SM24A-01 INVITED
Realistic Plasma Sheet Models: Kinetic Reconstructions Using Cluster
The Harris model has served for many years as the de-facto standard kinetic model for the tail plasma sheet. However, the model only rarely agrees with detailed satellite observations beyond roughly fitting the current profile. We present a reconstruction technique which can be used to fit the generalized quasi-isotropic class of models (which includes the Harris model and many others) to Cluster data. The resulting time-stationary reconstructions are self-consistent, with kinetic ions and fluid-like electrons. We apply the reconstruction technique to 30 Cluster plasma sheet crossings when the scale size was comparable to the ion inertial length. The 30 reconstructions show 3 different typical cases. Most commonly, the current is carried by diamagnetic ion currents, with small embedded perturbations from electron currents. Less common are thin, bifurcated sheets driven by electron Hall currents. A few cases are supported by anisotropic electron currents. The reconstructions point out the essential properties of a realistic plasma sheet model, and give insight into the stable thin current sheets that provide the initial conditions for reconnection and/or current disruption
SM24A-02
Cluster View of the Plasma Sheet Boundary Layer and Bursty Bulk Flow Connection
What is commonly referred to as the "plasma sheet boundary layer," or the PSBL, is a high-latitude region of recurring bursts of central plasma sheet-like ions (and electrons) with complex filamentary structure and more or less distinct energy dispersion. As confirmed by the multipoint Cluster observations, these ion bursts, mostly protons, have a transverse fine structure that may span but a few gyroradii of a 10-keV proton at 4 to 7 Earth radii (RE) distance. As proven by Polar (and other) ion spectral measurements, the proton energy dispersions, when distinct, are temporal in nature. The rate of energy dispersion varies a great deal (minutes to tens of minutes), indicating a burst source that is more than 100 RE down the tail at times, but may be inside of 20 RE at other times. The latter is where most observations of earthward directed "bursty bulk flows" of protons, or BBFs, have been made near the equatorial plane. Provided that similar BBFs occur over a large length of the magnetotail, these flow structures are most likely the source of the PSBL flow bursts. That there is a physical connection between the two kinds of proton flows is suggested by (1) the similarly bursty and structured nature of both, including gyroradii-sized gradients, (2) the earthward bulk flow of the BBFs, and (3) the fact that the BBFs are invariably associated with a transient local increase of the northward tail Bz component and a local decrease of the plasma density. Point 2 explains the lack of latitudinal energy dispersion of the PSBL protons, and Point 3 allows the equatorial plasma to expand earthward along higher-latitude field lines. These points are illustrated here by Cluster magnetic field and ion data from as far as 19 RE near local midnight in the equatorial plane.
SM24A-03
The plasma sheet PV5/3 and nV and associated particle and energy transport for different convection strengths and AE levels
The spatial variations of PV5/3 and nV, where P is total plasma pressure, n is number density, and V is the flux tube volume per unit magnetic flux, are crucial to understanding the large-scale plasma transport and its associated energy transfer in the plasma sheet. We have used 10 years of Geotail data and a formula for estimating V to investigate statistically the equatorial distributions of ion PV5/3 and nV in the nightside plasma sheet between r = 10 to 30 RE. We have examined the distributions under three different conditions according to the cross polar-cap potential drop and AE index: (1) weak convection and low geomagnetic activity, (2) enhanced convection and low geomagnetic activity, and (3) enhanced convection and high geomagnetic activity. The overall nV is seen to decrease significantly with increasing convection or activity, but the overall PV5/3 remains similar. The decrease of nV can be seen when convection increases during the same low activity, suggesting that a decrease of source particles, likely the cold particles from the flanks, may play an important role. We compared the variations of PV5/3 and nV along drift paths with the physical bases of ideal MHD and the Rice Convection Model (RCM). We found that PV5/3 drops significantly earthward along the estimated electric drift paths near midnight, indicating that it is far from the assumption d(PV5/3)/dt = 0 used in ideal MHD. Examination of PkV5/3 and the electric and magnetic drift paths of different energy invariants, where Pk is the partial pressure of a specific energy invariant, shows that the strong duskward drift of the above thermal-energy particles due to the magnetic drift, together with there being significantly fewer higher-energy particles from the dawn flank than from the tail, result in the strong earthward decrease of PV5/3. We also found that PkV5/3 does not change significantly along the electric and magnetic drift paths of particles with energies above the thermal-energy, indicating d(PkV5/3)/dt = 0 used in the RCM is a good approximation for the pressure-bearing plasma sheet ions. These comparisons show that the RCM physics can better account for large-scale plasma sheet thermodynamics.
SM24A-04
The Boltzmann H Function and Entropy in the Plasma Sheet
The Boltzmann H function was evaluated by integrating the product [f*(lnf)] based on ten years of 1-minute averaged electron and ion velocity distribution functions, f(v). Data came from the magnetometer and Comprehensive Plasma Instrumentation on the Geotail satellite. Long term averages of H, of particle densities, and of the magnetic field were used to determine the entropy density, entropy per unit flux tube, and entropy per particle. Spatial variations near the neutral sheet of the long term averages of these parameters are being studied in the -30 < x < -10 Re region. The observations contain information about irreversible processes that take place in the plasma sheet. The data also provide information about the spatial distribution of deviations from equilibrium. The current status of this project will be described.
SM24A-05
New Features of Electron Phase Space Holes Observed by the THEMIS mission
Observations of large-amplitude (>100 mV/m) electron phase-space holes by the THEMIS satellites indicate several new features including a magnetic perturbation (δ B∥) parallel to the ambient magnetic field (B). We interpret this signal as diamagnetic currents from the electron density cavity. Under this interpretation, δ B∥ is directly proportional to the electron density perturbation and thus can be used as a diagnostic of the density structure. Many of the structures also have magnetic signature (δ B⊥ ~ 0.1 pT) perpendicular to B, which we interpret as due to high speeds (>107 m/s) along B. The high speeds suggest that the electron holes are generated by an electron two-stream interaction, but a Buneman instability cannot be ruled out. In addition, we show that electron holes can be physically large in the direction along B (50-100 Debye lengths) and, when combined with the large electric field signal, have a signification potential well associated with them (Φ ~ kBTe/e, on the order of a kV). This large potential suggests strong, nonlinear behavior nearby, such as double layers or magnetic reconnection. Another rare property is also observed. A set of the high-speed electron holes indicate a "cigar" shape (E⊥ / E∥ > 1) in which the perpendicular scale appears to be only a few electron gyroradii. The majority, however, have a extended perpendicular direction (E⊥/E∥ < 1). The THEMIS "burst" (high time resolution) waveforms are selected on board by the electric field amplitude, so we study a subset of these structures with high amplitudes. Almost all occasions of electron phase space holes observed by the THEMIS satellites during a three month magnetotail phase were found at the boundary between the dense plasma sheet and the tenuous lobe plasma. Almost all events were at periods of increased auroral activity so they seem to be associated with reconfiguration of the magnetotail due to substorms. These results suggest that electron phase-space holes are an indicator of the large-scale behavior of the plasma sheet.
SM24A-06
Anisotropies of the Taylor Scale, Correlation Scale, and Effective Magnetic Reynolds Number Determination from Plasma Sheet and Magnetic Field Fluctuations
Cluster data from many different intervals in the magnetospheric plasma sheet are employed to determine
the magnetic correlation scale and the Taylor microscale from simultaneous multiple point measurements.
For this study we define the correlation scale as the exponential decay constant of the correlation coefficient
as a function of spacecraft separation and the Taylor scale as the radius of curvature of the correlation
coefficient values at zero separation. The present determination of the Taylor scale makes use of a novel
extrapolation technique to derive a statistically stable estimate from a range of measurements at small spatial
separations [Weygand et al., 2007]. Using all the plasma sheet data the Taylor scale length is found to be
largest (about 3,000 km) in the direction parallel to the magnetic field and smallest (about 1,200 km) in the
direction perpendicular to the magnetic field. Similarly, the correlation scale length is found to be largest
(about 16,000 km) in the direction parallel to the magnetic field and smallest (about 9,000 km) in the direction
perpendicular to the magnetic field. However, the parallel correlation scale systematically decreases with
increasing geomagnetic activity while the perpendicular correlation systematically increases. The effective
magnetic Reynolds number can be expressed in terms of the correlation scale and the Taylor scale. The
difference in the Taylor and correlation scale in the parallel and perpendicular direction indicates that the
effective magnetic Reynolds number varies with the direction of the magnetic field and has values between
10 and 200. Knowledge of the effective magnetic Reynolds number may be useful in magnetohydrodynamic
modeling of the magnetosphere and the solar wind and may provide constraints on kinetic theories of
dissipation in space plasmas.
http://www.igpp.ucla.edu/jweygand
SM24A-07
Aurora secondary electrons in the plasma sheet: A Cluster study
A population of bidirectional low energy (~100 eV) electrons is commonly observed within the magnetotail plasma sheet by Cluster, in coexistence with the hot plasma sheet electron population. We interpret the cold electron population to be secondary electrons formed by the precipitation of higher energy particles [Kleitzing and Scudder, 1999]. Subsequent diffusion in pitch angle spread the cold electrons out of the loss cone to form a trapped population in the plasma sheet. Using an automated procedure of spectral fitting, we identify the presence (or absence) of this population, as well as parameters such as temperature and beam half-width. We expect the temperature of the cold population to correlate with the temperature of the collocated hot plasma sheet if this precipitation of the latter is really the source. The pitch angle width of the cold population will provide information on the rate of pitch angle scattering. We will also compare the plasma sheet state during the presence and absence of the cold electron population, in an attempt to understand the generation mechanism better. This cold electron population may be useful as a tracer for diffuse aurora, in a region where the precipitation loss cone cannot be distinguished with current instrumentation.
SM24A-08
Counter-Streaming Beams and Flat-top Electron Distributions Observed with Langmuir, Whistler and Compressional Alfvén Waves in Earth's Magnetic Tail
Low energy (< 1keV) counter-streaming and flat-top electron distributions accompanying fast ion bulk speed flowing tailward (~350 km/s) have been observed by Cluster spacecraft in the vicinity of the current sheet in Earth's magnetic tail (-18 Re). These distributions were detected simultaneously with Langmuir (2-6 kHz) and whistler mode (0-50 Hz) waves. The streaming and thermal energies of the electron beams vary from ~150 to 800 eV and ~50 to 100 eV where the higher values correspond to measurements made closer to the current sheet (CS). In the central CS, the electron distributions are isotropic and the beams disappear as do the Langmuir and high frequency (8-50 Hz) whistler waves. At the edges of this region, the magnetic field (B) and density gradients are strong and can increase the propagation angle of the whistler waves that preferentially propagate parallel or anti-parallel to B. Thus, the strong magnetic and density gradients could prevent the whistler waves from propagating deeper into the CS. Instead, low frequency (< 2 Hz) whistler waves (compressional Alfvén waves) are observed with perpendicular or oblique wave vectors. These Alfvén waves fade away in the central part of the central CS where the plasma beta (β=2μ0nkT/B2) reaches ~ 100. Theoretical explanations for these observations must take into account that ions are non-magnetized and that dynamics occur on scales smaller than the ion Larmor radius.