It is shown that the widely used equilibrium in which the nighttime F region is supported by E×B drifts is unstable if, in addition to the supporting eastward field, a north-south electric field component exists. The instability, which takes the form of rising and falling sheets of ionization, has a growth rate γ ≈ 3 × 10−4 νin−1 sec−1, where νin is ion-neutral collision frequency at the F region peak. These conclusions are based on a set of new moment equations that govern the time evolution of the Pedersen conductance and the plasma content of a flux tube. It is argued that temperate- and high-latitude spread F are a result of this instability. The new moment equations also predict: (1) that there is a maximum height-integrated Pedersen current that the F region can carry; and (2) that barium clouds deform in agreement with observations of recent releases.