity estimated from dE/dB. Their agreement shows the waves are electromagnetic. The steepening is most likely due to nonlinear steepening of ion cyclotron waves.
SH23A-1148
The Lagrangian Map and Lie Symmetries in Magnetohydrodynamics and Gas Dynamics
We explore the role of the Lagrangian map for Lie symmetries in magnetohydrodynamics and gas dynamics. By converting the Eulerian Lie point symmetries of the Galilean group to Lagrange label space, in which the Eulerian position is regarded as a function of the Lagrange fluid label and time t, one finds that there is an infinite class of symmetries in Lagrange label space that map onto each Lie point symmetry. This involves the solution of the Lie determining equations for the fluid relabeling symmetries. We also consider a class of scaling symmetries for a gas with a constant adiabatic index. These symmetries map onto a modified form of the fluid relabeling symmetry determining equations with non-zero source terms. We investigate under what conditions the scaling symmetries give rise to conservation laws, and find that the conservation laws depend on the initial entropy, density and magnetic field of the fluid. Lie algebraic structures in Lagrange label space corresponding to the symmetries are investigated.
SH23A-1149
Energetic particles' diffusion, numerical simulations in dynamical turbulence and the comparison with theories
Nonlinear guiding center theory (NLGC, Matthaeuss et al. 2003) is considered the first theory to be able to describe energetic particles' perpendicular diffusion in good agreement with numerical simulations. Recently we present a nonlinear theory of parallel diffusion of charged particles with perpendicular scattering and dynamical turbulence to show good agreement with numerical simulations. However, in all the above simulations turbulence is not taken time-varying. In this paper we will study energetic particles' diffusion in dynamical turbulence with numerical simulations. Furthermore, we will compare the simulation results with the theories to check their agreement.
SH23A-1150
Turbulence Transport Throughout the Heliosphere
We employ a turbulence transport model to compute distributions of turbulence throughout the heliosphere. The model determines the radial dependence of three (coupled) quantities that characterize interplanetary turbulence - the energy per unit mass, the cross helicity or Alfvénicity, and a similarity length scale. A fourth integrated quantity, the plasma (proton) temperature, is modified by heat deposition due to a von Kármán type turbulence dissipation that represents decay of turbulence due to cascade to small scales. The model includes effects of two types of driving; 1) a simple model of stream shear, which is more active in the inner heliosphere; and 2) wave energy injection due to pickup protons of interstellar origin, which is more active in the outer heliosphere. Parameters for the model have been tuned using observation data from Voyager and Ulysses. We analyze the constraining observations to provide boundary conditions and parameters that vary with heliocentric latitude, with some extrapolations. The fully assembled model permits the computation of the distribution of turbulence throughout the entire heliosphere, and we present solutions for several appropriate parameter sets.
SH23A-1151
Kolmogorov Versus Iroshnikov-Kraichnan Spectra: Consequences for Ion Heating in the Solar Wind
Whether the phenomenology governing MHD turbulence is Kolmogorov or Iroshnikov-Kraichnan (IK) remains an open question, theoretically as well as observationally. The ion heating profile observed in the solar wind provides a strong, if indirect, observational constraint on the relevant phenomenology. Recent studies of a solar wind heating model based on Kolmogorov spectral scaling have produced reasonably good agreement with observations, provided the effect of turbulence generation due to pickup ions is included in the model. Without including the pick-up ion contributions, the Kolmogorov scaling predicts a proton temperature profile that decays too rapidly. In the present study, we incorporate in this heating model the energy cascade rate based on IK scaling, and show that the model yields higher proton temperatures, within the range of observations, with or without the inclusion of pickup ions. Furthermore, the turbulence correlation lengths governed by IK scaling seem to follow better the trend of observations. In addition to a self-consistent treatment of pickup ions, we also include in our study the effects of finite cross-helicity. This work is supported by DOE, NSF and NASA.
SH23A-1152
Measurements of the phase speed of the solar wind turbulence at short wavelengths
In this study we present results of electric and magnetic field fluctuation spectras in 14 intervals observed in the ambient solar wind near 1 AU, using electric- and magnetic field and plasma measurements from Cluster and ACE spacecrafts. The main focus is particularly on the energy transfer in the small scales (k ρ > 1), usually referred as "dissipation range" in literature. It is however likely that it is a second inertial range caused by the change in the dispersion relation as energy cascades to smaller wavelengths from Alfven waves in some nonlinear process and heats the surrounding plasma. In the inertial range electric and magnetic fluctuation spectras follow the classical fluid-like Kolmogorov behavior as is previously observed. The fluid-like behavior breaks down near the ion thermal gyro-radius ( ρi) and we observe an enhancement in the electric spectrum while the magnetic spectrum becomes steeper. Special interest is paid to the in-situ measurements of the phase speed (vphi(k ρi)) of the turbulent plasma and comparison of the approximated dispersion of the waves to the theoretical predictions, goal being to find out the nature of the observed waves in the "dissipation range".
SH23A-1153
Collisionless Dissipation in Hybrid Simulations of the Orszag-Tang Vortex
The nature of the spectral break in the solar wind at the ion inertial scales is one of the unanswered questions in solar wind physics because of the limitations of in situ measurements and limitations of MHD models to explore dynamics at ion inertial length scales. We study a simple system which gives us insight into dissipation processes at these length scales as a first step to answer this question. We study the dissipation processes in a turbulent plasma, set up as an Orszag-Tang vortex in a hybrid code. We do not include any explicit dissipation terms such as, viscosity or resistivity. Clear evidence of dissipation is seen in magnetic and bulk flow energies. We study the time evolution of the energy in different channels in an attempt to quantify the dissipation in our system. In the turbulent regime, the energy is seen to dissipate mostly through the magnetic channel and the bulk of the dissipated energy goes into perpendicular heating of the ions.
SH23A-1154
Solar-Wind Discontinuities and the Potential Role of Alfvénic Turbulence
Magnetohydrodynamic (MHD) simulations of Alfvénic turbulence show that a cross-field cascade predominates and generates small scale current sheets across the magnetic field. In these current sheets turbulent energy significantly dissipates. In collisionless plasmas, the width of these sheets should approach the proton inertial length or proton gyroradius and dissipation within the sheets may also occur in association with wave-particle interactions. The nearly collisionless solar wind has long been known to contain discontinuities of these widths, but the identity of these discontinuities and the originating source became unclear after Cluster spacecraft measurements established that discontinuity normals were nearly perpendicular to the background magnetic field. In addition to static tangential discontinuities, the possibility that the discontinuities arise in association with turbulence needs to be considered. We have identified over 6000 discontinuities from a 27-day period using magnetic field data at 1/3 per second resolution with the ACE spacecraft. We conclude that turbulence can account for the origin of the discontinuities, their small or zero normal field components, their small intensity change, and their correlated velocity and magnetic field fluctuations. Using cross-product normals and plasma data, we have found that discontinuity width averages about 4 proton inertial lengths at small proton β(= ratio of gas to magnetic pressure) and 4 proton gyroradii at large proton β. The distribution of separations between successive discontinuities is lognormal which can arise in association with a multiplicative random cascade. In contrast, synthetic phase-random magnetic fields are found to contain less coherent discontinuities confined mostly to small field rotations and a Poisson distribution of successive separations. Solar-wind discontinuities are then substantially coherent which is consistent with sheets generated by turbulence. This work is performed in association with the Living With A Star focus team on Heliospheric Magnetic Fields.
SH23A-1155
Quantifying the scaling properties and the crossover between the inertial and dissipation ranges in solar wind turbulence with CLUSTER magnetic field observations.
The CLUSTER high resolution magnetometer observations in the solar wind cover timescales that potentially span the inertial and dissipation ranges of local intermittent turbulence. One may anticipate a crossover in behaviour from intermittent MHD turbulence to that dominated by ion kinetic effects. Theoretical predictions for the dissipation range typically center around the power spectrum, and these include either a modified turbulent cascade with an associated range of power law scaling, or a phenomenology dominated by damping implying an exponential rolloff. Motivated by the need to distinguish these predictions, we perform statistical analyses (PDF rescaling and generalized structure functions) to quantify the scaling of fluctuations as we pass though the crossover from inertial range to dissipation range phenomenology. In particular we explore whether deviations from power law scaling are of a form that can be removed by Extended Self Similarity, so that scaling is manifested in ratios of the structure functions.
SH23A-1156
Solar Wind MHD Turbulence: Anomalous Scaling and Intermittency Effects in the Slow and Fast Wind
Although considerable progress has been made in the understanding of MHD turbulence over the past few decades through the analysis of in-situ solar wind data, two of the primary problems of solar wind MHD turbulence that still remain a puzzle are the nature of the nonlinear energy cascade, and the strong intermittent character of solar wind fluctuations in the inertial range. This intermittency modifies significantly the scaling exponents of actual power-law spectra, which are directly related to the physical nature of the energy cascade taking place in the solar wind. The identification of the most intermittent structures and their relation to dissipation represents then a crucial problem in the framework of turbulence. Anomalous scaling of both solar wind magnetic field and velocity fluctuations in the inertial range, as well as intermittency effects have recently been investigated in detail using Wavelet transforms on simultaneous WIND 3s resolution particle and magnetic field data from the 3DP and the MFi experiments respectively. Specifically, the Haar Wavelet transform is used to compute spectra, structure functions and probability distribution functions (PDFs). This powerful technique allows: (1) for a systematic study of intermittency effects on these spectra, structure functions and PDFs, thus for a clear determination of the actual scaling properties in the inertial range, and (2) for a direct and systematic identification of the most active, singular structures responsible for the intermittency in the solar wind. The analysis of structure functions and PDFs, as well as new results on the nature of the intermittent coherent structures will be presented. The turbulent properties and intermittency effects in different solar wind regimes will be also discussed.
SH23A-1157
Stochastic acceleration by MHD turbulence in Solar flares
We address the problem of particle acceleration in Solar flares. We account for turbulent energy cascade of MHD modes and their damping processes. We consider stochastic acceleration of particles by fast modes. We find that transit time acceleration dominates over the acceleration by gyroresonance. Acceleration rates are obtained with both quasilinear and nonlinear approaches and compared with energy loss rate. Scattering by fast modes appears to be sufficient to prevent the protons from escaping the system during the acceleration. Confinement of electrons, on the other hand, requires the existence of plasma waves. Electrons can be accelerated to GeV energies through the process. We estimate the total energy transferred into non-thermal particles, and show that our approach provides sufficiently accurate results.
SH23A-1158
Linear and Nonlinear Electromagnetic and Electrostatic Instabilities in a Plasma With two ion Beams
Linear and nonlinear properties of a plasma system with a proton and an alpha-particle beam, are studied. It is shown that the presence of a second beam can change the linear and nonlinear stability thresholds of the magnetosonic instabilities. Since the nonlinear dispersion relation of the nonlinear waves have 14 roots, the system is very reach in parametric decays. There are parametric decays which clearly cannot be present in a one beam system. These parametric decays are of the decay type, modulational, and essentially electromagnetic beat wave instabilities. In a previous paper [Gomberoff, 2007, JGR in press], the stabilization roperties of the magnetosonic instability triggered by the presence of a large amplitude Alfén waves for fix temperature were studied. Here the study is extended to large amplitude waves belonging to the dispersion branch having a resonance at the proton gyrofrequency, for several temperature values. It is shown that the threshold amplitude for stabilization can suffer drastic changes with respect to the one beam system. Finally, it is shown that when both beams trigger their own magnetosonic instabilities, the system can also be stabilized by the presence of the large amplitude wave.
SH23A-1159
Non-resonant heating of ions by Alfven waves with a spectrum
Non-resonant heating of ions by a monochromatic low-frequency Alfven wave, which propagates along the background magnetic field, has recently been investigated in a low beta plasma [Lu and Li, Phys. Plasmas 14, 042303(2007)]. Here, the monochromatic Alfven wave is generalized to a spectrum of Alfvén wave with random phase. It is found that ions can be non-resonantly heated with the same physical mechanism. First, ions are picked up in the transverse direction, and then phase difference (randomization) between ions due to their different parallel thermal motions leads to heating of ions. The heating is dominant in the direction perpendicular to the background magnetic field. The temperatures of ions at the asymptotic stage do not depend on individual waves in the spectrum, but are determined by the total wave amplitude. The effect of the initial ion bulk flow in the parallel direction on the heating is also considered.
SH23A-1160
Linear Mode Conversion of Langmuir to Electromagnetic Waves at Oblique Density Inhomogeneities
Linear mode conversion (LMC) of waves from one mode to another at constant frequency can occur in plasma irregularities because the natural modes in inhomogeneous plasmas need not be distinct but instead can be connected for restricted ranges of wave vectors k and frequencies f. Previous work suggests that the electrostatic (ES) Langmuir/z mode and free-space electromagnetic (EM) modes have two "radio windows", with multiple possible applications to solar radio bursts, planetary continuum radiation, auroral radio emissions, and ionospheric modification events. The first radio window is for Langmuir/z waves just above the local plasma frequency (fpe) with k almost aligned with the ambient magnetic field B0 and the density gradient \nabla n. The second involves upper hybrid/z waves with f ≈ fUH, the upper hybrid frequency, and k almost perpendicular to B0 and \nabla n. Here we use numerical simulations of the warm plasma fluid equations to investigate the detailed characteristics of LMC as functions of the outgoing EM o and x modes, k, the angles φ between B0 and \nabla N, B0, and the length scale of the density irregularity L. These results extend the recent demonstration that LMC produces both o and x-mode radiation when φ = 0 [Kim et al., Phys. Rev. Lett., 99, 015003, 2007], contrary to previous expectations that only the o mode is produced. Our new results include: (1) Only one radio window exists, smoothly linking the previous two windows as φ, f and k change. (2) Both o and x-mode radiation are produced in the unmagnetized to moderately magnetized regimes, as found for the φ = 0 case. (3) Less than 10% of the incident ES energy is transformed into EM radiation. (4) In weakly magnetized situations almost equal amounts of o and x-mode radiation are produced, but only the o mode is produced for large enough B0. Applications of these results to natural solar and planetary emissions and to ionospheric modification experiments are also explored.
SH23A-1161
Simulation of Multiple Harmonic Plasma Emission
Electromagnetic radiation at the plasma frequency and/or its second harmonic, the so-called plasma emission, is widely accepted as the fundamental process responsible for solar type II and type III radio bursts. The traditional theory based upon weak turbulence three-wave and nonlinear wave-particle interaction processes predicts radiation emission primarily at the fundamental and second harmonic of the plasma frequency. This paper presents the first demonstration of third and higher harmonic plasma emission on the basis of two- dimensional electromagnetic particle-in-cell simulation experiment. This finding indicates that under certain circumstances, fundamental-harmonic pair emission might also be accompanied by the third- and/or higher- harmonic component(s), a result that may be highly relevant to certain observations.
SH23A-1162
Simulated Coronal Type III Solar Radio Bursts: Source Region Dynamics and Remote Radiation Spectra
We have developed a 3D model for coronal type III bursts, including (1) the 3D structure of the source region, (2) a detailed simulation of the dynamics within the source of electron beams, Langmuir, ion-sound, and fundamental and second harmonic electromagnetic radiation, and (3) the propagation of the electromagnetic radiation to a remote observer. For realistic coronal conditions and beam acceleration parameters the simulated radiation dynamic spectra are consistent with the general characteristics (e.g., frequency drift rate, radiation flux and brightness temperature) of type III bursts observed at Earth. Moreover, the spectral characteristics depend quantitatively on the coronal conditions and beam acceleration parameters. We found that, in general, second harmonic emission dominates fundamental emission, since the latter is strongly damped by free-free absorption and scattering-induced damping. However, harmonic pairs may exist under some favorable conditions. A second set of simulation results imply that an interplanetary type III burst may be produced by merging of multiple coronal type III bursts as the beams propagate from the corona to the interplanetary medium.
SH23A-1163
The inertial range of MHD turbulence and Hall MHD turbulence in solar wind
Turbulent fluctuations within solar wind spectrum follows nearly Kolmogorov's power law spectrum below the ion cyclotron frequency fci. Above this frequency, the observed steeper power law is believed to be a 'dissipative range' of the solar wind turbulence. The inertial range is studied here in terms of the pseudo-energy flux, which can be shown to have a linear scaling law in isotropic, homogeneous, fully developed turbulence. Such linear scaling has recently been observed in polar wind measured by Ulysses, and represent a strong evidence that a nonlinear, turbulent cascade is at work in the solar windplasma. Moreover, we analyze magnetic field fluctuations measured onboard Cluster, lasting two decades above fci. Well defined power law and a strong increase of intermittency with frequency in this range indicates that turbulence cannot be characterized by a 'dissipative range'. Rather we conjecture that the presence of dispersive effects is responsible for a steepening of the spectral energy density, simply because a cascade can be realized in a time that is shorter than the usual eddy- turnover time. In the Hall MHD formulation magnetic power depends on the degree of plasma compressibility. Such results are compared with numerical smulation of Hall MHD.
SH23A-1164
Eigenmode Structure in Solar Wind Langmuir Waves
Bursty Langmuir waves associated with space plasma phenomena including type II and type III solar radio bursts, auroral field-aligned electrons, and radiation from shocks often exhibit localized beat-type waveforms. A consensus view on the modulation mechanism remains elusive. Current theories include multi-wave interactions, turbulence, or non-linear growth such as kinetic localization. Most of these theories start with the assumption that the density of the background plasma is near-uniform, in spite of numerous observations to the contrary. An alternative approach is to start with the assumption that density perturbations pre-exist. We construct an analytical electric field solution, describing Langmuir waves as a combination of trapped eigenmodes within a parabolic density well. This hypothesis is supported by discreet frequency structure in auroral Langmuir wave observations observed to be associated with density fluctuations, and by the high degree of localization observed in solar wind borne Langmuir waves. This simple, one-dimensional model can reproduce waveform and frequency structure of localized Langmuir waves observed by STEREO/SWAVES. The waveforms can be reasonably reproduced using linear combinations of only a few low-mode eigenmode solutions. The eigenmode solutions are sensitive to plasma environmental parameters such as the electron temperature and solar wind velocity. The trapped-eigenmode solutions can form a theoretical basis to explore the non-linear behavior of Langmuir waves which may allow for efficient conversion and escape of electromagnetic emissions and second harmonic production.
SH23A-1165
Search for Persistent Quasi-Periodicities in the Solar and Interplanetary Magnetic Fields
Previous analysis of the radial component of the interplanetary magnetic field from 1962 - 1998 has revealed a dominant frequency of 27.03 days to 0.02 day accuracy (Neugebauer, et al., 2000). We have repeated and extended this analysis with OMNI data from 1963 - 2007 obtained from the Coordinated Heliospheric Observations (COHO) database. Over this longer data string we find that the 27.03 day Lomb-Scargle periodogram peak is reduced while two side peaks near 26.8 days and 27.6 days become almost as strong. In the interval 1999-2007 there are two dominant periods near 26.5 days and 27.2 days. As a solar counterpart to the above analysis we have searched for persistent rotation periods near 27 days of global patterns of photospheric magnetic fields derived from Wilcox Solar Observatory synoptic Carrington rotation maps. Techniques applied include, principal components analysis, independent component analysis, singular spectrum analysis, wavelet spectral analysis, and complex demodulation. We find a variety of quasi- periodicities between 26 and 29 days that remain coherent for 1 - 2 years. In the southern solar hemisphere the strongest periodicity is at 28.2 days, while in the northern hemisphere it is around 26.5 days. Neugebauer, M., Smith, Smith, E.J., Ruzmaikin, A., Feynman, J., Vaughan, A.H. 2000, J. Geophys. Res., 106, A5, 8363.
SH23A-1166
On the Effect of Reduced Ion-Electron Mass Ratio in the Kinetic Plasma Simulation
In order to simulate the ion-time-scale phenomena, the reduced ion-electron mass ratio is commonly used in the fully kinetic plasma simulation. In this study, we use the contact discontinuity (CD) as an example to examine how the reduced ion-electron mass ratio might affect the simulation results under different initial conditions. We first derive the stable jump condition of the CD in ion-electron two-fluid plasma. We then use the electrostatic Vlasov simulation and the electromagnetic particle code simulation to study the CDs with different ion-electron mass ratio and with the initial conditions which satisfy the MHD jump conditions but may or may not satisfy the stable two-fluid jump conditions. Our simulation results indicate that the simulation results with reduced ion- electron mass ratio is reliable only when the initial condition is close enough to the final equilibrium state. Results obtain in this study can explain the reason why the conclusions are opposite to each other in the previous simulation studies of CDs (Wu et al., 1994; Lapenta and Brackbill, 1996).
SH23A-1167
Realistic Numerical Simulations of Solar Magneto-Convection and Oscillations
The objective of this research is to study how magnetic field affects the structure and dynamics of solar convection and the sources that drive the waves in the Sun. We use a 3D, compressible, non-linear radiative magnetohydrodynamics code developed by Dr. A. Wray for simulating the upper solar photosphere and lower atmosphere. This code takes into account several physical phenomena: compressible fluid flow in a highly stratified medium, radiative energy transfer between the fluid elements, magnetic phenomena, and a real-gas equation of state. Magnetic fields play a crucial role in the structure and heating of the outer atmosphere. Our objectives are to understand the processes related to magnetic fields in this zone. We investigate the interaction between convection, magnetic fields, and oscillations.
SH23A-1168
Helioseismic Observations of Active Regions Below the Solar Surface from SOHO/MDI
We apply the time-distance helioseismology technique to obtain 3D tomographic images of sound-speed variations and mass flow velocity maps below the visible surface of the Sun, for emerging and evolving magnetic active regions. In particular, using uninterrupted helioseismology observations from the MDI instrument on the SOHO spacecraft we investigate the development of the large complex of activity NOAA 10484-10488, which produced a series of giant proton flares in October, 2003. The flow maps reveal new interesting properties, such as strong divergent and shearing flows associated with the magnetic flux emergence and flaring activity. Using the sound-speed image we attempt to find the common roots and links of these remarkable active regions.
SH23A-1169
Temporal Intermittency with Spatially Coherent Rolls in 2D Boussinesq Magnetoconvection
We study a two-dimensional Boussinesq magnetoconvection numerically in order to make clear the difference between spatial and temporal intermittency of stochastic fluctuations near the critical bifurcation point. In this system, a temporal intermittent chaos with spatially coherent rolls is observed near the bifurcation point due to nonlinear interaction between magnetic fields and plasma convection like in the solar convection zone. To characterize the temporal intermittency, introducing a simple multiplicative stochastic model, we analyze the scale dependence of the probability density function (PDF) with an increasing coarse-grained scale of the time series. This approach has been introduced originally to study the intermittency problem of hydrodynamic turbulence. In our system, the PDF's near the critical bifurcation point display strongly non-Gaussian properties, specifying highly clustered intermittent behaviors of magnetic fluctuations. In addition, we find that the intermittent fluctuations near the bifurcation point show very slow convergence to a Gaussian similar to the Kolmogorov- Obukhov scenario for fully developed spatiotemporal turbulence, and that there exists a scaling law in the behavior of the non-Gaussian parameter as a function of the coarse-grained scale. We expect that our analysis method is applicable to the intermittent behavior of solar observational data and to the nonlinear generation of stochastic variable.
SH23A-1170
Statistical Analysis of the Nonlinear Mixing Correlations in Magnetohydrodynamic Turbulence and its Application to the Solar Wind
The velocity strain is related to the non-Gaussian nature of turbulence through the dynamics of vorticity. With the aid of a spectral closure theory coupled with the multiple-scale method, the nonlinear mixing correlations in the inhomogeneous magnetohydrodynamic turbulence is analyzed. Using the analytical results, a turbulence model for MHD turbulence is proposed. The model is expected to be useful in the MHD turbulence with the mean velocity shears. The system of model equations is applied to the solar wind, and shown to be appropriate to describe the radial evolutions of the cross helicity (velocity-magnetic-field correlation) and the residual energy (difference between the kinetic and magnetic energies) in solar-wind turbulence. The Alfven ratio (ratio of the kinetic to magnetic energies) of ~ 0.5 stationary in space in the outer heliosphere is elucidated as a stationary solution of the turbulence model.