The Pioneer Venus Orbiter measurements of the plasma and magnetic environment of the near tail of Venus show that the ionosphere becomes increasingly filamentary with increasing altitude, apparently forming cometlike tail rays that extend several thousand kilometers behind the planet. We call this region the ionotail of Venus. The tail rays are envisioned as plumes of high‐beta plasma of ionospheric origin that are surrounded by regions of low‐density, low‐beta plasma. The ionotail appears to be in quasi‐equilibrium, with the plasma pressure in the rays approximately balanced by the magnetic pressure of the region surrounding the rays. The magnetic field in this region is approximately sunaligned as we assume are the tail rays. Magnetic field reversals observed in the tail ray boundaries suggest the presence of strong current sheets there. Unlike the lower ionosphere whose major ion is thermal O+, a detailed study of tail ray plasma between 2000 and 2500 km altitude shows that the major ions are superthermal O+, with energies in the range of 9‐16 eV. The electrons are much cooler, with energies of about 1 eV. A minor, more energetic ion component, having energies exceeding 40 eV is also observed within the tail rays and occasionally between the rays as well. These Pioneer Venus Orbiter measurements reveal an ionotail that is highly dynamic, a region in which solar wind induced magnetic fields configure the ionospheric structures and accelerate the ions beyond the planetary escape velocity. We estimate a total planetary O+ escape rate of 5 × 1025 ions/s, and we infer an H+ escape rate of about half that value, about a factor of 2 below the hydrogen escape rate due to H+ charge exchange with the hydrogen exosphere of Venus.

Ion composition and electron temperature data obtained from AE-C during a magnetically quiet period centered on the June 1976 solstice have been used in a statistical study of the southern winter F region poleward of-50 ø A at 300-km altitude. Prominent ionospheric features revealed by topographic maps of O +, NO +, and O•. + concentration and Te include the nightside main trough, an ionization 'hole' poleward of the nightside auroral zone, and ionization and Te enhancements in the dayside auroral zone-cusp region. The main trough, in which O + was the dominant ion, extended throughout the night between -60 ø and -70 ø A, the lowest trough densities, • 1 X 10 • cm -•, being detected near dusk. We attribute these low concentrations to the opposition, in the dusk sector, of plasma corotation and solar wind induced plasma convection velocities, leading to long plasma residence (and decay) times. That the distributions of NO + and O•. + in the trough region exhibited little correlation with O + suggests that drift-enhanced O+loss via reactions such as O + + N•. --, NO + + N played a minor role in the formation of the trough during this period. A band of enhanced electron temperature coincided with the trough throughout the night; this T, peak, which has been observed previously in the topside ionosphere, is attributed to heat conducted downward from the protonosphere. The ionization hole, a region poleward of the nightside auroral zone between -70 ø and -80 ø A, was characterized by depletions in all the measured ion densities and by a minimum in T,. The total ion concentration measured in this region exhibited extreme temporal variability, ranging from values as low as 2 X 10 •' to 6 X 10 • cm -•. We have c, oncluded that the hole forms as a result of slow antisunward plasma drift across the dark polar cap and attendant ion recombination; an average drift velocity of •0.1 km/s, corresponding to a convection electric field of less than 5 mV/m, could produce the deepest holes observed. The ion density variability in the hole is attributed to changes in the transpolar plasma convection configuration and the distribution of energetic particle fluxes. The dayside auroral zone-cusp region was characterized, in general, by enhanced levels of ionization and electron temperature associated with energetic particle precipitation. On some passes through this region, however, localized O + depletions and corresponding molecular ion increases were detected; we attribufe these features to the reactions O + + N•. --, NO + + N and O + + O•. --, O•. + + O, whose rates are enhanced by the high-speed plasma drifts observed in the cusp region.

The Venus bow shock location has been measured at nearly 2000 shock crossings, and its dependence on solar EUV, solar wind conditions, and the interplanetary magnetic field determined. The shock position at the terminator varies from about 2.14 Venus radii at solar minimum to 2.40 Venus radii at solar maximum. The location of the shock varies little with solar wind dynamic pressure but strongly with solar wind Mach number. The shock is farthest from Venus on the side of the planet in which newly created ions gyrate away from the ionosphere. When the interplanetary magnetic field is perpendicular to the flow, the cross section of the shock is quite elliptical.This effect appears to be due to the anisotropic propagation of the fast magnetosonic wave.When the interplanetary magnetic field is aligned with the flow, the bow shock cross section is circular and only weakly sensitive to changing EUV flux.

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.

Measurements of electron density and temperature by the Pioneer Venus orbiter electron temperature probe have been employed to examine the characteristics and morphology of ionospheric holes in the antisolar ionosphere of Venus. The holes apparently exist as north‐south pairs which penetrate the ionosphere vertically down to altitudes as low as 160 km. Magnetic field measurements show that the holes are permeated by strong radial fields whose pressure is sufficient to balance the plasma pressure of the surrounding ionosphere. The electron temperature in the holes is substantially cooler than the surrounding ionosphere, except in the lowest density regions of the holes where the temperatures greatly exceed the ionosphere temperature. The low temperatures and the low densities of the holes are consistent with the strong radial magnetic fields which inhibit horizontal transport of plasma and thermal energy from the surrounding ionosphere. Plasma depletion processes associated with magnetotail electric fields may be important in the formation of the holes.