On the structure of the magnetotail current sheet

Abstract

A self-consistent tail current sheet model described by an exact analytic solution of the time independent Viasoy-Maxwell equations is presented. The model has a 'slingshot' field configuration with field lines outside the plasma, .sheet slightly flared in the antisolar direction. It is pointed out that when the model parameters are adjusted to agree with the spatial variation along the tail the required thickness of the neutral sheet must be •2.5 Rr, instead of _•1 Rr as indicated by observations. Furthermore, it is shown qualitatively that a considerable velocity shear must be present in the tail current sheet if the plasma sheet is indeed much thicker than the neutral sheet. Satellite observations indicate that the thickness of the neutral sheet in the undisturbed geomagnetic tail is of the order of 1 Rr, while the plasma sheet thickness is 5-8 R• [Speiser and Ness, 1967; Mihalov et al., 1970; Meng and Mihalov, 1972]. The magnetic field in the lobes of the tail decreases from ~19 7 at ~20 R• to ~10 7 at ~60 Rr [Behannon, 1968; Mihalov and Sonerr, 1968]. The field component perpendicular to the neutral sheet also decreases along the tail; e.g., B z ~ 3.52 7 at ~20 Rr and decreases to ~0.55 7 at ~60 R• [Behannon, 1970]. Recently, Siscoe [1972] and Rich et al. [1972] concluded, based on the stress balance condition, that either the true B z is much smaller than the reported average value (3.52 7 at 20 R•) or the neutral sheet is much thicker than 1 R• or the pressure in the plasma sheet is anisotropic with P• > Pl or there is a substantial plasma flow in the quiet tail. A similar conclusion has been reached independently by Cole and Schindler [1972]. On the other hand, Bird and Beard [1972] calculated a tail current system based on the first-order guiding center theory and claim that their results are in close accord with the observed plasma sheet and neutral sheet thicknesses. However, their definition of the neutral sheet thickness is different from the definition used in some observations [e.g., Mihalov et al., 1970]. Soop aad Schindler [1972] presented a general discussion of a self-consistent current sheet for magnetospheric structures. In this paper we present a quantitative dis-Copyright ¸ 1973 by the American Geophysical Union. cussion of the tail current sheet based on an analytic equilibrium solution (O/Ot --0) of the Vlasov-Maxwell equations. This solution describes a 'slingshot' field configuration with field lines outside the plasma sheet slightly flared in the antisolar direction. If the reported field variation along the tail and the reported field magnitudes are adopted, the present model admits a neutral sheet with thickness 2h ~ 2.5 Rr, which is. somewhat large compared with the observed value (2h •< 1 Rr). In this sense, our results confirm the qualitative conclusions of Siscoe [1972], Rich et al. [1972], and Cole and Schindler [1972]. Furthermore, it will be shown that for a given neutral sheet thickness the plasma sheet can be made thicker than the neutral sheet by introducing a velocity shea...

Stability analysis of the plasma sheet using Hall magnetohydrodynamics

“…The specific value of this critical • depends on the particular field line selected for the ballooning test and/or whether the ballooning mode under consideration is compressible or incompressible. The ballooning mode seems to be stable in the very stretched field models like the Kan [1973] model. Second, we emphasize that this Hall-MHD result is not much different from the stability result of the ideal MHD.…”

MHD ballooning stability of a sheared plasma sheet

“…Clearly, for this solution form to be physically valid we 2 For our purposes we must have that q2 + 2fi2 < a n. [1996] show that if boundary effects can be neglected the purely 2-D equilibrium model of Lemb•ge-Pellat [1982] becomes unstable to the interchange mode. The other analysis of Lee and Min [1996] demonstrates that the purely 2-D model of Kan [1973] is stable to antisymmetric modes due to field line bending.…”

Comparison of multi-point measurements of current sheet structure and analytical models

“…In general the condition B z =0 requires two-dimensionality (∂ ∂x =0, ∂ ∂z =0). This problem was solved by Schindler (1972), Kan (1973), Lembege and Pellat (1982) and Manankova (2003), who presented 2-D kinetic current sheet models. 2-D fluid models have been built as a solution of Grad-Shafranov equation Schindler, 1983, 2002).…”