ELECTRIC-FIELD OBSERVATIONS OF EQUATORIAL BUBBLES

We present here results from the double floating probe experiment carried on the San Marco D satellite, with emphasis on the observation of large incremental changes in the convective electric field vector at the boundary of equatorial plasma bubbles. This study concentrates on isolated bubble structures in the upper ionospheric F region and divides these observed bubble encounters into two types, type I (live bubbles) and type II (dead bubbles). Type I bubbles show varying degrees of plasma density depletion and upward velocities ranging from 100 to 1000 m/s. Type II bubbles show plasma density depletion but no appreciable upward convection. Both types of events are often surrounded by a halo of plasma turbulence extending considerably outside the region's plasma depletion. Most type I events show some evidence for local continuity in the eastward (y) electric current, where the y component of the observed electric field (E(y)) shows hyperbolic correlation with the plasma density (n), as dictated by horizontal current continuity. This model stresses the importance of including magnetic field aligned currents in deriving the electric potential equation from the divergence equation del . j = 0. All of the type I (live) events examined exhibit a striking and systematic lack of conservation of the vertical component (x) of the electric field vector (E(x)) on crossing these structures. This lack of conservation of E(x) is of the order of 1.5 mV/m from west to east, directly implying that type I bubbles are not steady state plasma structures. A straightforward interpretation of this jump phenomenon in E(x) leads to the conclusion that the walls of most of the type I bubbles are collapsing inward at the rate of some 50 m/s. Since the average east-west dimension of the bubble structures we have examined here is of the order of 40 km, we conclude that the average lifetime of the strong upward convection phase is about 15 min. This suggests that after 15 min or so these type I events may be pinched off from the low densities of the bottomside F region and the bubbles perhaps become type II events which continue to drift eastward with the general background zonal plasma flow during the equatorial night. It is argued that the collapse motions may be driven by an asymmetry between the upwind (west) and downwind (east) E region drag on the F region eastward dynamo motions. Such an asymmetry appears to be of the proper magnitude and direction to produce the observed (approximately 1.5 mV/m) jump in the tangential (vertical) component of E on crossing these events.