Polar cap electric fields patterns are presented from times when the S3‐2 Satellite was near the dawn‐dusk meridian and IMF data were available. With Bz ≥ 0.7 γ, two characteristic types of electric field patterns were measured in the polar cap. In the sunlit polar cap the convection pattern usually consisted of four cells. Two of the cells were confined to the polar cap with sunward convection in the central portion of the cap. The other pair of cells were marked by anti‐sunward flow along the flanks of the polar cap and by sunward flow in the auroral oval. These observations are interpreted in terms of a model for magnetic merging at the poleward wall of the dayside polar cusp. The sunward flow in the auroral zone is not predicted by the magnetic model and may be due to a viscous interaction between the solar wind and magnetosphere. The second type, which was observed in some of the summer hemisphere passes and all of the winter ones, was characterized by an electric field pattern which was very turbulent, and may be related to inhomogeneous merging.

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A dc electric field experiment on the Air Force satellite S3‐2 has occasionally detected intense localized electric fields near the ionosphere projection of the plasmapause. These poleward directed electric fields were observed in the pre‐midnight local time sector, seem to be related to substorm activity, and typically exceeded 100mV/m. In one case the field was 280mV/m corresponding to a drift velocity of 9.8 km/s at an altitude of 1463 km and a potential drop of 25 kilovolts. A possible source lies in the interaction between hot plasma freshly injected near magnetic midnight and the cold plasmaspheric particles. Since the potential drop is the order of the mean ring current energy, this structure may have important consequences for the understanding of magnetospheric flow patterns under disturbed conditions.

Thermal plasma density measurements obtained by using the spherical electrostatic analyzer probe carried by the polar orbiting satellite Isis 1 have been spectrally analyzed to provide quantitative results on the amplitude and scale size distribution of the plasma irregularities in the high‐latitude top side ionosphere. The range of scale sizes examined, 200 m to over 100 km under normal conditions, is more than an order of magnitude greater than that available from ground‐based measurements. Data in the altitude range 574–3523 km and from the period February 1969 to April 1972 have been analyzed. Irregularities in the high‐latitude region from 40° invariant latitude up to the invariant L pole have been studied. In over 90% of the cases examined, the power spectra are found to fit a power law of the form P = P0f−β, where the index β varies within the range 1.5–2.5. The power spectral density was integrated over the scale size range 1–10 km in order to analyze quantitatively latitudinal variations of the irregularity amplitude. While the boundaries of the high‐latitude irregularity zone on particular orbits examined by using this technique are in agreement with earlier morphological studies, it is also evident that the amplitude of the irregularities within the high‐latitude irregularity region itself shows irregular enhancements above a generally raised amplitude level.

The main trough in the topside ionosphere has been studied using the thermal positive ion and electron densities measured on 15,000 orbits over a 3-year period (1969)(1970)(1971)(1972) by means of spherical electrostatic analyzers aboard the Isis I and Injun 5 satellites in the 560-to 3600-km altitude range. The trough is found to be a persistent feature at night with an occurrence frequency of approximately 95%. The occurrence frequency decreases to approximately 60% near the dawn-dusk meridian and to approximately 48% near local noon. At altitudes below about 1500 km during quiet to moderate conditions (Kp _< 3o) the trough equatorward boundary is found at L = 3.5 + 0.5 near midnight and L = 12.5 + 1.0 near local noon. The equatorward edge of the trough in the nighttime sector lies near the L shell where the plasmapause has been observed. Near local noon the trough occurs at the equatorward edge of the magnetospheric cleft and is at significantly higher L values than those reported for the dayside plasmapause. The seasonal variation of the trough location at a given local time is negligible except near sunrise. With increasing altitude between 1500 and 3600 km the equatorial boundary of the trough moves to continually lower latitudes during the night hours. The equatorward trough wall becomes a dominant feature of the trough, often extending from 15 ø to 20 ø in width during quiet magnetic periods. The poleward edge of the trough becomes less well marked with increasing altitude, often being defined only by a sharp spike in ionization extending over a few degrees within the auroral zone. At altitudes above 1500 km on the dayside, two independent troughs or density gradients are observed. The trough at high latitudes located at the equatorward edge of the cusp gradually decreases in amplitude with increasing altitude. This trough is tentatively identified as the dayside analog of the high-latitude trough observed on the nightside by Ogo 6. Its development is attributed to the depleting effects of enhanced ion chemical reactions in the presence of convective electric fields. The lower latitude dayside trough, observed between L = 2 and 6, results from the partial depletion of flux tubes in the outer plasmasphere.