Direct measurements of the distributions of the thermal positive ions H+ and He+ in the magnetosphere reveal a distinct variability in the position and structure of the plasr^^apause. Such variability is observed to be most pronounced in the afternoon-dusk local time sector ,: and is indicative of magnetospheric irregularities in the same region. As the OGO-3 satellite made progressive dusk-side (1500 - 1900 L.T.) and night-side (2200 - 0100 L.T.) passes during June-July 1966, the dusk-side plasmasphere was observed to exhibit an outward expansion or . bulge, accompanied in some cases by considerable fine structure. In particular, the plasmapause was observed at L positions as distant as L = 7-8 in the afternoon-dusk sector, in contrast to positions near L = 5-6 observed near midnight on the same day and at comparable levels of moderate magnetic activity (Kp < 3). Within the bulge and just above the initial plasmapause structured plasma recoveries are observed, wherein n(H+ ) returns to concentrations of the order of SO-100 ions/cm3 over intervals of 0.5 - 1.5L. Both the dusk-side IRREGULAR

ABSTRACTGlobal midday measurements of the primary ionic constituents of theearth's topside ionosphere between June and October, 1966, have revealed strong longitudinal variation in the altitude of the 0+-H+ transitionlevel resulting from a corresponding variation in the distribution of 0+.While the attitudinal variation of the transition level at European andPacific longitudes is nearly symmetric about the geomagnetic equator, atAmerican longitudes the levy ) drops by almost 1000 km between the north-ern and southern hemispheres. This behavior is interpreted as an effectcaused by atmospheric winds. Concentration profiles for 0+ and H+ alonga midlatitude field line in the northern and southern hemispheres havebeen calculated, considering the atmospheric wind field and its compo-nent parallel to the magnetic field. It is found that the ion-wind inter-action produces an important interhemispheric proton flux, which isactually responsible for the asymmetry in the 0+-H+ transition levelobserved at American longitudes.

The concentrations of neutral hydrogen within the atmosphere of Venus are investigated for the period 1979–1980. During this period, the planet made nearly three orbits about the Sun, so that nearly three complete diurnal cycles were observed from the Pioneer Venus Orbiter (PVO). Values of n(H) are derived from in-situ ion and neutral composition measurements from the Orbiter Ion Mass Spectrometer (OIMS) and the Orbiter Neutral Mass Spectrometer (ONMS) using a charge exchange relationship involving O+, H+, O and CO2. The dawn bulge in the diurnal distribution of n(H), reported from the first diurnal cycle by Brinton et al., is found to persist with n(H) peaking at levels near 2 - 5 × 107/cm3 at altitudes below 165 km. At peak levels, the bulge exhibits a concentration ratio up to 400/1 relative to dayside values. Large day to day variations of up to a factor of five in n(H) are frequently encountered, and are attributed to perturbations induced by the solar wind interaction. These short term variations, plus a suggestion of some local time variation in the bulk location, make precise assessment of interannual variations in the n(H) difficult. Between the first diurnal cycle in early 1979 and the third in mid 1980, the decline in solar euv flux was of the order of 10% or less. Allowing for uncertainties due to short term variations, no clear evidence is found for an interannual variation in the hydrogen concentrations.

The predawn bulge in the diurnal distribution of neutral hydrogen at Venus is found to persist over the first three Venus years of the Pioneer Venus mission. Concentrations of H, derived from in situ mass spectrometer measurements of O+, H+, O, and CO2 show a diurnal bulge with peak levels near 2–5 × 107/cm³ and a night to day concentration ratio of about 200/1. The presence of short‐term fluctuations of as much as a factor of three from day to day reflects the variability in the ion and neutral concentrations often detected on the nightside. Allowing for this superimposed variability, relatively little difference in the magnitude of n(H) between 1979 and 1980 is indicated. This appears to be consistent with the fact that the solar euv flux level changed very little between successive transits of the bulge. On the other hand, there is a suggestion in some of the modulation observed in n(H) that short‐term variation in the euv flux due to solar rotation may influence the bulge characteristics. This speculation on the effectiveness of solar radiation cannot be substantiated as yet, and there is also evidence that changes in the solar wind ρυ² parameter are linked to significant changes in n(H). Although the ion and neutral variability complicates the identification of chemical equilibrium, rather similar behavior in the distributions of n(He), which is measured directly, lends confidence to the n(H) results. Overall, the attempt to understand the hydrogen and helium variability provides a stimulus for more detailed analysis of the nightside aeronomy and its relationship to both solar and solar wind inputs.

The ionization rate of helium in the thermosphere is inferred from AE‐E ion and neutral particle composition measurements made on 20 days between January 1979 and June 1980. The variations in the inferred rate are consistent with the variations in the ultraviolet flux measured during the same period by the EUV spectrometer on board the AE‐E satellite.

A comparison of ion and neutral composition measurements at Venus for periods of greatly different solar activity provides qualitative evidence of solar control of the day-to-night transport of light ion and neutral species. Concentrations of H+ and He in the predawn bulge near solar maximum in November, 1979, exhibit a depletion signature correlated with a pronounced modulation in the solar F10.7 and EUV fluxes. This perturbation, not observed in the predawn region during an earlier period of relative quiet solar conditions, is interpreted as resulting from pronounced changes in solar heating and photoionization on the dayside, which in turn modulate the transport of ions and neutrals into the bulge region.

A series of experiments was conducted in December 1979 to investigate the structure of plasma depletions in the low latitude, nightime ionosphere. The measurements included all sky imaging photometer (ASIP), ionosonde and amplitude scintillation observations from the AFGL Airborne Ionospheric Observatory (AIO), and in situ ion density measurements from the Atmosphere Explorer (AE‐E) Bennett Ion Mass Spectrometer (BIMS). The AIO performed two flights along the Ascension Island (−18° MLAT) magnetic meridian: one in the southern hemisphere and one near the Ascension conjugate point in the northern hemisphere. During these flights, measurements from the AE‐E satellite at 434 km altitude are compared with simultaneous remote ionospheric measurements from the AIO. Density biteouts of approximately one order of magnitude in the dominant ion O+, were mapped to lower altitudes along magnetic field lines for comparison with 6300‐Å and 7774‐Å O I airglow depletions. Because of the different airglow production mechanisms (dissociative recombination of O2+ for 6300 Å and radiative recombination of O+ for 7774 Å) the 6300‐Å depeletions reflect plasma depletions near the bottomside of the F layer, while those at 7774 Å are located near the peak of the layer. The O+ biteouts map directly into the 7774‐Å airglow depletions in the same hemisphere and also when traced into the opposite hemisphere, which indicates magnetic flux tube alignment over north‐south distances of ∼2220 km. The 6300‐Å (bottomside) depletions are wider in longitude than the 7774‐Å (F‐peak) depletions near the equatorward edge of the Appleton anomaly. This difference in topside and bottomside structure is used to infer large‐scale structure near the anomaly and to relate this to structure, commonly observed near the magnetic equator by the ALTAIR radar.

In-situ measurements of the ion composition and concentration of the ionosphere of Venus are obtained with the Bennett rf ion mass spectrometer (OIMS) on the Pioneer Venus Orbiter (PVO). Dayside ion profiles exhibit considerable variability in the height of the ionopause as well as the scale heights of the ion constituents, which reflect the compression and expansion of the ionosphere in response to solar wind variations. Near the dayside upper boundary of the thermal O+ distribution, superthermal (E ⋍ 10–90 ev) ions are detected by the OIMS, presenting a complication for identifying the ion signature of the ionopause. Correlated with the presence of the superthermal ions, the ac electric field detector (OEFD) detects regions of intensified signals, with peak response in the 100 Hz frequency channel. A limited set of comparisons indicates that both the OIMS and the OEFD detect more pronounced enhancements in the superthermal ions and 100 Hz fields, respectively, at midlatitudes (∼ 30°N), relative to low latitudes (∼ 5°N), and that wave like variations are sometimes present. These characteristics of the superthermal ion-plasma wave results suggests that these phenomena are generated in the vicinity of the ionopause, possibly by the turbulent acceleration of planetary ions at the lower boundary of the ionosheath. It is expected that further analysis of the superthermal ion-electric field signatures will contribute to a clearer understanding of the physical processes underlying the formation of the ionopause.

Measurements of ion current, electron temperature, and density and values of satellite potential from the U.S. Air Force Satellite S3‐2 together with ion composition measurements from the Atmosphere Explorer (AE‐E) satellite were used to examine the variation of the ratio α=[I+(wake)]/[I+(ambient)]| (where I+ is the ion current) with altitude and to examine the significance of the parametric interplay between ionic Mach number, normalized body size RD (=R0/λD, where R0 is the satellite radius and λD is the ambient debye length) and normalized body potential ϕN (=eϕS/KTe, where ϕs is the satellite potential, Te is the electron temperature, and e and K are constants). It was possible to separate between the influence of RD and ϕN on α for a specific range of plasma parameters. Uncertainty, however, remains regarding the competition between RD and S(H+) and S(O+) (where S(O+) and S(H+) are oxygen and hydrogen ionic Mach numbers, respectively) in determining the ion distribution in the nearest vicinity to the satellite surface. A brief discussion relevant to future experiments in the area of body plasma flow interactions to be conducted on board the Shuttle/Spacelab facility, is also included.

In situ measurements of the thermal ion composition of the ionosphere of Venus have been obtained for a period of two Venus years from the Bennett rf ion mass spectrometer on the Pioneer Venus Orbiter. Ion measurements within an altitude interval of 160 to 300 kilometers, corresponding to an overall latitude interval of about −4° to 34°N, are assembled from the interval December 1978 to March 1980. This time interval corresponds to two revolutions of Venus about the Sun, designated as two “diurnal cycles”. The distributions of several ion species in this data base have been sorted to identify temporal and spatial variations, and to determine the feasibility of an analytical representation of the experimental results. The first results from the sorting of several prominent ions including O+, O2+, and H+ and several minor ions including CO2+, C+, and H2+ reveal significant diurnal variations, with superimposed modulation associated with solar activity and solar wind variations. The diurnal variation consists of strong day to night contrast in the ion concentrations, with differences of one to two orders of magnitude, depending upon ion mass and altitude. The concentrations of O2+, O+, CO2+ and C+ peak throughout the dayside decreasing sharply at the terminators to nightside levels, lower by one to two orders of magnitude relative to the dayside. The diurnal variations of the light ions H+ and H2+ peak during the night, exhibiting asymmetric nightside bulges favoring the pre-dawn sector, near 0400 solar hour angle. Superimposed upon the diurnal distributions are modulation signatures which correlate well with modulation in the F10.7 index, indicating a strong influence of solar variability on the ion production and distribution. The influence of solar wind perturbations upon the ion distributions are also indicated, by a significant increase in the scatter of the observations with increasing altitude as higher altitudes, approaching 300 kilometers, are sampled. Together, these temporal and spatial variations make the task of modelling the ionosphere of Venus both very interesting and challenging.

The ionization rate of helium in the thermosphere is inferred from AE‐E ion and neutral particle composition measurements, made on several days during solar cycle 21, in the context of a steady state photochemical model for helium ions. The inferred ionization rate varies by a factor of 4 between July 1976 and January 1979, a value consistent with the variation in the flux measured during the same period by the EUV spectrometer on board the AE‐E satellite.

Comprehensive model calculations of the dayside ion density distributions were carried out and compared with results from the Pioneer Venus ion mass spectrometer. The coupled continuity and momentum equations were solved for O-2(+), O+, CO2+, C+, N+, He+, and H+ densities for altitudes well away from the ionopause, where the horizontal transport terms are negligible. Chemical equilibrium solutions, describing conditions below about 200 km, were also obtained for N-2(+), NO+, and CO+. The agreement between the model calculations of ion density and the measurements is good for some species, such as O+, and rather poor for others, such as CO+ indicating that while a basic understanding of the major chemical and physical processes controlling the composition and vertical distribution of the dayside Venus ionosphere, well below the ionopause, has been achieved, there are many important details requiring further investigations.

Measurements of electron temperature, satellite potential, ion density and ion composition from the cylindrical electrostatic probe and the Bennett ion mass spectrometer on board the AE‐C satellite were used to investigate the influence of the body size paramter RD = R0/λD (where R0 is the satellite radius and λD is the ambient Debye length) on ion distribution in the very near wake. The investigation focused on [O+] plasmas. It was found that the ratio (β) of density in the wake to ambient density varies with RD and that the variation can be described by a simple exponential relationship of the form β = a0 exp (a1RD) for 37 ≤RD ≤ 47 and a0 = 0.06, a1 = −0.009. The present study extends that of Samir et al. [1979a].

We compare the southward directed neutral meridional wind and the nighttime ion density simultaneously measured on AE‐E for altitudes from 250 to 290 km and find them to correlate when the magnetic field inclination is negative and anticorrelate when the inclination is positive. This effect is attributed to neutral winds transporting the ions up or down a field line depending upon the sign of the field‐aligned wind and the inclination. Model calculations for the ion density in the presence of a field‐aligned wind and ambipolar diffusion are presented. Six examples of this ionospheric response are analyzed and found to be in qualitative and approximate quantitative agreement with the theoretical results.

In situ measurements of neutral thermospheric composition derived from orbiter neutral mass spectrometer (ONMS) and ion mass spectrometer (OIMS) experiments on Pioneer Venus are interpreted. Observed day to night density variations with asymmetries between dawn and dusk contain pronounced signatures of various transport processes due to winds, exospheric flow, and vertical diffusion. The relative magnitudes of these processes depend significantly on the rotation rate of the thermosphere and its turbulent properties. On the basis of a theoretical three‐dimensional multiconstituent model describing solar diurnal tides in a rotating atmosphere, the analysis leads to the following conclusions:(1) The day‐night temperature contrast on Venus is associated with wind velocities of about 200 m/s (for the lowest order harmonic) which transport 0, He, and H toward the nightside. (2) Mass exchange with the mesosphere, commensurate with an eddy diffusion coefficient of K=3 × 107 is required to buffer that horizontal advection such as to reproduce the observed daytime bulge in 0 as well as the comparatively small diurnal variations in He. (3) Exospheric flow significantly affects the H distribution. (4) The observed time response and magnitude of the day‐night density variations require that transport processes are only effective over time periods between 5 to 10 days, which, in comparison with the 243‐day rotation rate of the solid body, implies a super rotation rate or prevailing winds in excess of 50 m/s at the equator. (5) Nonlinear mass transport results in wave steepening and contributes to amplify the density extrema in H and He; owing to the long time constant for He transport, higher order tidal modes overtake the fundamental harmonic and shift the density maximum toward dawn.

Time‐dependent model studies of the aurora show that various ionospheric parameters respond to the onset of auroral ionization with different time histories. While satellite‐borne instruments sample the spatial morphology of these parameters, geostationary detectors are needed to follow the temporal morphology at a point in space. A pass of the Atmosphere Explorer C satellite over Poker Flat, Alaska, and ground‐based photometric and photographic observations have been used to resolve the time‐space ambiguity of a specific auroral event. The density of the O+, NO+, O2+, and N2+ ions, the electron density, and the electron temperature observed at 280‐km altitude in a 50 km wide segment of an auroral arc are predicted by the model if particle precipitation into the region commenced about 11 min prior to the overpass. Continuous photometric measurements and all‐sky photographs taken at 1‐min intervals from Poker Flat show that the arc formed and became bright approximately 11 min before the satellite measurements were made.

The concentration of atomic hydrogen in the Venus thermosphere near 165km altitude and ∼18° north latitude has been derived from Pioneer Venus in situ measurements of n(H+), n(O+), n(O), and n(CO2), under the assumption of chemical equilibrium. Altitude profiles of derived n(H) suggest that chemical equilibrium prevails to an altitude of at least 200km on the dayside and to 165 km on the nightside. Measurements below these limits were made by the ion and neutral mass spectrometers on the orbiter spacecraft between December 1978 and July 1979, while periapsis traversed a complete diurnal cycle. The hydrogen concentration is found to rise sharply at both terminators from a dayside value of ∼5 × 104 cm−3, and to exhibit an asymmetric nightside distribution with a peak density in the predawn sector approximately 400 times greater than the dayside value. Analysis suggests that wind‐induced diffusion, combined with exospheric return flow, can account for the observed hydrogen behavior. The large day‐night temperature contrast enhances advective transport, which produces the large n(H) diurnal variation; the shift of the n(H) nighttime maximum toward dawn is caused by atmospheric superrotation.

The first in situ measurements of the details of the global composition and dynamics of the ionosphere of Venus have been obtained from the Bennett ion mass spectrometer on the Pioneer Venus orbiter during the period December 1978 through August 1979. These results include observations of three related plasma regimes, including (1) the bowshock‐ionosheath region, (2) the thermal ionosphere, and (3) a superthermal flowing ion layer interfacing with the ionosphere at the ionopause and extending outward to variable heights above the planet. During quiet periods an abundant ionosphere is observed both on the dayside and on the nightside, generally dominated by O+ above 200 km (except for a predawn region where H+ exceeds O+) and by O2+ down to typical periapsis heights of about 160 km. A sampling of some of the less disturbed data exhibits strong day to night variations in the distributions of the most prominent ions, including O+, O2+, CO2+, C+, N+, CO+(N2+), NO+, H+, He+, O18+, O++, and H2+. Important features of the day‐night variation measured at 200 km and at a fixed latitude of about 8°N include asymmetric nightside bulges in the distributions of the light ions H+ and He+, peaking near dawn at 110° and 90° solar zenith angle, respectively. These asymmetries are associated with dawn‐dusk asymmetries in the distributions of O+, O2+, and other molecular ions, which exist in higher concentrations near dusk, relative to dawn. Associated observations of the bowshock‐ionosheath and the ionosphere‐superthermal plasma layer regions indicate close coupling among these plasma regimes. The ionopause, identified as the boundary between the thermal ionosphere and the superthermal flow layer, is encountered near 250–400 km near the subsolar point and extends at times to heights greater than 1000 km in the flanks and nightside regions. Under disturbed nightside conditions, particularly noticeable in the dusk region, the ionosphere may exhibit randomly spaced concentration gradients of an order of magnitude associated with complex patterns of ion flow with velocities up to 10 km/s encountered within the main body of the ionosphere.

Identical Bennett radio frequency ion mass spectrometer instruments on the Pioneer Venus Bus and Orbiter have provided the first in-situ measurements of the detailed composition of the planet's ionosphere. The sensitivity, resolution, and dynamic range are sufficient to provide measurements of the solar-wind-induced bow-shock, the ionopause, and highly structured distributions of up to 16 thermal ion species within the ionosphere. The use of adaptive scan and detection circuits and servo-controlled logic for ion mass and energy analysis permits detection of ion concentrations as low as 5 ions/cm3 and ion flow velocities as large as 9 km/s for O+. A variety of commandable modes provides ion sampling rates ranging from 0.1 to 1.6 s between measurements of a single constituent. A lightweight sensor and electronics housing are features of a compact instrument package.