ELECTRIC-FIELD DIAGNOSTICS OF THE DYNAMICS OF EQUATORIAL DENSITY DEPLETIONS

During its life of 10 months, the San Marco D satellite crossed a larg e number of plasma density depletion channels in the nightside F-regio n equatorial ionosphere. In-situ measurements of vector electric field s from San Marco D reveal convection velocity variations inside such c hannels and thus can be used as diagnostics of the dynamics of these p lasma depleted regions. Furthermore, in some cases, the temporal evolu tion of the channel can be inferred from the measurements. In this pap er the electric field data are converted to plasma drift velocities in order to illustrate cases where the plasma how is directed upward or downward in the channel, the channel itself is oriented vertically upw ard or tilted eastward/westward, or the channel is experiencing a bifu rcation or pinching-off process. Although the E x B plasma drift veloc ities within the depleted channels are commonly a few hundred m s(-1) on some occasions electric fields corresponding to speeds as large as 2-3 km s(-1) have been observed. The implications for such highly supe rsonic convection are discussed, including the possible constriction o f such high-speed depletion channels at higher altitudes. (C) 1997 Els evier Science Ltd.