This paper explores the consequences of enforcing the quasi-neutrality (QN) condition on a magnetotail and auroral plasma. We show that this condition has never been handled correctly in the literature when the bounce motion of particles is involved, for either adiabatic or stochastic ion motion. Correct treatment yields a global electrostatic potential which is constant along the magnetic field line. We describe how the stability results of Hurricane et al. (1995) are modified. We show that the QN condition modifies the classical MHD analysis of symmetric interchange/ballooning modes with respect to low-frequency MHD waves. An additional consequence of the QN condition on MHD-like waves is a modification of the plasma equation of state. More precisely, we show that the plasma remains isothermal during such wave propagation. For antisymmetric modes there is no associated electrostatic potential, but we give the necessary corrections to the standard analysis upon entering a stochastic ion regime. Last, we show how the electrostatic potential couples to the ionosphere, revealing a new understanding of auroral arcs. ¿ American Geophysical Union 1995.