On large‐scale rotational motions and energetics of auroral shear layers

Abstract

The stability, dynamics and energetics of an auroral shear layer are considered in the framework of incompressible, one‐fluid magnetohydrodynamics, under conditions where current flow through the system is limited by the finite Pedersen conductivity and an enhanced field‐aligned resistivity. The model includes a magnetospheric region where currents resulting from polarization electric fields and viscous forces are important, an ionospheric substrate of uniform conductivity, and a force‐free acceleration region, characterized by a linear current‐voltage relation and located at an intermediate altitude between the magnetospheric viscous/polarization layer and the ionosphere. It is assumed that the Alfvén wave transit time across the viscous/polarization layer is small compared with the eddy time. Neutral stability of the model system is determined for a class of one‐dimensional equilibria in which a specified current distribution at the upper boundary of the viscous/polarization layer produces a potential structure with convergent, localized reversals in the transverse (E×B) electric field. The calculated neutral curves depend on three nondimensional parameters related to the intensity of the imposed field‐aligned current, the shear layer scale size, and the ratio of resistive to viscous drag at equilibrium. Numerical simulations of unstable configurations show that (1) two‐dimensional quasi‐steady rotational states arise when the equilibrium is weakly unstable; (2) eddy shedding turbulent states can arise when the equilibrium is strongly unstable; and (3) the flow kinetic energy and energy input/dissipation rates in the model system are reduced as a consequence of the instability. Power spectral densities for the electric and magnetic fields are also evaluated along sample “satellite” cuts through the shear layer. An application to postnoon auroral forms confirms the tendency for 2D rotational motion and periodic bright spots, although the observed intensity of the upward field‐aligned current suggests that the effective resistivity of the system is not sufficient to suppress inductive fields in the vortex dynamics.

Stabilization of the Kelvin‐Helmholtz instability by the transverse magnetic field in the Magnetosphere‐Ionosphere Coupling System

“…Previous electrostatic models studying the stability of the velocity shear layer in the M-I coupling system included the ionospheric coupling, but they are not fully 3-D, because the magnetosphere is either height-integrated [Keskinen et al, 1988] or a current-voltage relationship has been used [Lotko andShen, 1991' Wei andLee, 1993].…”

Coupling of magnetopause‐boundary layer to the polar ionosphere

“…FACs and found that the potential profile corresponds to observed inverted V events. The two-dimensional model for the nonsteady boundary layer flow dynamics including the ionospheric drag and a parallel electric field was also developed [Lotko et al, 1987;Lotko and Shen, 1991]. Lotko et al [1987] showed that the sheared plasma flow couples to the polar ionosphere and may be responsible for the formation of V shocks in and above the auroral acceleration region.…”

“…Lotko et al [1987] showed that the sheared plasma flow couples to the polar ionosphere and may be responsible for the formation of V shocks in and above the auroral acceleration region. The stability, dynamics, and energetics of an auroral shear layer are studied by Lotko and Shen [1991]. Nonlinear evolution of the KH instability in the polar ionosphere due to the sheared plasma flow in the magnetosphere including the finite Pedersen conductivity has also been studied [Keskinen et al, 1988].…”

Hybrid Kelvin‐Helmholtz/Rayleigh‐Taylor instability in the plasma sheet

“…The electrostatic Kelvin‐Helmholtz (KH) instability has been very often invoked as a possible mechanism for the auroral deformations [e.g., Hallinan and Davis , 1970; Miura and Sato , 1978; Wagner et al , 1983; Murphree et al , 1994; Yamamoto et al , 1994]. For realistic modeling, proper attention should be paid to the effect of the ionospheric Pedersen conductivity coupling [e.g., Lotko et al , 1987; Keskinen et al , 1988; Lotko and Shen , 1991; Wei and Lee , 1993; Lysak et al , 1995]. Particularly, Keskinen et al showed a stability criterion to the auroral phenomena in the plasma sheet: the outer plasma sheet is likely to be unstable to the KH instability because the inertial relaxation rate v is usually smaller than the shear frequency, while the inner part is stable because of v exceeding the shear frequency.…”