A comprehensive set of experimental observations of a high β (2.4), supercritical (Mf = 3.8), quasi‐perpendicular (ΘBn1 ∼ 76°) bow shock layer is presented, and its local geometry, spatial scales, and stationarity are assessed in a self‐consistent, Rankine‐Hugoniot‐constrained frame of reference. Included are spatial profiles of the ac and dc magnetic and electric fields, electron and proton fluid velocities, current densities, electron and proton number densities, temperatures, pressures, and partial densities of the reflected protons. The transformation of the apparent time scales to the actual spatial scales is performed with unprecedented accuracy. The observed layer profile is shown to be nearly phase standing and one dimensional in a Rankine‐Hugoniot frame, empirically determined by the magnetofluid parameters outside the layer proper. One or both of these properties appear to collapse at the time resolution of 1.5 s in the specific geometry considered in this study. Several pieces of evidence are used to show this stationarity: (1) the similarity of the average magnetic structures seen on the two ISEE spacecraft; (2) the close agreement between the electric currents directly determined from the plasma data and those inferred from the magnetic data assuming the layer is one dimensional and time stationary; (3) the close agreement of the empirically determined scale lengths of the most prominent substructures with those determined by numerical simulations and previous laboratory studies; and (4) the close agreement between the theoretical Rankine‐Hugoniot‐determined normal plasma pressure jump and that of the observed electron and proton fluids. The resolved cross‐field electrical currents (with empirical error estimates) are observed to peak within the main magnetic ramp at a level well below the first stabilization threshold for ion acoustic turbulence suggested for low β shocks by Galeev (1976); clear evidence is also provided for smaller parallel currents throughout the main ramp and overshoot, with a predominant sense as if the shock electric field has caused the lighter electrons to lead the ions along the local magnetic field direction. The width of the shock depends on what structures are used to define it. The upstream pedestal or “foot” is nearly two upstream ion skin depths wide, but the main magnetic ramp is only 1/5 the upstream ion skin depth and thus considerably smaller than “conventional wisdom” and most simulations. The ramp scale length is directly corroborated by the current densities determined from the plasma instruments.

The DE 2 satellite observed electric field fluctuations on the topside of the nighttime midlatitude ionosphere. They extended several hundred kilometers in the latitudinal direction with wavelengths of several tens of kilometers, and their amplitudes were a few millivolts per meter. Such fluctuations were often observed at magnetically conjugate points in the northern and southern hemispheres. These electric field fluctuations are perpendicular to the geomagnetic field. They are not accompanied by any significant plasma depletion or electron temperature v•riations. Magnetic field fluctuations are sometimes observed simultaneously with electric field fluctuations. We interpret that these fluctuations are caused by fieldaligned currents which flow from the ionosphere in one hemisphere to the conjugate point in the other hemisphere. The power spectrum of these midlatitude electric field fluctuations follows a power law of the form Power c• f-•, with the spectral index n of 3.5 to 4.5, which is steeper than that of the electric field fluctuations in the high-latitude ionosphere or in the equatorial ionosphere. This phenomenon may be related to other ionospheric phenomena, for example, the F region field-aligned irregularities or spre•d-F, observed by ground-b•sed methods such •s the MU r•d•r, but the relationship is not clear. (MEFs), which often appear simultaneously at magnetically conjugate points, and discuss the relation between these electric field fluctuations and the F region FAIs. In situ observations of the midlatitude ionospheric electric field by satellites play an important role in understanding the mechanism of the ionospheric irregularities which have been observed by ground-based techniques. Observation The DE 2 satellite flew in polar orbit at about 250-km to 900-km altitudes. It observed the ionospheric electric field at midlatitudes from August 1981 to February 21,439 21,440 SAITO ET AL.' MIDLATITUDE ELECTRIC FIELD FLUCTUATIONS a sampling rate of two samples per second [Krehbiel et al., 1981]. Vector magnetic field data were obtained by dinate (SPC) system, where the x axis is in the direction of the satellite velocity and the y axis is downward, with the z axis completing a right-handed coordinate system.

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A large (30‐mV/m peak) impulsive westward electric field was observed in the midnight, low latitude, dipole L = 7.5 region of the earth's magnetospheric tail at the onset of a large substorm. The measurements were made with the long, 179‐m baseline, cylindrical electric field probes carried by ISEE 1. The electric field impulse was coincident with a sharp 60 nT steplike change in the x component of the magnetic field at the satellite and the onset of a sharp 60 nT decrease in the H component of the field at a magnetic observatory near the subconjugate point. The 2‐min envelope of the westward Ey field correlates with the time derivative ∂Bx/∂t of the collapsing magnetic field attributed to the decrease in the cross‐tail current. Associated with this inductive impulse, large electric field variations are also observed on time scales of tens of seconds to tens of milliseconds. The low‐frequency (10 s) wave variations show a coherent phase relationship between the electric and magnetic field that changes from correlative to anti‐correlative during the event. Higher frequency (t < 1 s) turbulent electrostatic fields of similar magnitude are present throughout the event. These large time‐dependent electric fields seem to have the proper amplitude, duration, and timing relative to the auroral substorm sequence to explain the energetic proton enhancements frequently observed near midnight both at synchronous orbit and in the magnetotail at times of substorm onsets. The azimuthal direction of Ey (westward) for this event appears to conflict with the IMP 6 observations of an eastward Ey at higher latitudes under similar conditions. However, these observations are in agreement when magnetotail collapse is viewed in terms of a sharp reduction in the anti‐solar tension on auroral field lines as opposed to interpretations based solely on the effects of a sudden decrease in the cross‐tail current sheet.

We present plasma and electric field observations from two satellite encounters with equatorial plasma bubbles updrafting at velocities of-2 km/s. These large, upward velocities are consistent with an adaptation of Chandrasekhar's model for the motion of plasma blobs supported against gravity by a magnetic field; that is,/fz • -g. Vector magnetic field measurements, available during one of the bubble encounters show a perturbation of -150 nT, directed radially outward from the Earth, near the western wall of deepest plasma depletion. This magnetic variation is too large to be caused by simple shunting of the g x B current along the bubble's edge. Rather, it is Alfv6nic in nature, radiating from a generator located near the magnetic equator, in the plasma outside the bubble's leading edge. A heuristic model of a depleted flux tube with constant circular cross section moving upward through a background plasma predicts most of the measurements' qualitative features. INTRODUCTION Plasma bubbles, responsible for equatorial spread F originate in the bottomside of the F layer where plasma density gradients are unstable to the growth of Rayleigh-Taylor turbulence [Hudson and Kennel, 1975]. In a local approximation the linear growth rate reverses sign at the peak of the F layer [Balsley et al., 1972]. G. Haerendel (unpublished manuscript, 1973) stressed that plasma bubbles act as twodimensional entities whose line-integrated characteristics determine the motion of the entire flux tube. Upward moving density depletions have been detected below and near the peak of the F layer [McClure et al., 1977]. They are, however, also found well above the F layer peak [Burke et al., 1979; Young et al., 1984]. Computer simulations show that with proper background conditions Rayleigh-Taylor turbulence grows into the nonlinear regime and penetrates through to the topside ionosphere [Ossakow et al., 1979; Anderson and Haerendel, 1979; Zalesak et al., 1982]. An analytical model for the nonlinear evolution of plasma bubbles was developed by Ott [1978] and Kelley and Ott [1978]. Plasma bubbles are field-aligned density depletions that percolate through the equatorial ionosphere after sunset. They are represented as two-dimensional structures whose buoyancy is supplied by eastward electric fields within the depletions. The electric fields result from steps in g x B currents at the edges of depleted flux tubes. In the absence of dissipation, positive (negative) charge accumulates on the west (east) wall of the flux tube until it reaches a flotation level. During the charge accumulation phase a bubble's E x B velocity should increase at a rate nearly opposite to the acceleration due to gravity [Chandrasekhar, 1960]. The model of Chandrasekhar [1960] concerns plasma enhancements, rather than depletions, supported against gravity by a magnetic field. Ossakow and Chaturvedi [1978] pointed out a formal similarity between the equations of single [Ossakow and Chaturvedi, 1978] and multiple [Chen et al., 1983] bubbles have been obtained assuming that ...