Instability of the Lembege-Pellat equilibrium under ideal magnetohydrodynamics

The magnetotail-like equilibrium model of Lembege and Pellat [Phys. Fluids, 25, 1995 (1982)] has been used in a number of stability calculations over the past 13 years. Most of these computations examine collisionless tearing-a mode of tail instability first suggested by Coppi et al. [Phys. Rev. Lett. 16, 1207 (1966)]-or other kinetic modes and involve both analytical and computer analysis. In this Communication it is shown, analytically, that the equilibrium model of Lembege and Pellat is unstable according to ideal magnetohydrodynamics (MHD). More precisely, for the ideal MHD stability theory of Bernstein et al. [Proc. R. Soc. London Ser. A 244, 17 (1958)] it is proven that Lambda = p' + Gamma p (upsilon ''/upsilon') < 0. (C) 1996 American Institute of Physics.