PHYSICAL NATURE OF A NEW CROSS-FIELD CURRENT-DRIVEN INSTABILITY IN THE LOWER IONOSPHERE

The physical nature of a new instability predicted earlier in the fram ework 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 le ad to the instability via two independent mechanisms. One mechanism co nsists in the reversal of the sign of pressure perturbations, and the other is associated with perturbations of the electron Pedersen conduc tivity due to the temperature dependence of the electron-neutral colli sion frequency. The instability gives rise to the excitation of suffic iently long waves (several meters and longer) in the upper D/lower E r egions. The unstable zone covers two broad ranges of flow angles aroun d the bisectors between the directions of the ambient electric field /-E(0) and the E(0) x B drift. The data available from rocket measurem ents in polar regions indicate possible evidence for the new instabili ty.