SM11B-01 INVITED
Entropy and Plasma Sheet Transport
Different entropy-related parameters are compared, along with the assumptions involved in their approximate conservation in Earth's plasma sheet. Our discussion centers on the entropy parameter PV5/3, where V=volume per unit magnetic flux. It affects the structure and dynamics of the plasma sheet in several ways: (1) Earthward convection along with conservation of the entropy parameter leads to extreme stretching of inner- plasma-sheet flux tubes ("pressure crisis"). (2) Regions with an earthward gradient in the entropy parameter tend to be unstable to interchange, although there is still controversy about the exact instability criterion. (3) A localized region of reduced entropy parameter (bubble) moves earthward, causing a flow burst, in some instances; bubbles also apparently play a key role in ring current injection. The chief obstacle to understanding the role of entropy in plasma-sheet dynamics lies in the difficulty of measuring the flux tube volume V. We describe a method for estimating local V and entropy parameter from single-spacecraft measurements. It is not possible to test the algorithm against observations, but tests against a variety of models indicate that the mean error in the estimated V is about 8 per cent. The algorithm is based partly on the assumption of equilibrium and substantially underestimates the terminal value of the entropy parameter on flux tubes that are moving rapidly earthward.
SM11B-02
Substorm plasma sheet ion pressure profiles
The plasma sheet pressure, temperature, and density profiles inferred from DMSP observations are used to investigate substorm growth, expansion, early-recovery, and late-recovery phases. During the growth phase, the pressure peaks at the inner edge of the plasma sheet. The premidnight pressure peak is associated with the temperature peak, while the postmidnight peak is associated with the density enhancement. After the substorm onset, the pressure at the inner edge diminishes. Instead, the pressure peaks at premidnight from X = –10 to –40 RE, which can be associated with temperature enhancement. During the early and late recovery phases, the pressure peaks at postmidnight, which is associated with a cold, dense ion population, possibly resulting from ion outflow and the substorm current systems. In the near-Earth region, the entropy decreases after substorm onset, but the specific entropy appears to be roughly conserved.
SM11B-03 INVITED
Entropy conservation and entropy loss governing substorm phases and tail transport
The role of entropy conservation and loss during substorm phases is discussed on the basis of MHD theory and simulations, using a comparison with PIC simulations for validation. Entropy conservation appears to be a crucial element leading to the formation of thin embedded current sheets in the late substorm growth phase and the potential loss of equilibrium. Entropy loss (in the form of plasmoids) is essential in the earthward transport of flux tubes (bubbles, bursty bulk flows), while entropy gain from general magnetic reconnection may contribute to the tailward transport of plasmoid/flux ropes (blobs). Entropy loss may also change the tail stability properties and render ballooning modes unstable and thus contribute to cross-tail variability. We illustrate these effects through results from theory and simulations. Entropy conservation also governs the accessibility of final states of evolution and the amount of energy that may be released.
SM11B-04
Comparing Different Models for Fast Earthward Flows in the Magnetotail: Moving Flux Ropes, Unsteady Reconnection, Pressure-Depleted Plasma Bubbles, and Atypical Currents Sheets
The physics of fast earthward flows or BBFs, a major mechanism of bursty transfer of the plasma and magnetic flux in the terrestrial magnetotail, remains uncertain and controversial. A part of observations can be explained as signatures of earthward moving flux ropes or secondary plasmoids dragged by the earthward part a larger-scale reconnection region [Slavin et al., 2003]. The statistics of variations of the z-component of the magnetospheric magnetic field in the central plasma sheet [Ohtani et al., 2004] suggest no changes of the magnetic field topology for another group of BBFs. These observations can be explained as signatures of either unsteady reconnection, which remains located tailward of the spacecraft, or other phenomena that are connected but not identical to reconnection in its active phase. These are the plasma bubbles, flux tubes with the reduced specific entropy that may move earthward faster than the neighboring flux tubes due to the buoyancy force. However, the original model of bubbles arising from local reductions of the plasma pressure [Pontius and Wolf, 1990] also explains only a part of observations. Another part [Angelopoulos et al., 1992] reveals no reduction of the plasma pressure in BBFs. One more model, which explains both missing magnetic topology changes and no reduction of the plasma pressure [Sitnov et al., 2005] describes the bubble as a seam in the body of the tail plasma, which appears after the formation and tailward retreat of a small plasmoid, and which is composed of atypical, embedded and bifurcated thin current sheets. Signatures of such atypical current sheets have been convincingly demonstrated recently in CLUSTER observations [Runov et al., 2003]. In this presentation we elaborate the BBF models and compare them with 2001 and 2002 tail CLUSTER observations in the central plasma sheet. These include full-particle simulations of the secondary plasmoid formation in tail-like systems, two- and three- dimensional features and dynamical properties of atypical current sheets that constitute plasma bubbles. Comparison with data is focused on the distinction between plasma flows moving in the earthward part of a neutral line (which may be both stable and moving earthward or tailward) and similar motions of plasma and magnetic field structures associated with plasma bubbles.
SM11B-05 INVITED
Connections Between Plasma Sheet Transport, Region 2 Currents, and Entropy Changes During Substorms and SMCs
Modeling shows that energy-dependent magnetic drift leads to a divergence of particles and particle energy flux that gives significant violation of entropy conservation along the direction of the plasma bulk velocity. This is of critical importance to understanding the formation of the substorm growth-phase plasma sheet and the maintenance of a stable plasma sheet during SMC periods of prolonged enhanced convection. The particle divergence also is a divergence of the cross tail current, leading to the field-aligned currents that form the Region 2 system. Thus the Region 2 current system provides direct evidence that the modeled variations in entropy are reasonable. Shielding of the convection electric field from the inner magnetosphere is an aspect of the Region 2 system, so that the well-known development of shielding after a convection enhancement can be viewed as an additional signature of violation of entropy conservation. Region 2 field-aligned currents dramatically enhance during the substorm expansion phase in the vicinity of the Harang electric field reversal, signaling a region of greatly enhanced plasma sheet particle and current divergence. This enhanced divergence should also be a region of greatly enhanced divergence of particle energy flux, which should lead to significant reduction of plasma sheet entropy and pressure within the substorm current wedge, and such a reduction has been observed with Geotail. The observations also show a reduction of the flux tube content of particles over a broad range of invariant energies, despite the apparent injections of energetic particles inferred from fluxes measured at fixed particle energies. Evidence suggests that a reduction in the strength of earthward convection can initiate the enhanced divergence within the current wedge. Observations from the new ground radars and THEMIS will provide critical information for the evaluation and further understanding of the above processes.
SM11B-06
Transport of particles from the flanks by electric drift and diffusion within the plasma sheet
Cold particles (<1 keV) from the flanks are an important source for the plasma sheet population. However, the transport of these particles within the plasma sheet and how they affect the entropy of the plasma sheet is not well understood quantitatively. We have analyzed 11 years of Geotail data to estimate drift velocities and diffusion coefficients as a function of plasma sheet locations and the direction of the IMF Bz. The drift directions are in general toward the Earth and toward the flanks, and the drift speed is stronger during southward IMF. The diffusion coefficients are found to be higher with increasing distance from the Earth, higher near midnight than near the flanks, and higher during southward IMF. We have used the estimated drift velocities and diffusion coefficients to simulate the spatial distributions of phase space densities at different energies resulting from drift and diffusion of particles from flank sources. The comparisons between the simulation results and the Geotail distributions indicate that diffusion is important to the transport of cold particles and that the flank particle sources are stronger during northward IMF. The simulations show that strong diffusion, which is directed mainly toward midnight, allows the cold particles from the flanks to overcome electric drift, which is directed mainly away from midnight, to have access to midnight. The number of particles resulting from the combined transport decreases with increasing distance from the flanks. Since they are cold, the inward transport of the particles from the flanks leads to a significant increase in the plasma sheet density, but its contribution to pressure is small. Thus the flank particles are not likely to significantly enhance entropy of the plasma sheet or lead to a tail pressure crisis.
SM11B-07 INVITED
Plasma Entry and Kelvin-Helmholtz Modes at the Flanks of the Magnetosphere
The physics of the magnetospheric boundaries is central to the transport of mass, energy, and momentum into magnetosphere. However, for northward IMF the transport of mass into the magnetosphere is not well understood. The two major models for plasma transport into the magnetosphere are cusp reconnection and reconnection within nonlinear Kelvin-Helmholtz (KH) vortices. These transport mechanism have attracted much attention, however, related to KH modes much work has focused on two-dimensional dynamics. In three-dimensions the system has an additional degree of freedom which is important because in a 3D system the orientation k vectors of instabilities is not restricted to a two-dimensional plane. Specifically magnetic reconnection and KH modes do not have to operate in the same two-dimensional plane. Here we review the properties and the plasma transport associated with Kelvin-Helmholtz modes and magnetic reconnection for the flank boundaries of the magnetosphere in two and three dimensions. We will derive the associated mass diffusion coefficient and address aspects of the further transport of plasma in the magnetotail for northward IMF.
SM11B-08
The specific entropy of the LFM plasma sheet
The specific entropy of the Earth's plasma sheet is central to the theory of transport within the plasma sheet, relating to the stability criterion for interchange motions and thus "bubble" formation. We here use the specific entropy parameter to characterize the stability of a global MHD model in the plasma sheet during a simulated substorm. We demonstrate that the earthward-penetrating fast-flow channels are predominantly depleted of entropy, and show this non-adiabaticity is likely caused by their association with localized reconnection regions.