SM11A-1163 0800h
Instabilities Driven by Ion Shell Distributions in the Plasma Sheet Boundary Layer
Recent Cluster satellite observations made in the Earth's plasma sheet boundary layer region have shown the presence of ion shell distributions coincident with broadbanded electrostatic waves. The shell distributions are observed as a field-aligned beam with a perpendicular thermal spread and are related to velocity dispersed ion structures (VDIS). We have examined ion shell instabilities in the presence of a cold ion and electron background using a magnetized 2-1/2D electrostatic particle in cell code with full dynamics ions for both the energetic shell and cold background, and guiding center electrons with full parallel dynamics. Second order accurate time integration of the ion and electron orbits is achieved through a centered combination of leapfrog and mid-point trapezoidal predictor-corrector methods. Preliminary results from the numerical simulations indicate that a combination of lower hybrid waves and ion Bernstein modes are excited accompanied by electron energy gain parallel to the ambient magnetic field as well as cold ion heating in both the parallel and transverse directions. The shell distribution tends to smear in velocity due to the wave-particle interactions. Simulation results will be compared with relevant Cluster particle and wave observations.
SM11A-1164 0800h
Modeling non-Harris Current Sheets to fit Tail Cluster Observations
Four-spacecraft Cluster mission provided for the first time an unambiguous spatial-temporal picture of thin current sheets in the tail of Earth's magnetosphere. Some new elements of this picture, such as bifurcated current sheets and their flapping motions, could not be explained by the former current sheet theory and simulations and strongly stimulated their further development. It becomes more and more clear, in particular, that the classical theory of the current sheet equilibrium proposed by Harris [1962], which was used for years in current sheet and reconnection studies, needs to be strongly generalized to fit the new observations. Here we present the new results on a generalized Harris theory, which explicitly takes into account the effects of plasma anisotropy and non-gyrotropy, as well as particle simulations based on the new family of current sheet equilibria. We explore in particular the structure of the ion pressure tensor and the distinctive features of the ion distribution function. We present a modification of the previous model of the bifurcated current sheet [Sitnov et al., 2003] with a very small plasma anisotropy outside the sheet. We show that in contrast to Harris-type models, the presence of a dawn-dusk magnetic field makes the new models asymmetric along the north-south direction. We also consider the features of anisotropic current sheet with the thickness less than the thermal ion gyroradius in the field outside the sheet, which are most relevant to 2003 tail Cluster observations. It is shown that the spatial equilibrium structure of such sheets is much less susceptible to the ion anisotropy as compared to the thicker sheets. On the other hand, different dynamics of ions and electrons in these super-thin anisotropic sheets result in their strong charging. Another interesting effect is a relatively quick further collapse of the super-thin sheets prior to their inflation and distortion in the form of kink-type flapping motions.
SM11A-1165 0800h
Multifractal analysis of magnetic field fluctuations inferred Cluster and solar wind measurements: Evidence for intermittent turbulence in the plasma sheet and solar wind.
Solar wind magnetometer and plasma data and magnetometer data acquired by Cluster in the magnetospheric plasma sheet are employed to construct probability distribution functions (PDFs) of velocity fluctuations and magnetic field fluctuations over various temporal and spatial scales. This technique, often used in analysis of laboratory plasmas, is used to look for intermittent plasma turbulence. We examined the distribution of the magnetic field and plasma fluctuations for a single spacecraft, and between pairs of highly correlated spacecraft time series data in both the plasma sheet and the solar wind (fast and slow speed streams). We demonstrate that the plasma sheet fluctuations are multi-fractal and do not rescale to a single master curve. The plasma and magnetic field PDFs are fit with the Castaing et al. [1990] energy cascade model. This model is able to capture the non-Gaussian behavior at small temporal and spatial separations in the wings of the distribution and non-self similar scaling behavior over a range of temporal and spatial separations. The model results provide an experimental test for the Castaing et al. model and allow for a quantitative comparison between different systems and events.
http://www.igpp.ucla.edu/jweygand
SM11A-1166 0800h
Temperature Anisotropy Constraint In The Plasma Sheet
Geotail observations of proton temperature anisotropies in the magnetotail show a sharp upper bound at - 1 > 0 vs. where and // are directions in reference to ambient magnetic field and . The same upper bound has been observed in the magnetosheath, in the solar wind, and in the outer magnetosphere. Hybrid simulations have shown that this constraint is due to proton scattering by the electromagnetic proton cyclotron anisotropy instability. Our observations show a spread of three decades in domain and effectively extends previous results to high region. The bi-Maxwellian assumption is tested and found to be valid in high region while it breaks down in low regions, thus it holds in the plasma sheet, which is a necessary condition for above theory and simulations. For the opposite sense of proton temperature anisotropy, the upper bound of 1- > 0 vs. in = [1,10] domain is also shown; simulations predict that this constraint is due to scattering by the electromagnetic proton firehose instability. However, waves excited by these temperature anisotropies are often obscured by background fluctuations.
SM11A-1167 0800h
Observations of ion velocity space holes associated with magnetic field fluctuations in the plasma sheet
During geomagnetically active periods, magnetic field fluctuations are frequently observed. We present Cluster ion observations obtained in the near-Earth magnetotail from 15 to 20 Re. When field fluctuations are prominent, ion distributions become irregular. In particular, holes appear in ion velocity space distributions in the tailward direction. On the other hand, ions moving in the Earthward direction are accelerated and form beam-like distributions. The holes in velocity space are gyrophase dependent and suggest a sink of particles nearby the spacecraft. The directions of ion holes and beam-like distributions are quite steady even though magnetic field direction is changing. This results in high anisotropy in V$_\perp$, which yields large velocity moments perpendicular to the magnetic field. The presence of large $\partial$B/B suggests that a temporal mechanism (for example, scattering by waves) plays a role in the formation of the gyrophase-dependent holes and beam-like distributions. Possible mechamisms responsible for these observations will be investigated.
SM11A-1168 0800h
Magnetotail thin currents sheet equilibrium: Influence of electron pressure anisotropy
The magnetotail current sheet is the site of magnetic energy storage and subsequent release during substorms, and thus plays a key role in the evolution of the magnetosphere. Recent multi-spacecraft measurements, especially by CLUSTER, have motivated many studies, in particular their small scale and multi-dimensional structure. A self-consistent 1D analytical model of thin current sheets in which the tension of the magnetic field lines is balanced by the ion inertia rather than by the plasma pressure gradients was developed earlier and this model is used to study the effect of electron anisotropy and the electrostatic field that ensures quasi-neutrality. It is assumed that the electron motion is fluid-like in the direction perpendicular to the magnetic field and fast enough to support quasi-equilibrium Boltzmann distribution along the field lines. Electrostatic effects lead to a narrow but sharp peak of electron current in the very center of the sheet due to fast curvature drift of the particles in this region. The corresponding magnetic field profile becomes much steeper near the neutral plane although the total cross-tail current is dominated by the ion contribution. The dependence of electrostatic effect on the ion to electron temperature ratio, the curvature of the magnetic field lines, and the average electron magnetic moment are also analyzed. The implications of these effects on the fine structure of thin current sheets and their impact on substorm dynamics will be presented.
SM11A-1169 0800h
Three Dimensional Energetic Electron Distributions Observed During Substorms: Using a New Mode the L3DD Data of the Cluster RAPID Experiment
As of May 13, 2004, new software patches had been uploaded to the four RAPID (Research with Adaptive Particle Imaging Detectors) instruments having an electron energy range of 20-400 kev to produce the L3DD data. The L3DD product allows the data from energy channel 1 and channel 3 (counting 1-8) to be fully resolved three dimensionally for all four CLUSTER satellites. Here we report our examination of the 3-D pitch angle distributions (PAD) of energetic electrons as the CLUSTER satellites cross the earth neutral sheet near their apogee in the nightside magnetosphere during the tail season (July, August, September, October of year 2004) . We identify several energetic electron ejections associated with substorm events and analyze their PAD evolution in the two available energy channels for all four satellites. The PAD evolution and its energy dependent or independent dispersion provide us with an understanding of the particle motion and insights into acceleration processes in the neutral sheet where the topology of the magnetic field is changing dynamically. Energy spectra at different pitch angle are studied in detail as well. Finally we will compare those events with the previous Nov.13, 2003 event observed by the CLUSTER SC2 satellite and produce a timeline for each event.
SM11A-1170 0800h
Motion of near-Earth neutral line in substorm recovery phase
A primary characteristic of the plasma sheet boundary layer(PSBL) is the presence of high speed field-aligned ion beams with abrupt low-speed cutoffs. Onsager et al.[\textit{JGR,96},20,999,1991] proposed a model of the formation of the PSBL which assumed a steady spatially extended source of plasma, together with steady equatorward ExB convection due to reconnection at a downtail neutral line. Elphic et al. [\textit{JGR,100},1857,1995] applied this model to ISEE 2 observations to estimate the downtail distance of the neutral line. The estimated locations were more than 65 $R_E$ downtail from the Earth, which seem to correspond to the distant neutral line(DNL). In this study, we estimated the distance of a near-Earth neutral line(NENL), which is inferred to be formed during substorms, by applying the velocity filter effect model of plasma sheet boundary layer(PSBL). The method was applied to counter-streaming ion beams observed by Geotail during the substorm event at 11:02 UT on Octorber 5, 2000. In the best case,we can estimate the downtail distance of the NENL with an accuracy of 10%. The estimated values range from 20 to 55 $R_E$ downtail, and increased with the total pressure decrease. This is consistent with previous statistical studies [\textit{Nagai et al.,JGR,101},4419,1998;\textit{Miyashita et al.,JGR,108},1353,2003]. In contrast to the past studies, however, our approach allows us to monitor the distance of the NENL continuously and to address its dynamics in the course of substorms.
SM11A-1171 0800h
Formation of Plasmoids and Magnetic Flux Ropes during Southward IMF
We have shown previously that 2D global hybrid (electron fluid, kinetic ions) simulations capture correctly the physics of the many of the important boundaries in the Earth's magnetosphere. Given that 2D simulations are much less CPU intensive than 3D, we have used them more extensively to study kinetic structure of magnetospheres. Here we report on the results of simulations during southward IMF and the structure of plasmoids and flux ropes formed at the magnetopause. These structures undergo large changes as they are convected away from their point of origin. This can include coalescence of multiple magnetic islands, changes in the core field and other internal structures. There are two effects that can lead to differences in regards to dynamics of reconnection in 2D versus 3D global simulations: (1) In 2D, only plasmoids can form whereas in 3D it becomes possible to form magnetic flux ropes with varying pitch angle. (2) In 3D the plasma flow around the obstacle is more complex whereas in 2D it is confined to one plane. The consequences and relative importance of these two effects will be illustrated.
SM11A-1172 0800h
Electron Physics in Slow-Mode Shocks
Electron dynamics and dissipation in collisionless slow-mode shocks are examined using one-dimensional hybrid (kinetic ions, massless fluid electrons) and full particle (kinetic ions and electrons) simulations. The dynamics of slow shocks at very oblique shock angles (84 degrees) are explored for the upstream ion and electron beta value of 0.1. For these very oblique angles, results from hybrid simulations using an adiabatic electron fluid differ from results using the full particle code, which indicates that the ion dissipation alone is inadequate to set up the shock, and that additional electron physics is needed. Full particle simulations show that the downstream electron temperature becomes anisotropic at very oblique angles, i.e.,T\_e,par $>$ T\_e,per, where the subscripts are directions (parallel, perpendicular) with respect to the local magnetic field. The anisotropy results from both the large mirror effects and the electron heating due to the parallel electric field of very obliquely propagating kinetic Alfven waves. These primarily electrostatic waves that provide the heating give rise to steepened, spiky density fluctuations in the shock ramp. As a consequence, finite off-diagonal electron pressure tensor terms (quasi-viscous effects) are generated in the simulation frame. Inclusion of quasi-viscous effects in hybrid simulations with a model for the parallel wave heating allows the very oblique slow shocks observed in the distant magnetotail to be efficiently modeled.
SM11A-1173 0800h
FUV remote sensing of the isotropic boundary and magnetotail stretching
Several studies attempted to identify the Isotropic Boundary (IB) defining the limit between the adiabatic and non-adiabatic trajectories of the trapped protons along closed magnetic field lines. This boundary is an indicator of the amount of magnetic field lines' stretching in the magnetotail. These studies are based on in situ measurements, resulting in spatially and temporally restricted samples. To avoid this limitation, we propose to use global data obtained with the FUV-SI12 proton imager on board IMAGE spacecraft. We determine at each magnetic local time the position of an optical boundary equivalent to IB and thereby the stretching of the magnetic field lines. We show that the correspondence between the latitude of the maximum proton precipitation observed by SI12 and the IB measured by DMSP satellites is statistically established and depends on the magnetic local time. The relation between the position of the maximum proton precipitation as well as the intensity of this maximum and the magnetic field's distortion is determined by comparison with GOES-8 data. We thus suggest that SI12 can be used as a tool for the global determination of the isotropic boundary and to monitor the amount of stretching in the magnetotail.
SM11A-1174 0800h
A Theoretical Model for Bursty Bulk Flows
The bursty bulk flows (BBFs) are known to have abrupt increases of the plasma flow velocity and the intensity of the northward/southward magnetic fields, occasionally followed by periodic variations with 1-3 minute characteristic time. In this paper, a theoretical model for the BBFs is proposed. In our model, a region of dawnward electric field is assumed to propagate earthward , which corresponds to the propagation of the fast-mode wave front generated by magnetic reconnection. The coupling between the magnetosphere and the ionosphere is modeled by means of an equivalent electric circuit. We made a series of test particle simulations with an assumption that each test particle corresponds to a magnetic flux tube. We further obtained an analytic solution that approximates the numerical results and suitably explains the main characteristics of actual BBFs.
SM11A-1175 0800h
Liapunov Stability of Plasmas: An Application to the Earth's Magnetotail at Substorms
Methods of understanding stability of complicated dynamical systems without the need to find a particular solution for the system date back to Riemann, Poincare, and Liapunov. These methods exploit the fact that the equations governing dynamics of many physical systems are Hamiltonian in nature, and have been successfully applied to many types of physical systems. Here, we present the general (nonlinear) stability analysis of the near-Earth magnetotail during late growth phase of substorms just prior to the onset. We use the Grad-Shafranov equilibrium constrained by the CANOPUS photometer data to obtain the configuration of the region of the magnetotail in the near-equatorial region between 5 and 30 earth radii. Then we apply Liapunov stability analysis to find a possible coincidence between the onset and the transition from stable to unstable configurations. Also, we analyze the influence of the topology of the magnetic field on the stability of the system, and we discuss a possible scenario for the onset dynamics.
SM11A-1176 0800h
Ballooning Instability in the Magnetotail: MHD Simulations
Recent progress in observations and modeling of magnetotail dynamics has strengthened the case of ballooning instability as a trigger mechanism for substorm onset. We report our findings on the ballooning instability of the near-Earth magnetotail from initial-boundary value MHD simulations. The simulations focus on the stability of analytic 2D static magnetotail equilibria developed by Voigt. The linear stage of the pressure-driven instability has been extensively studied by examining the parametric dependence of the growth rate of a single Fourier eigenmode in 2D MHD simulations on the wavenumber $k_y$, plasma $\beta$, and the thin current sheet width. The thinning of the current sheet is found to effectively enhance the regime of unstable $\beta$ as well as the growth rate of the linear ballooning instability of the near-Earth magnetotail, suggesting a new scenario for the substorm trigger within the context of the linear theory. To study the nonlinear behavior, particularly the possible explosive growth of the ballooning instability that can cause current disruption at near-Earth distances, 3D MHD simulations with sufficiently high resolution along all three coordinate directions have been developed in the PETSc framework. Results from linear and nonliner simulations will be presented.
SM11A-1177 0800h
Observations of a plasma transient on lobe field lines during a substorm. Interball Tail observations on October 3, 1995
On October 3, 1995 the Interball Tail spacecraft was located in the Northern hemisphere on lobe field lines (XGSM | -14.8 RE, YGSM | -8.0 RE and ZGSM| 18.4 RE). Solar wind conditions monitored by the WIND and Geotail spacecrafts were quite stable with a long lasting southward magnetic field, which is favorable for formation of the plasma mantle behind the cusp region. Indeed, the Interball Tail observed a typical plasma mantle. However, at ~15:07 UT a strong plasma transient with enhanced number density up to 6-8 cm-3, velocity of ~ 400 km/s and a depressed magnetic field was detected. This transient is quite unexpected because of solar wind conditions. However, we have found that the magnetic field profile of the transient correlates well with the H component of the geomagnetic field measured by the Tixie Bay station. Ground based measurements indicate that this transient was observed a during strong substorm. We argue that this transient is probably more dense mantle plasma, which can be observed at the Interball Tail location provided that the current on the tail magnetopause is depressed. As a result the magnetic field is depressed also and the magnetic field pressure becomes less than the plasma pressure. This depression probably reflects the response of the tail magnetopause to a changing of the global current system of the magnetosphere caused by the substorm.
SM11A-1178 0800h
Spatial Structure of Alfven and Sound Speeds in the Magnetotail
Magnetohydrodynamic (MHD) waves are believed to be a global indicator of magnetotail processes. Even if the wave source is localized, the wave propagates to various parts of the magnetosphere and thus dynamical phenomena in the magnetotail would be monitored. Since the plasma density and magnetic field are spatially variable, the phase fronts of the MHD waves are highly distorted due to inhomogeneity. Thus, it is important to understand the global spatial structure of MHD wave speed. To calculate the wave speed in the magnetotail, we use the plasma and magnetic field data acquired from the Geotail spacecraft in two years (1996-1997). From these statistical values, we present the map of Alfven and sound speeds in the near-Earth magnetotail ($\vert \rm X_{GSM} \vert = 10 \sim 30 R_E$, $\vert \rm Y_{GSM} \vert < 10 R_E$). We also investigate the effects of seasonal variations and geomagnetic activities on these speed profiles.
SM11A-1179 0800h
A Statistical Comparison of the Geotail and MHD Plasma Sheet
We present a statistical comparison of the near-earth central plasma sheet properties derived from Geotail observations and MHD simulations. We bin Geotail observations from 1995 to mid-1998 in the XY$_{GSM}$ plane and compare them with binned parameters from a 2 month simulation of the Lyon-Fedder-Mobarry (LFM) model. The distributions of solar wind inputs driving the simulation are statistically similar to the solar wind inputs during the 3.5 years of Geotail observations. This study improves on an earlier Guild et al. result by comparing the Geotail results to a longer simulation, yeilding a better statistical solar wind comparison between the two studies. Similar results from the data and model analysis serve as the basis for a statistical validation of the LFM model in the plasma sheet, whereas differences illuminate model shortcomings.
SM11A-1180 0800h
Determination of the Quiet-Time Far-Tail Current Sheet Thickness Using Geotial CPI Data and Nonlinear Dynamics Modeling
Computer simulations of nonlinear charged particle dynamics in magnetotail0like magnetic fields have pointed towards the existence of an ion distribution function signature that manifests itself as a serried of peaks and valleys. The separation of the peaks has been shown to scale as the fourth root of the normalized energy, where the normalization in turn depends on the parameters that describe the magnetic field structure, i.e. the current sheet half-thickness and the ratio of the magnetic field strength at the mid-plane to the asymptotic magnetic field strength. Using ion distributions functions and magnetic field data from the GEOTAIL spacecraft, it has been shown that this signature may be used to determine the quiet-time (Kp < 2-) current sheet thickness. In this paper, we extend our previous work in this area to include measurements in the regime -100$R_E$ < $X_{GSE}$ < -20$R_E$. We have also improved our ability to estimate the magnetic field strength at the mid-plane by employing a minimum variance analysis to obtain the magnetic field component normal to the current sheet. Preliminary analysis of the data indicates that the current sheet is remarkably uniform throughout the region