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We indicate how existing Pioneer Venus Orbiter (PVO) data might be used to gain a better understanding of nightward ion flow in the Venusian ionosphere. Calculations based on PVO measurements made at solar maximum suggest that the global nightward flow of O(+) may be signifcantly greater than is required to maintain the observed nightside ionosphere densities. The validity of this conclusion depends upon (1) the accuracy with which the flow can be determined from the PVO ion density and velocity measurements and (2) the validity of the ionosphere theory used to estimate the required downward O(+) flux on the night side. If the measurements and theory are assumed to be accurate, the excess nightward flow implies a significant rate of ion escape from the planet, particularly at times of low solar wind dynamic pressure, Psw, when the ionopause rises to allow increased nightward flow. To illustrate a potentially important mechanism for ion escape from Venus, we present Orbiter Electron Temperature Probe (OETP) observations of plasma clouds and scavenged ionospheric plasma observed above the ionopause. We then employ OETP and Orbiter Retarding Potential Analyzer (ORPA) data to reexamine the global ion flow for average Psw conditions.

A global spectral model of electron temperature and density is developed in which only the latitudinal variations observed in narrow altitude and local time bands are used. The measurements for this purpose are obtained during intervals that are short as compared to a season. By suppressing all variables except latitude the model can use 17th-order polynomials and generates a latitudinal structure that is similar to that described by the observational data. The treatment is designed to correct for the eccentricity and slow evolution of the data-collection satellite orbit so that small-scale features can be resolved. By comparing the results of a global model and the present latitudinal model to observational measurements of electron temperature and density it is shown that auroral-zone and midlatitude trough structures are not resolved in latitudinal models. The observational data can be used to study the response of electron density and temperature to solar EUV flux variations and geomagnetic activity.

Pioneer Venus in situ measurements of N(e), total solar EUV flux, and solar wind dynamic pressure P(sw) are used to investigate the solar cycle and orbit-to-orbit variations of the Venus nightside ionosphere. It is found that the density of the main body of the nightside ionosphere below 600 km responds to the solar wind dynamic pressure variations in much the same way that the density of the ionotail does: both regions are depleted at times of high P(sw). Solar cycle variations in the EUV flux near solar minimum produce very large changes in the electron density in ionotail, changes that are consistent with the ionotail being supplied by nightward ion flow. However, short-term variations in the EUV flux produce a weak negative response in the density at altitudes between 150 and 600 km, at least at solar maximum. The nightside variations are consistent with enhanced solar wind scavenging of plasma from the nightside ionosphere during periods of higher P(sw).

Newly developed global models of the electron density of (Ne) and (Te) in the F-region at solar maximum, based on Langmuir probe measurements from the Dynamics Explorer-2 satellite, are compared with solar minimum models that were developed earlier from Atmosphere Explorer data. Spherical harmonics are used in both models to describe the variations with geomagnetic latitude and local time, but the solar maximum model also includes longitudinal variations. The solar minimum models were for the fixed altitudes of 300 km and 400 km, while the solar maximum model covers all altitudes between 300 and 1000 km. In this paper, the global patterns of Ne and Te at 400 km at solar maximum and minimum are compared with the IRI model for the corresponding parts of the solar cycle. In most respects, the IRI model describes the empirical models quite well at both solar maximum and solar minimum.

This paper describes the observations by the Pioneer-Venus-Orbiter Langmuir probe of the low-density Venus bow shock precursor, i.e., a region of electron and ion-current enhancements just upstream from the Venus bow shock. It is suggested that the precursor signatures represent the effects of a small population of energetic ions in this region. Several possible planetary sources of these ions are considered. The paper also discusses the Langmuir probe measurement technique itself as it applies to the very low densities of the precursor.

Data from the Pioneer Venus Langmuir probe, collected since 1979 (and thus, including the period between solar maximum in 1979-1980 and solar minimum in 1986-1987) are examined. Calculations show that about 51 percent of the solar emission at Venus is due to Lyman alpha (1216 A), 46 percent is produced by wavelengths between 550 and 1100 A, and less than 3 percent is due to wavelengths longer than Lyman alpha. The photocurrents were found to exhibit variations related to the solar cycle and solar rotation, as well as a major 7.2-month periodicity. Three different indices of solar EUV behavior at Venus were derived, which include the photoemission current itself, the total EUV flux, and an F(10.7)-like solar index, and are compared with related measurements made simultaneously at earth.

Empirical models such as the International Reference Ionosphere (IRI) are synthesized from large data bases. They can be viewed as analytical tools to facilitate accessing information stored in the data banks. However, in establishing the models, one has to apply smoothing and averaging procedures that in effect reduce the original information content. This study evaluates the agreement between the data base and the model at two opposite extremes of time resolution. Electron densities and temperatures in the altitude range of 300 to 400 km predicted by the IRI and measured by the AE-C and DE 2 satellites on the level of individual orbits as well as on the level of mission averages are compared. Whereas the averages show excellent agreement, the comparison for individual measurements indicates the limitations of empirical models.

The configuration and morphology of the plasma clouds in the ionotail of Venus (revealed by the Pioneer Venus Orbiter) are studied, and the rate of planetary ion escape, which may be associated with the dissipation and removal of the ionospheric plasma, is estimated. The data supplied by the Orbiter's instruments, the Orbiter electron temperature probe, the ion mass spectrometer, the neutral mass spectrometer, the magnetometer, and the plasma analyzer, are analyzed, and the results of the observations are discussed.

Langmuir-probe measurements made at solar maximum from the DE-2 satellite in 1981 and 1982 are used to examine the latitudinal variation of electron temperature at altitudes between 300 and 400 km and its response to 27-day variations of solar EUV. A comparison of these data with models based on solar-minimum measurements from the AE-C suggests that the daytime electron temperature does not change very much during the solar cycle except at low latitudes where a particularly large 27-day variation occurs. It is found that the daytime electron temperature near the F2 peak is more responsive to short-term variations in F10.7 than to any longer-term changes that may occur between solar minimum and maximum.

Ionosphere electron temperature data gathered by the AE-C, AEROS, Isis-1 and -2 spacecraft are employed to define linear models for the average conditions. Account is taken of evidence for seasonal, altitudinal, solar activity and density-temperature effects. Notably, use is made of the high negative correlations between the electron temperature and density, thereby allowing either to be calculated if data are available on the density.

Using global empirical models of Venus ionospheric conditions, the plasma flow field consistent with the horizontal momentum equation in both viscid and inviscid forms is solved for. It is found that plasma viscosity is negligible except at low altitudes and that the observed plasma flows are consistent with the inviscid solution above 300 km but are larger than the calculated flows at lower altitudes. This is probably due to downward momentum advection. The strong vertical shear in the calculated and observed flows just beyond the terminator may produce turbulence there, manifested as the observed transterminator waves in plasma density and magnetic field. The implications of ionospheric superrotation are discussed.

Pioneer Venus Orbiter (PVO) electron temperature probe measurements from the Venus years between December 1978 and December 1982 have been used to construct new empirical models of electron temperature and density. The models are used to obtain a two-dimensional solution of the momentum equation for the nightward ion flow velocities believed to be largely responsible for the maintenance of the nightside ionosphere. The velocities at the terminator rise from the neutral atmospheric wind velocity of about 300 m/s at 150 km to a peak velocity exceeding 2000 m/s above 500 km, in general agreement with PVO measurements of ion drift in that region.

On the basis of in situ measurements of solar EUV flux, an investigation is conducted on the extent of EUV-contributed Venus dayside condition modulation as found in the Pioneer Venus Orbiter's Langmuir probe experiment. In addition, a novel method for Venus EUV flux measurement is introduced which relies on the Langmuir probe sensor's photoelectron emission in regions far above the ionosphere. It is found that while EUV flux strongly affects ionospheric number density, its electron temperature effects are minor. An examination of the role of ionospheric magnetic fields in dayside condition modulation shows that large scale horizontal field presence or absence has no effect on electron number density or temperature at these altitudes, due to the collision domination of ions and the fact that vertical diffusive transport is unimpeded by magnetic fields of the observed magnitudes.

Data from the Atmospheric Explorer-C recorded during the rise in solar activity from 1975 to 1978, and Dynamic Explorer-2 data recorded at solar maximum are used to examine how the relation between electron temperature and density in the F-region of the ionosphere is altered by solar activity. It is found that the solar maximum temperature Te is a factor of two larger than the solar minimum Te for the same values of electron density. Temperature does not necessarily increase in response to solar activity because electron density increases enough to approximately cancel the effect of higher solar extreme ultraviolet heating. The effect of solar activity is accounted for by a simple function of the F (10.7 cm) index which multiplies the solar minimum equation from a previous study (Brace and Theis, 1978).

It is pointed out that the thermal structure of the ionosphere represents a quasi-static balance between a variety of heat sources and sinks which vary spatially and temporally on a wide range of time scales. The present investigation has the objective to present selected early results from the Dynamics Explorer-2 (DE-2) Langmuir probe instrument and to make an initial evaluation of how the thermal structure of the ionosphere at solar maximum differs from that observed at solar minimum. Bowen et al. (1964) and Brace and Reddy (1965) devised early empirical models of the F region electron temperature (Te), based on satellite Langmuir probe measurements at low levels of solar activity. The global structure of Te and the electron density (Ne) obtained in the current investigation is not very different from that reported by Brace and Reddy. The primary difference at solar maximum is that Ne is everywhere much higher, but Te differs only in detail.

The global distribution of the plasma clouds observed by the Pioneer Venus Orbiter suggest that they originate at the dayside ionopause as wavelike structures which may become detached and swept downstream in the ionosheath flow. Alternatively, the clouds may actually be attached streamers analogous to cometary structure. Estimates of the total ion escape rate from Venus by this process yields values up to 7 x 10 to the 26th ions/s. Preliminary analysis shows that such an escape flux could be supplied by the upward diffusion limited flow of O(+) from the entire dayside ionosphere. If such an escape flux were to continue over the entire lifetime of Venus, the effects upon the evolution of the primitive atmosphere may have been significant.

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.

Langmuir probe measurements of electron temperature, T sub e, in the vicinity of 300, 400, 1400 and 3000 km from the Atmosphere Explorer-C and the ISIS satellites have been employed to construct empirical models of the global distribution of T sub e at each of these altitudes. Legendre polynomials are employed to describe the observations at solstice and equinox in terms of dip latitude and local time. Sources of T sub e variations, such as solar activity, magnetic activity and longitude are found to be of second order importance, although they are resolvable in some cases by comparisons of the data with the model. The behavior of T sub e at the altitudes of these models is discussed in terms of its implications for our understanding of the energy exchange between the F-region and the plasmasphere.

The Dynamics Explorer Langmuir probe instrument (DE-LANG), a spare unit from the Pioneer Venus mission that has been modified to work with the DE spacecraft, is described. Two independent sensors are connected to individual adaptive sweep voltage circuits that continuously track the changing electron temperature and spacecraft potential while autoranging electrometers adjust their gain in response to the changing plasma density. The control signals used in achieving this automatic tracking provide a continuous monitor of the ionospheric parameters without telemetering each volt-ampere curve. In addition, internal data storage circuits permit high-resolution, high-data-rate sampling of selected volt-ampere curves for transmission to the ground to verify or correct the inflight processed data.

Pioneer Venus Orbiter Electron Temperature Probe measurements of hundreds of bow shock and ionopause crossings are employed in describing the configuration of these two boundaries and their variations in response to changes in solar wind pressure. The average bow shock configuration is found to be well represented by an Archimedian hyperboloid whose altitude at the subsolar point is 0.46 Venus radii, a value slightly greater than that derived from Pioneer Venus magnetometer data using a fit to a general conic section. It is noted that the average bow shock configuration exhibits a high degree of azimuthal symmetry near the terminator. The orbit to orbit variability of the shock location is unexpectedly large, the standard deviation being about 10%. A tendency is noted for the bow shock and the ionopause to expand and contract simultaneously, but the weakness of their orbit by orbit correlation suggests that the ionopause of Venus is not the only obstacle to the solar wind. It is thought that such processes as photoion pickup and charge exchange with neutrals may be important in diverting the solar wind plasma around the planet.

A steady-state two-dimensional heat balance model is used to analyze the night side Venusian ionospheric electron temperatures given by the Pioneer Venus orbiter electron temperature probe. The energy calculation includes the solar EUV heating at the terminator, electron cooling to ions and neutrals, and heat conduction within the ionospheric plasma. An optimum magnetic field is derived by solving for the heat flux directions which force energy conservation while constrained by the observed temperatures within the range of 80-170 deg solar zenith angle and 160-170 km. The heat flux vectors indicate a magnetic field that connects the lower night side ionosphere to the day side ionosphere, and connects the upper ionosphere to the ionosheath. The lower ionosphere is heated through conduction of heat from the dayside, and the upper ionosphere is heated by the solar wind in the ionosheath with heat flowing downward and from the nightside to the day side.

The dynamics of the Venus ionosphere relates to the variations in the solar wind and the ionosheath magnetic fields as demonstrated by the electron density and temperature measurements of the Pioneer Venus orbiter electron probe. The mean ionopause height increases from 330 km at the subsolar point to 700 km at the dusk terminator, and to 1000 km at the dawn terminator; the dayside ionopause expands and contracts with solar wind pressure variations. Extreme spatial irregularities in the shape of holes, horizontally stratified layers, and detached plasma clouds are observed in the nightside ionosphere. The ion pickup on the dayside is described in terms of solar wind discontinuities inducing a wavelike pattern at the ionopause which is penetrated by the ionosheath plasma and magnetic fields which remove the plasma in the form of detached plasma clouds.

The electron temperature and density of the Venus ionosphere is characterized using data from the Pioneer Venus orbiter electron temperature probe experiment for the full range of solar zenith angles and local times. Values for the electron density are nearly uniform across the day side with a sharp decrease in the vicinity of the terminator. The model shows a substantial night side ionosphere which can be accounted for by a combination of several transport processes, and by local production by precipitating particles. The electron temperature model demonstrates the high temperatures seen on both the day side and night side. It is found that the night side is much more variable than the day side, and that there is no obvious north-south asymmetry in either the temperature or the density.

Recently atomic nitrogen densities of about one million per cu cm were measured at 400 km by the open source mass spectrometer on the Atmosphere Explorer-C satellite (AE-C). Daytime N densities about 50 million per cu cm at 160 km have also been inferred from airglow and other measurements on AE-C. It is shown that atomic nitrogen densities of this magnitude result in significantly lower values for the O2(+) concentration than those measured on AE-C over the altitude range to 160 to 200 km, because of the removal process O2(+) + N k3 yields NO(+) + O. The discrepancy can be explained in terms of latitudinal variations in both the N and O2 densities. Evidence is presented which indicates that k3 could be as low as 0.1 billionth per cu cm at ionospheric temperatures. K3 is the rate constant for the reaction of O2(+) with N(4-S).

Observations of the electron concentration and the temperature from the electrostatic probes on the Isis 1 satellite were used to examine the location and behavior of the plasmapause at about 3000-km altitude in the vicinity of L = 4. At these altitudes the electron concentration measurements are equivalent to measurements of H(+), since the satellite is well into the protonosphere. The plasmapause is evident as a sharp drop in electron concentration by a factor of 100 as the satellite passes into the polar cap, and a corresponding increase is observed as it enters the plasmasphere on the opposite side of the earth. An enhancement of temperature is also observed at the plasmapause, an effect that is most visible at night, when the temperatures at latitudes above and below the plasmapause are usually very low. The position of the plasmapause decreases with magnetic activity but is found to be somewhat less sensitive to Kp than is the equatorial plasmapause.

The Orbiter Electron Temperature Probe (OETP) instrumentation and measurement technique has been designed to perform in-situ measurements of electron temperature and electron and ion density in the ionosphere of Venus. Adaptive sweep voltage circuitry continuously tracks the changing electron temperature and spacecraft potential while auto-ranging electrometers adjust their gain in response to the changing plasma density. Control signals used in the instrument to achieve this automatic tracking provide a continuous monitor of the ionospheric parameters without telemetering each volt-ampere curve. Internal data storage permits high data rate sampling of selected raw characteristic curves for low rate transmission to earth. These curves are used to verify or correct the inflight processed data. Sample in orbit measurements are presented to demonstrate instrument performance.

The paper reports in situ direct measurements of regularly observed thermosphere neutral particle (i.e., N2) temperature maxima that occur predominantly after midnight. The measurements were made using the Neutral Atmosphere Temperature Instrument aboard Atmosphere Explorer-E in circular orbit (inclination 19 deg) at about 275 km. Hundreds of midnight temperature maxima were observed, some having values exceeding the well-known afternoon maximum. Simultaneous in situ direct measurements of the wind component normal to the orbit plane have also been obtained and observed to correlate in a systematic way with the temperature maxima.

The AE-C satellite measurements of electron temperature and ion concentration are used to construct a model of the relationship between these parameters and its variation with altitude in the daytime, nonauroral ionosphere. The major features of the model are that electron temperature is independent of ion concentration at altitudes below 200 km and that electron temperature varies inversely with ion concentration above perhaps 250 km. This behavior is qualitatively consistent with current theoretical models of ionospheric heating and cooling.

In situ measurements of local vertical neutral particle motions have been made using the Neutral Atmosphere Temperature Instrument (NATE) on Atmosphere Explorer-C from observations of the direction of flow of neutral particles into the antechamber of the sensor (mass spectrometer). Values ranging from a few to more than 80 meters per second have been observed. The data show vertical motions greater than a few meters per second to be present most of the time, the magnitude being a function of many factors including magnetic activity, location, and magnetic storm history. In a specific case, it is concluded that the observed vertical motions and kinetic temperature are evidence of a travelling disturbance originating as a gravity wave in the auroral zone.

Observations of the electron concentration, N sub e, and temperature, T sub e, from the electrostatic probes on the ISIS-1 satellite were used to examine the location and behavior of the plasmapause at about 3000 kilometers altitude in the vicinity of L = 4. At these altitudes, the N sub e measurements are equivalent to measurements of H(+) since the satellite is well into the protonosphere. The plasmapause as is evident as a sharp drop in N sub e by a factor of 10 to 100 as the satellite passes into the polar cap, and a corresponding increase is observed as it enters the plasmasphere on the opposite side of the Earth. An enhancement of T sub e is also observed at the plasmapause, an effect that is most visible at night when the temperatures at latitudes above and below the plasmapause are usually very low. The position of the plasmapause decreases with magnetic activity but is found to be somewhat less sensitive to K sub p than is the equatorial plasmapause. Also unlike its equatorial behavior, the mid-latitude plasmapause exhibits no detectable late afternoon bulge. These differences imply rather complex coupling of the thermal plasma along the field lines that link these two regions of the plasmasphere. An additional factor may be the recently observed axial asymmetry in the geomagnetic field at high altitudes.

This paper briefly describes the cylindrical electrostatic probes to be used on the new Atmosphere Explorer-C, -D, and -E, and outlines the methods to be employed to analyze the experimental data in terms of electron temperature and ion and electron concentration. Two independent cylindrical sensors and partially redundant electronic systems permit greater reliability in measurements. Measurements are made at 1-sec and 2-sec intervals along the orbit. The sensors themselves have been modified from previous applications to cope with the lower electron temperature expected in these low-altitude orbits and to counter possible surface contamination caused by the hydrazine engines used for orbit adjustments.

Description of a method that would permit a satellite-borne neutral mass spectrometer to measure the atmospheric temperature. The spectrometer examines the partial pressure variations that occur as the wake of a small rectangular baffle is swept across the entrance orifice of the spectrometer. For a given baffle size and for a mounting distance from the orifice, the depth of the resulting pressure minimum depends only on the thermal velocity or temperature of the observed species. The validity of the method can be checked by measuring the wake characteristics of more than one species and/or by employing each of several baffle sizes. The theory includes the effect of a finite orifice size, finite baffle length, and the backscattering of particles from the baffle into the orifice. It is found that a suitable baffle arrangement can be achieved that will permit the temperature to be measured over at least the range normally encountered in the thermosphere (200 to 2000 K) and, depending on the sensitivity and background pressure of the spectrometer, over an altitude range of about 140 to 600 km.