Electric field observations of equatorial bubbles

Abstract

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 (Ey) 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 ▽ · 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 (Ex) on crossing these structures. This lack of conservation of Ex 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 Ex 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 (∼1.5 mV/m) jump in the tangential (vertical) component of E on crossing these events.

ROCSAT 1 ionospheric plasma and electrodynamics instrument observations of equatorial spread F: An early transitional scale result

“…Hanson and Bamgboye, 1984]. More recent studies of bubble plasma dynamics by Aggson et al [1992] have shown that bubble plasma may indeed rise vertically and be well correlated with the overall density variations, but the bubble plasma may later stagnate and even on occasions be seen to move downward. It may therefore be appropriate to consider bubble A in terms of a stagnated bubble, where the large-scale plasma drift inside the depletion is very small.…”

Some new features of ionospheric plasma depletions over the Indian zone using all sky optical imaging

“…Subsequently, they diffuse down the magnetic field lines to lower altitudes at low latitudes. The plasma inside the depletions has been found to have velocities both perpendicular and parallel to the geomagnetic field (Hanson and Bamgboye, 1984;Aggson et al, 1992). The post-sunset equatorial F layer can become unstable under the influence of any disturbance produced by gravity waves, neutral winds or some other source and can generate plasma irregularities through Rayleigh-Taylor instability (RTI) (Hysell et al, 1990;Singh et al, 1997a).…”

Low-latitude ionospheric turbulence observed by Aureol-3 satellite

“…In particular, vector electric fields in the frequency range f from 10 to 500 Hz have been measured on the 400 km altitude polar orbiting OV1-17 satellite (Kelley andMozer, 1972), andHoltet et al (1977) demonstrated the association between spread-F properties and electrostatic noise near lower hybrid frequency (f LH R ∼10-15 kHz). Recently, Aggson et al (1996) presented electric field data from the San Marco D spacecraft in terms of structure and dynamics of ionospheric plasma (between 500 km and 800 km) depletions associated with night side equatorial spread-F. They measured two DC electric field components in the zonal flow, westward and eastward, and found bifurcations of these depletions.…”