H. A. Taylor

Ionosphere of Venus - First observations of day-night variations of the ion composition

The Bennett radio-frequency ion mass spectrometer on the Pioneer Venus orbiter is returning the first direct composition evidence of the processes responsible for the formation and maintenance of the nightside ionosphere. Early results from predusk through the nightside in the solar zenith angle range 63� (dusk) to 120� (dawn) reveal that, as on the dayside, the lower nightside ionosphere consists of F1and F2 layers dominated by O2+ and O+, respectively. Also like the dayside, the nightside composition includes distributions of NO+, C+, N+, H+, He+, CO2+, and 28+ (a combination of CO+ and N2+). The surprising abundance of the nightside ionosphere appears to be maintained by the transport of O+ from the dayside, leading also to the formation of O2+ through charge exchange with CO2. Above the exobase, the upper nightside ionosphere exhibits dramatic variability in apparent response to variations in the solar wind and interplanetary magnetic field, with the ionopause extending to several thousand kilometers on one orbit, followed by the complete rertnoval of thermal ions to altitudes below 200 kilometers on the succeeding orbit, 24 hours later. In the upper ionosphere, considerable structure is evident in many of the nightside ion profiles. Also evident are horizontal ion drifts with velocities up to the order of 1 kilometer per second. Whereas the duskside ionopause is dominated by O+ H+ dominates the topside on the dawnside of the antisolar point, indicating two separate regions for ion depletion in the magnetic tail regions.

Disappearing ionospheres on the nightside of Venus

Abstract Instruments on the Pioneer Venus Orbiter have detected a substantial ionosphere on the nightside of Venus during most orbits. However, during some orbits the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low-density plasma. The solar wind dynamic pressure on these occasions is greater than average. We have correlated data from several instruments (Langmuir probe, ion mass spectrometer, retarding potential analyzer, magnetometer, and plasma analyzer) for a number of orbits during which the nightside ionosphere had disappeared. The magnetic field tends to be coherent, horizontal, and larger than usual, and the electron and ion temperatures are much larger than they usually are on the nightside. We suggest mechanisms which might explain the reasons for the disappearance of the ionosphere when the solar wind dynamic pressure is large.

A theoretical model of the ionosphere dynamics with interhemispheric coupling

Abstract The ionospheric plasma is described by means of the momentum and continuity equations for O + , He + , H + and the energy equations for T e and T i . The ion production rate profiles and the photoelectron heating rates to the plasma are computed separately and serve as source functions in the continuity and energy equations respectively. The neutral atmosphere (composition, winds, temperature) as well as the incident photon spectrum are part of the inputs. Charge transfer, collisional energy loss processes, ion-neutral drag, diffusion and electron heat conduction are among the physical processes included in the calculations. Ion heat conduction is neglected. Assuming steady state, the equations are transformed into integral equations and then solved iteratively along geomagnetic field lines from the region of chemical equilibrium up to the equatorial plane. In this scheme non-local heating is included in a self-consistent way. The lower boundary conditions are obtained by satisfying photochemical and local energy equilibrium. Under conditions of asymmetry with respect to the equatorial plane the solutions are carried out for both hemispheres, and the upper boundary conditions are determined by requiring continuity of the physical parameters across the equatorial plane. In this way inter hemispheric plasma transport is introduced in a natural way. For symmetric conditions the transport fluxes are assumed zero at the Equator. An extension of this model to include time-dependent phenomena is presented.

Ionosphere of venus: first observations of the dayside ion composition near dawn and dusk.

The first in situ measurements of the composition of the ionosphere of Venus are provided by independent Bennett radio-frequency ion mass spectrometers on the Pioneer Venus bits and orbiter spacecraft, exploring the dawn and duskside regions, respectively. An extensive composition of ion species, rich in oxygen, nitrogen, and carbon chemistry is idenitified. The dominant topside ion is O+, with C+, N+, H+, and He+ as prominent secondary ions. In the lower ionosphere, the ionzization peak or F1 layer near 150 kilometers reaches a concentration of about 5 x l03 ions per cubic centimeter, and is composed of the dominant molecular ion, O2+, with NO+, CO+, and CO2+, constituting less than 10 percent of the total. Below the O+ peak near 200 kilometers, the ions exhibit scale heights consistent with a neutral gas temperature of about 180 K near the terminator. In the upper ionosphere, scale heights of all species reflect the effects of plasma transport, which lifts the composition upward to the often abrupt ionopause, or thermal ion boundary, which is observed to vary in height between 250 to 1800 kilometers, in response to solar wind dynamics.

Evidence of solar geomagnetic seasonal control of the topside ionosphere

Abstract Ion composition results obtained from the polar orbiting OGO-4 satellite during 1967–68 reveal a pronounced longitudinal variation in the composition of the topside ionosphere. This variation is in the form of a large scale wobble or shift in the latitudinal distributions of the major topside ions H + , O + , He + and N + , observed as the earth rotates beneath the fixed satellite orbit. Both the location and prominence of distinct ionospheric features, including the O + -H + transition level, the H + /He + ratio, the high latitude depletion of H + and He + , and the winter bulge in He + , are found to change significantly between longitudes for which the angle between the Earth-Sun line and the dipole equator has its greatest variation. Similarly, it is found that the ambient ion concentrations at a given latitude may change by as much as an order of magnitude between contrasting longitudes, even though the altitude and magnetic activity remain nearly constant. The overall result is that seasonal variations, such as the decrease in production of ionization at winter latitudes, are maximized at the location of extreme ‘solar-geomagnetic’ season. The persistence of the longitudinal asymmetry over a range of local times, seasons, and magnetic conditions reveals that the topside ionosphere is dependent upon a ‘solar-geomagnetic’ rather than simply a solar seasonal control. This variation, which may involve large scale transport of both ions and neutral particles, presents an added complication to studies of the topside ionosphere. These results indicate that investigations of both long and short term changes in the ion composition must, to be rigorous, take into consideration the ‘solar-geomagnetic’ seasonal effects.

Ionosphere of venus: first observations of the effects of dynamics on the dayside ion composition.

Bennett radio-frequency ion mass spectrometers have returned the first in situ measurements of the Venus dayside ion composition, including evidence of pronounced structural variability resulting from a dynamic interaction with the solar wind. The ionospheric envelope, dominated above 200 kilometers by O+, responds dramatically to variations in the solar wind pressure, Which is observed to compress the thermal ion distributions from heights as great as 1800 kilometers inward to 280 kilometers. At the thermal ion boundary, or ionopause, the ambient ions are swept away by the solar wind, such that a zone of accelerated suprathermnal plasma is encountered. At higher altitudes, extending outward on some orbits for thousands of kilometers to the bows shock, energetic ion currents are detected, apparently originating from the shocked solar wind plasma. Within the ionosphere, observations of pass-to-pass differences in the ion scale heights are indicative of the effects of ion convection stimlulated by the solar wind interaction.

The Venus ionosphere

Abstract Physical properties of the Venus ionosphere obtained by experiments on the US Pioneer Venus and the Soviet Venera missions are presented in the form of models suitable for inclusion in the Venus International Reference Atmosphere. The models comprise electron density (from 120 km), electron and ion temperatures, and relative ion abundance in the altitude range from 150 km to 1000 km for solar zenith angles from 0° to 180°. In addition, information on ion transport velocities, ionopause altitudes, and magnetic field characteristics of the Venus ionosphere, are presented in tabular or graphical form. Also discussed is the solar control of the physical properties of the Venus ionosphere.

Location and source of ionospheric high latitude troughs

Abstract One prominent feature of the high latitude topside ionosphere is the existence of sharp latitudinal depletions in the total ion (electron) concentrations within the auroral/cusp regions. These high latitude troughs, as seen by the Bennett ion mass spectrometer observations on the satellite OGO 6 at altitudes between 400 and 1100km correspond to depletions in the atomic ions which are accompanied by localized enhancements of the minor molecular ion densities. All of the high latitude troughs traversed by OGO 6 (1969–1970) were recorded and the average invariant latitude-magnetic local time (M.L.T.) distribution was determined. The troughs on the average were found at all local times to be in the vicinity of the auroral oval and to move equatorward in response to increasing magnetic activity. The average trough location was compared to the average polar cap boundary as defined by the convection electric field reversal and the electron trapping boundary as well as to the maximum horizontal magnetic disturbance associated with the large scale field aligned currents. The high latitude troughs on the average best followed the maximum magnetic disturbance distribution. It is concluded that the troughs are the result predominantly of enhanced chemical 0+ losses in regions with high convection velocities.

Thermal ion perturbations observed in the vicinity of the Space Shuttle

Abstract During the Spacelab-2 Shuttle mission the University of Iowa Plasma Diagnostic Package (PDP) probed the plasma environment of the Space Shuttle by maneuvers at the end of the extended Remote Manipulator System (RMS) arm. Also the PDP was operated as a free flying satellite which remained in the vicinity of the Shuttle as the Shuttle was maneuvered about it. During this portion of the mission, the Bennett thermal ion mass spectrometer on the PDP measured some distinctive effects of the large and gaseous emitting Shuttle upon the ambient thermal plasma environment. Within the open cargo bay when thrusters were not firing there was typically an absence of measurable thermal ions when the bay was in the wake of the spacecraft. Just above the top of the bay when traces of ions are detected in the wake the dominant ion was the contaminant water. When the PDP was released in the wake of the Shuttle and moved away, the spin modulation of the ion flux into the spectrometer was initially different for the O + and H 2 O + ions. The O + ions were streaming into the spectrometer from the spacecraft velocity direction whereas the water ions were flowing from a direction as much as 77° from this. The concentration of water ions in the near wake decreased with increasing distance, becoming less than the predominant ion O + at wake distances of the order of 30 m. However, traces of contaminant ions were present as far as the maximum distance of several hundred meters explored by the free-flying PDP. The neutral gases from the Shuttle extend in all directions as was shown by the presence of water ions, not only in the immediate vicinity of the Shuttle and in its wake but also even several hundred meters upstream. The presence of contaminant NO and O + 2 ions brings into question whether reliable ambient ion measurements can be made from the Shuttle.

Venus Ionosphere: Photochemical and Thermal Diffusion Control of Ion Composition

The major photochemical sources and sinks for ten of the ions measured by the ion mass spectrometer on the Pioneer Venus bus and orbiter spacecraft that are consistent with the neutral gas composition measured on the same spacecraft have been identified. The neutral gas temperature (Tn) as a function of solar zenith angle (χ) derived from measured ion distributions in photochemical equilibrium is given by Tn (K) = 323 cos1/5χ. Above 200 kilometers, the altitude behavior of ions is generally controlled by plasma diffusion, with important modifications for minor ions due to thermal diffusion resulting from the observed gradients of plasma temperatures. The dayside equilibrium distributions of ions are sometimes perturbed by plasma convection, while lateral transport of ions from the dayside seems to be a major source of the nightside ionosphere.