T. L. Aggson

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 regions 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.

Magnetospheric boundary dynamics: DE 1 and DE 2 observations near the magnetopause and cusp

A broad spectrum of particle and field measurements was taken near local noon by the Dynamics Explorer satellites during the magnetic storm of September 6, 1982. While at apogee, DE 1 sampled the magnetospheric boundary layer at mid southern latitudes and, due to the passage of an intense solar wind burst, briefly penetrated into the magnetosheath. In the boundary layer and the adjacent magnetosheath the plasma flow was directed toward dawn. Variance and de Hoffmann-Teller analyses of electric and magnetic field data during the magnetopause crossing showed the magnetopause structure to be that of a rotational discontinuity or an intermediate shock with a substantial normal magnetic field component. This is consistent with an open magnetosphere model in which significant magnetic merging occurs at the local time of the spacecraft. The orbit of DE 2 carried it through the morning sector of the low-altitude, southern cusp. The measurements show a well-defined, cusp current system occurring on open magnetic field lines. At both cusp and subcusp latitudes the electric field was equatorward indicating a strongly eastward plasma flow. The boundary between these two regions was marked by the onset of magnetosheath precipitation and an electric field structure containing both poleward and equatorward spikes. The poleward spike has associated field-aligned currents which are closed by Pedersen currents and, from force balance considerations, is interpreted as the signature of a magnetic merging event at the magnetopause. The equatorward spike has the characteristics of a down-coming and reflected Alfven wave packet of finite dimensions. The high-altitude measurements suggest that the dayside boundary layer is made up of closed magnetic flux tubes, a large fraction of which drift to the magnetopause where merging with the IMF occurs. The merging line maps to the ionosphere as a “gap” across which the polar cap potential is applied to the magnetosphere. The potential is applied from a magnetosheath generator to the polar ionosphere by means of the cusp, field-aligned current system. The electric fields provide an ionospheric indicator of the mapping of the merging line location.

Average equatorial zonal and vertical ion drifts determined from San Marco D electric field measurements

San Marco D electric field measurements have been averaged in terms of their equivalent ion drift to produce an average pattern of equatorial zonal and vertical ion drifts. Variations with season, solar activity, Kp, lunar phase and longitude have been analyzed. Similarities and some differences from previous Jicamarca, DE 2 and AE-E results are seen. Confirmation is given of the dominance of the F region dynamo in the 1900-2100 local time region. The daytime zonal ion drift is larger for high F10.7 values than that for low values. There is little variation between high and low values of Kp. Superrotation is evident in this data set and is larger at equinox compared to solstice. At the June solstice there are significant differences between the average ion drifts in the longitude sector where the geomagnetic equator is north of the geographic equator (Indian sector) and the sector where the geomagnetic equator is south of the geographic equator (Peruvian sector). The daytime upward velocity is larger in the Indian sector than in the Peruvian sector, and it reverses later in the evening in the Indian sector. Daytime westward zonal velocities are larger and the nighttime eastward velocities are smaller in the Indian sector. A presunrise enhancement is seen in the downward velocity in the Indian sector but not in the Peruvian sector. Significant variations are also seen with the phase of the moon. In light of current theory, the lunar variations suggest a complex interaction of E and F region dynamo sources with conductivity, changing in phase and character with latitude.

Effects of a lightning discharge detected by the DE 2 satellite over Hurricane Debbie

We report the satellite observation of a large, ∼40 mV m−1, transient electric field disturbance over Hurricane Debbie in September 1982. The event lasted less than a second and correlated closely with a burst of highly field-aligned, upward moving electrons with nearly 1 keV of energy. The electric field event is viewed as a spheric disturbance from a lightning discharge in the active weather system located beneath the satellite. A spheric interpretation of the observed electric field transient is consistent with a subsequent observation of energetic electrons precipitating from the radiation belts. Measured quasi-dc electric fields and cold plasma density variations are only roughly consistent with model predictions for ULF wave propagation from a storm system to the ionosphere. To understand this first observation of upward moving electrons in the ionosphere associated with a lightning event, we compare the several mechanisms for electron acceleration by electric fields with components (E∥) along the magnetic field. In our scenario, “runaway” electrons were accelerated in ∼1 ms by a downward directed E∥ pulse of ∼1 V m−1 magnitude. Such fields can result from rapidly exposed, negative space charges near the tops of clouds during positive cloud-to-ground discharges. High-frequency Fourier components of the E∥ pulse must propagate through the low-conducting nighttime atmosphere to the ionosphere with little dissipation.

In situ observations of bifurcation of equatorial ionospheric plasma depletions

Vector electric field measurements from the San Marco D satellite are utilized to investigate the bifurcation of ionospheric plasma depletions (sometimes called {open_quotes}bubbles{close_quotes}) associated with nightside equatorial spread F. These depletions are identified by enhanced upward ExB convection in depleted plasma density channels in the nighttime equatorial ionosphere. The in situ determination of the bifurcation process is based on dc electric field measurements of the bipolar variation in the zonal flow, westward and eastward, as the eastbound satellite crosses isolated signatures of updrafting plasma depletion regions. The authors also present data in which more complicated regions of zonal velocity variations appear as the possible result of multiple bifurcations of updrafting equatorial plasma bubbles. 10 refs., 7 fig.

Downdrafting plasma flow in equatorial bubbles

The electric field experiment carried aboard the San Marco D equatorial ionospheric satellite regularly measured updrafting in plasma depletion channels or “equatorial bubbles” which form on the bottomside of the nightside equatorial F region. We report here observations of downdrafting vertical plasma velocities inside such depletion regions in the nightside equatorial ionosphere. Both updrafting and downdrafting motions can be expected on the basis of a generalized gradient drift/collisional Rayleigh-Taylor instability process in the ionospheric F region. Although the gravitation can only drive upward plasma flow in plasma depletion regions, both background westward zonal electric fields and upward vertical neutral winds can cause an occurrence of downdrafting (i.e., a downward motion of the plasma within the bubble) if those parameters are strong enough. We show that as the background zonal electric field becomes westward (often after ∼2100 LT) in the equatorial ionosphere, the plasma interior to an existing bubble at altitudes of ∼400 km and less at the magnetic equator may assume a downdrafting motion, while at higher altitudes in the same bubble channel, the plasma flow remains upward. Such a simultaneous occurrence of the updrafting and downdrafting plasma flow in a single bubble channel may lead to the pinching off of the upper part of the depletion region from the lower altitude regions, causing the decay of a bubble or the formation of a “dead” bubble.

A Comparison of in situ measurements of E→ and −V→×B→ from Dynamics Explorer 2

Dynamics Explorer-2 provided the first opportunity to make a direct comparison of in situ measurements of the high-latitude convection electric field by two distinctly different techniques. The vector electric field instrument (VEFI) used antennae to measure the intrinsic electric fields and the ion drift meter (IDM) and retarding potential analyzer (RPA) measured the ion drift velocity vector, from which the convection electric field can be deduced. The data from three orbits having large electric fields at high latitude are presented, one at high, one at medium, and one at low altitudes. The general agreement between the two measurements of electric field is very good, with typical differences at high latitudes of the order of a few millivolts per meter, but there are some regions where the particle fluxes are extremely large (e.g., the cusp) and the disagreement is worse, probably because of IDM difficulties. The auroral zone potential patterns derived from the two devices are in excellent agreement for two of the cases, but not in the third, where bad attitude data may be the problem. At low latitudes there are persistent differences in the measurements of a few millivolts per meter, though these differences are quite constant from orbit to orbit. This problem seems to arise from some shortcoming in the VEFI measurements. Overall, however, these measurements confirm the concept of “frozen-in” plasma that drifts with velocity E→×B→/B2 within the measurement errors of the two techniques.

Satellite observations of zonal electric fields near sunrise in the equatorial ionosphere

We report here on a number of examples of anomalous enhancements of eastward electric fields near sunrise in the equatorial ionospheric F-region. These examples were selected from the data base of the equatorial satellite, San Marco D (1988), which measured ionospheric electric fields during a period of solar minimum. The eastward electric fields reported correspond to vertical plasma drifts. The examples studied here are similar in signature and polarity to the pre-reversal electric field enhancements seen near sunset from ground-based radar systems. The morphology of these sunrise events, which are observed on about 14% of the morning-side satellite passes, are studied as a function of local zonal velocity, magnetic activity, geographic longitude and altitude. The nine events studied occur at locations where the zonal plasma flow is generally measured to be eastward, but reducing as a function of local time and at satellite longitudes where the magnetic declination has the opposite polarity as the declination of the sunrise terminator.

Electric field diagnostics of the dynamics of equatorial density depletions

Abstract During its life of 10 months, the San Marco D satellite crossed a large number of plasma density depletion channels in the nightside F-region equatorial ionosphere. In-situ measurements of vector electric fields from San Marco D reveal convection velocity variations inside such channels and thus can be used as diagnostics of the dynamics of these plasma depleted regions. Furthermore, in some cases, the temporal evolution of the channel can be inferred from the measurements. In this paper the electric field data are converted to plasma drift velocities in order to illustrate cases where the plasma flow is directed upward or downward in the channel, the channel itself is oriented vertically upward or tilted eastward/westward, or the channel is experiencing a bifurcation or pinching-off process. Although the E × B plasma drift velocities 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 supersonic convection are discussed, including the possible constriction of such high-speed depletion channels at higher altitudes.

Observations of ionospheric electric fields above atmospheric weather systems

We report on the observations of a number of quasi-dc electric field events associated with large-scale atmospheric weather formations. The observations were made by the electric field experiment onboard the San Marco D satellite, operational in an equatorial orbit from May to December 1988. Several theoretical studies suggest that electric fields generated by thunderstorms are present at high altitudes in the ionosphere. In spite of such favorable predictions, weather-related events are not often observed since they are relatively weak. We shall report here on a set of likely E field candidates for atmosphere-ionosphere causality, these being observed over the Indonesian Basin, northern South America, and the west coast of Africa; all known sites of atmospheric activity. As we shall demonstrate, individual events can often be traced to specific active weather features. For example, a number of events were associated with spacecraft passages near Hurricane Joan in mid-October 1988. As a statistical set, the events appear to coincide with the most active regions of atmospheric weather.