The ionospheric disturbance dynamo

A numerical simulation study of the thermospheric winds produced by auroral heating during magnetic storms, and of their global dynamo effects, establishes the main features of the ionospheric disturbance dynamo. Driven by auroral heating, a Hadley cell is created with equatorward winds blowing above about 120 km at mid‐latitudes. The transport of angular momentum by these winds produces a subrotation of the mid‐latitude thermosphere or westward motion with respect to the earth. The westward winds in turn drive equatorward Pedersen currents which accumulate charge toward the equator, resulting in the generation of a poleward electric field, a westward E × B drift, and an eastward current. When realistic local time conductivity variations are simulated, the eastward mid‐latitude current is found to close partly via lower latitudes, resulting in an ‘anti‐Sq’ type of current vortex. Both electric field and current at low latitudes thus vary in opposition to their normal quiet‐day behavior. This total pattern of disturbance winds, electric fields, and currents is superimposed upon the background quiet‐day pattern. When the neutral winds are artificially confined on the nightside, the basic pattern of predominantly westward E × B plasma drifts still prevails on the nightside but no longer extends into the dayside. Considerable observational evidence exists, suggesting that the ionospheric disturbance dynamo has an appreciable influence on storm‐time ionospheric electric fields at middle and low latitudes.