Plasma Sheet II
Presiding: S Wing, Applied Physics Laboratory, Johns Hopkins University Applied Physics Laboratory; M O Fillingim, University of California, Berkeley
SM52A-01 10:30h
Dawn-dusk asymmetry in the northward IMF plasma sheet
During periods of northward IMF, as a result of large influx of the magnetosheath ions, the plasma sheet becomes cold and dense. During these periods, a large number of the plasma sheet ions have two components: hot (magnetospheric origin) and cold (magnetosheath origin). Based on their spectral distributions: one-component Maxwellian, two-component Maxwellian, and kappa (k), the characteristics of the plasma sheet ions were studied with DMSP satellites and a method of inferring plasma sheet ion properties from the ionospheric observations. The cold-component constituent of the two-component ions is hotter in the dawn than the dusk sector, consistent with the in situ studies that suggest that the magnetosheath ion is heated upon its entry along the plasma sheet dawn flank. This temperature asymmetry leads to a dawn-dusk asymmetry in the ion spectral distribution. The cold and hot components are closer together in temperature space, which increases the proportion of ions having (apparent) one-component distribution in the dawn flank while, in the dusk flank, the influx of the magnetosheath ions increase the density of the two-component ions. The dawn-dusk asymmetry in the cold magnetosheath ion profile should help determine the roles of various proposed magnetosheath entry mechanisms.
SM52A-02 10:45h
FAST Observations of the Formation of the Cold Dense Plasma Sheet
FAST observations during extended periods of northward IMF provide a global picture of the formation of the Cold Dense Plasma Sheet (CDPS). These observations suggest that dual lobe reconnection, as described by Song and Russell (JGR, 1992), is the most likely source of the CDPS. One aspect of dual lobe reconnection that has not been appreciated is the dual injection of ions at both lobe reconnection sites. These ions continue to bounce-drift with the flux tube forming velocity dispersed ion signatures (VDIS) that continue around the flanks of the auroral oval. These bouncing ions have a signature with energy increasing with latitude, and form overlapping dispersive signatures as the flux tubes convect to the nightside. These observations suggest that most of the VDIS signatures observed by sounding rockets and satellites away from the cusp are actually cusp injections after several bounces. In addition, a VDIS near the cusp does not necessarily indicate the flux tube just reconnected since the observer may be seeing ions that have bounced. These results suggest that mechanisms involving impulsive ion penetration or flank magnetopause ion injections may be unnecessary to describe these VDIS signatures.
SM52A-03 11:00h
Superposed Epoch Analysis of Cold, Dense Plasma Sheet Access to Geosynchronous Orbit
We report on the occurrence of cold, dense plasma access to geosynchronous orbit. We performed a superposed epoch analysis of 1952 events of dense (> 2 /cc at onset) plasma observed by the MPA instruments onboard the Los Alamos satellites, for the period 1990-2002. The results show the temporal evolution of various plasma parameters as a function of local time. We show that dense plasma access to geosynchronous orbit mostly occurs near local midnight. Such access is also observed at slightly later times on the dawnside of geosynchronous orbit. We demonstrate that this latter population is, however, not the result of the dawnward transport of the population detected near midnight. The dense plasma population observed in the midnight region is shown to be freshly injected from the mid-tail region, colder than the typical plasma sheet and composed of a relatively small O+ component. This population is thus probably the result of cold, dense plasma sheet (CDPS) injection from the mid-tail region. The population observed on the dawnside of geosynchronous orbit is possibly the result of inward motion of the low-latitude boundary layer (LLBL). The observation of an enhanced Kp index around the arrival of the CDPS at geosynchronous orbit shows that the inward transport of these populations is allowed by an enhanced magnetospheric convection.
SM52A-04 11:15h
Comparison of the Current Sheet Thickness as Determined Using Nonlinear Dynamics Modeling and Cluster Measurements
Computer simulations of nonlinear charged particle dynamics in magnetotail like 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. Recently we have completed a survey of the current sheet structure in the region -100RE < XGSE< -20 RE. and found that the current sheet structure in remarkably constant throughput the entire region with a scale length of approximately 0.8 RE . In this paper, we compare the thickness measured by using nonlinear dynamics modeling with that made by direct measurements of the thickness using the four Cluster satellites in the vicinity of XGSE= -20 RE. This effectively serves as a benchmark for the veracity of the nonlinear modeling methodology.
SM52A-05 11:30h
Electron Acceleration in the Near Earth Plasma Sheet
In the near Earth plasma sheet, the three dimensional plasma and energetic particle investigation (3DP) on the Wind spacecraft has detected very rapid (< 1 second) changes in the electron distributions. The changes generally consist of a transition from a relatively high density, low temperature distribution to a low density, higher temperature distribution. These changes coincide with intervals of large magnetic field fluctuations, large ion velocity moments, and increases in the flux of energetic particles. The goal of this work is to determine whether the electrons are accelerated in situ or if two distinct electron populations are being sampled as the boundary between them is swept by the spacecraft. First results indicate that the change in the electron distributions is nonadiabatic in both the thermodynamic sense, i.e., the specific entropy is not constant, and in the kinetic sense: the first adiabatic invariant is not conserved. Since changes in the magnetic field coccur on timescales much longer than the electron gyroperiod, one would expect that for a local acceleration process the first adiabatic invariant would be conserved in the absence of invariant breaking processes such as wave-particle interactions. Although we cannot rule out nonadiabatic in situ acceleration, the simplest interpretation is that the electrons are accelerated elsewhere and transported to the observation point with the boundary between the ambient and accelerated electrons passing over the spacecraft in less than one second.
http://sprg.ssl.berkeley.edu/matt/AGUS2005/
SM52A-06 11:45h
Wave Particle Interactions of Velocity Dispersed Ion Structures
Cluster observations of the high latitude near-Earth plasma sheet show well defined velocity dispersed ion structures. They are associated with ion shell plasma distributions and are coincident with electrostatic and electromagnetic wave emissions. Shell distributions have a constant drift velocity in both the parallel and transverse directions, as opposed to a ring distribution, which has a drift only in the perpendicular direction. We have examined wave particle interactions that result from ion shell distributions like those observed by Cluster. We used a magnetized 2-1/2D electrostatic particle in cell (PIC) code with full ion dynamics for both the energetic shell and cold background, and guiding center electrons with full parallel dynamics. 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 isotropize in velocity space due to the wave-particle interactions. We are in the process of examining electromagnetic emissions driven by the shell distribution using an electromagnetic PIC code and also will use observed distribution functions from Cluster to drive the simulations. Simulation results will be compared with relevant Cluster particle and wave observations