Electrodynamics of midlatitude spread F 3. Electrohydrodynamic waves? A new look at the role of electric fields in thermospheric wave dynamics

The study of atmospheric tides has long recognized the importance of electrodynamic or ohmic losses, in the form of forces, on the strength of winds in the Earth's upper atmosphere. Now, recent observations of ionospheric and atmospheric phenomena (particularly, but not exclusively, at midlatitudes) are suggesting that electrodynamic processes may also be important for atmospheric buoyancy waves with periods as short as a few hours. During the daytime, passive ohmic losses may be responsible for a strong azimuthal filtering of buoyancy waves which limits their propagation in the upper atmosphere to near equatorward where there is a minimum of the drag. At night, the active development of plasma instabilities, whose growth rate is also strongly azimuthally dependent, may not only minimize ion drag but may also feed energy into particular buoyancy waves to counteract the nighttime's enhanced molecular and thermal dissipation, thus creating a strong equatorward and westward orientation to observed traveling ionospheric disturbances. In all, the strength of the apparent coupling between buoyancy waves and ionospheric electrodynamics in the upper atmosphere may suggest the classification and study of a new form of electrohydrodynamic waves.