[1] This paper presents a focused review of the role of entropy in plasma sheet transport and also describes new calculations of the implications of plasma sheet entropy conservation for the case where the plasma pressure is not isotropic. For the isotropic case, the entropy varies in proportion to log [PV 5/3 ], where P is plasma pressure and V is the volume of a tube containing one unit of magnetic flux. Theory indicates that entropy should be conserved in the ideal MHD approximation, and a generalized form of entropy conservation also holds when transport by gradient/curvature drift is included. These considerations lead to the conclusion that under the assumption of strong, elastic pitch angle scattering, PV 5/3 should be approximately conserved over large regions of the plasma sheet, though gradient/curvature drift causes major violations in the innermost region. Statistical magnetic field and plasma models lead to the conclusion that PV 5/3 increases significantly with distance downtail (pressure balance inconsistency). We investigate the possibility that the inconsistency could be removed or reduced by eliminating the assumption of strong, elastic pitch angle scattering but find that the inconsistency becomes worse if the first two adiabatic invariants are conserved as the particles drift. We consider two previously suggested mechanisms, bubbles and gradient/curvature drift, and conclude that the combination of the two is likely adequate for resolving the pressure balance inconsistency. Quantitatively accurate estimation of the efficiency of these mechanisms depends on finding a method of estimating PV 5/3 (or equivalent) from spacecraft measurements. Two present approaches to that problem are discussed.

.[1] Recent THEMIS spacecraft observations have shown longitudinally narrow, earthward moving, plasma sheet flow channels with large, abrupt magnetic field dipolarizations that have been referred to as dipolarization fronts. We undertake a detailed study of the subset of these events that had good quality auroral imaging. We find that many occurred during the substorm expansion phase after onset (determined by auroral intensification) and were related to auroral streamers. Both the fronts' relation to streamers and their plasma sheet changes are similar to what has been previously shown for other plasma sheet flow channels and to other flow channels seen during the expansion phases of the events considered here. Surprisingly, the dipolarization fronts examined made major contributions to substorm onset ground and space magnetic signatures. These include substorm near-Earth dipolarization, which indicates that the current wedge may develop via a series of well-defined, narrow, post-onset wedge-like structures. These also include the auroral zone ground magnetic field, which showed only modest responses to the onsets but abrupt, large responses to post-onset dipolarization-front-related streamers, and midlatitude positive bays, which started near the time of streamer formation. Our observations suggest that traditional magnetic signatures of substorm onset, such as near-Earth dipolarization, auroral-zone negative bays, and midlatitude positive bays, may misidentify the expansion phase onset determined by auroral intensification by up to 10s of minutes for some events. It should be interesting in the future to determine the generality of the above relations between dipolarization front signatures and onset signatures.Citation: Lyons, L. R., Y. Nishimura, X. Xing, A. Runov, V. Angelopoulos, E. Donovan, and T. Kikuchi (2012), Coupling of dipolarization front flow bursts to substorm expansion phase phenomena within the magnetosphere and ionosphere, J. Geophys.

[1] This paper presents THEMIS measurements of two substorm events to show how the substorm current wedge (SCW) is generated. In the late growth phase when an earthward flow burst in the near-Earth magnetotail brakes and is diverted azimuthally, pressure gradients in the X-and Y-directions are observed to increase in the pileup and diverting regions of the flow. The enhanced pressure gradient in the Y-direction is dawnward (duskward) on the dawnside (duskside) where a clockwise (counter-clockwise) vortex forms. This dawn-dusk pressure gradient drives downward (upward) field-aligned current (FAC) on the dawnside (duskside) of the flow, which, when combined with the FACs generated by the clockwise (counter-clockwise) vortex, forms the SCW. Substorm auroral onset occurs when the vortices appear, Near-Earth dipolarization onset is observed by the THEMIS spacecraft (probes) when a rapid jump in the Y-component of pressure gradient is detected. The total FACs from the vortex and the azimuthal pressure gradient are found to be comparable to the DP-1 current in a typical substorm. Citation: Yao, Z. H., et al. (2012), Mechanism of substorm current wedge formation: THEMIS observations, Geophys.

[1] We have combined two empirical numerical models to estimate the entropy parameter PV 5/3 in the plasma sheet, where P is the plasma sheet pressure and V = R ds B is the volume of a flux tube containing one unit of magnetic flux. The Tsyganenko and Stern (1996) magnetic field model is used to calculate the flux tube volume, and the Tsyganenko and Mukai (2003) plasma sheet statistical model is used to calculate the plasma sheet pressure. Contour plots for PV 5/3 and V in the equatorial plane are presented for various solar wind conditions. These empirical models suggest that, although both PV 5/3 and V generally increase tailward, their gradients are generally not parallel or antiparallel to each other, whereas most theoretical discussions of interchange instability assume that the vectors are lined up. The Vasyliunas equation implies that the Birkeland current is proportional to the cross product of rPV 5/3 and rV, so that region 1 and region 2 Birkeland current flow between plasma sheet and ionosphere confirms that the gradients of entropy parameter and flux tube volume are not generally parallel. We present analytical calculations to investigate the criterion for interchange instability in a quasistatic, low-b plasma that is connected to a conducting ionosphere and has rPV 5/3 and rV at an arbitrary angle a, which implies shear flows. A boundary layer is assumed to separate two regions with different but uniform PV 5/3 . The main conclusion is that the system is interchange unstable if a > p/2. The results suggest that the statistical average plasma sheet configuration is interchange stable since the statistical plasma sheet models indicate that a < p/2. Our conclusion is quite different from analyses that neglect shear flow since they imply instability unless rPV 5/3 and rV are exactly parallel.Citation: Xing, X., and R. A. Wolf (2007), Criterion for interchange instability in a plasma connected to a conducting ionosphere,