The physical nature of a new instability predicted earlier in the framework of a kinetic theory [Dimant and Sudan, 1995a, b, c> is clarified by using a simple fluid model. It is associated with ohmic heating of electrons by the modulated electric field. Temperature modulations lead to the instability via two independent mechanisms. One mechanism consists in the reversal of the sign of pressure perturbations, and the other is associated with perturbations of the electron Pedersen conductivity due to the temperature dependence of the electron-neutral collision frequency. The instability gives rise to the excitation of sufficiently long waves (several meters and longer) in the upper D/lower E regions. The unstable zone covers two broad ranges of flow angles around the bisectors between the directions of the ambient electric field ¿E0 and the E0¿B drift. The data available from rocket measurements in polar regions indicate possible evidence for the new instability.¿ 1997 American Geophysical Union