H. C. Brinton

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.

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.

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.

Bennett Ion Mass Spectrometers on the Pioneer Venus Bus And Orbiter

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 planets 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.

Dynamic variations observed in thermal and superthermal ion distributions in the dayside ionosphere of venus

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.

Comparison of calculated and measured ion densities on the dayside of venus.

Datafrom the Pioneer Venus ion mass spectrometers are compared with model calculations of the ion density distributions appropriate for daytime conditions. The model assumes diffusive equilibrium upper boundary conditions for the major ions (O2+, O+, CO2+, He+, and H+); the agreement between the calculated and measured gross behavior of these ions is reasonably good except for H+, which may be influenced strongly by convective transport processes. The distributions of five minor ions (C+, N+, NO+, CO+, and N2+) are also calculated for the chemically controlled region (≲ 200 kilometers); the agreements are, in general, poor, an indication that our present understanding of the Venus minor ion chemistry is still incomplete.

Temporal and spatial variations observed in the ionospheric composition of Venus: Implications for empirical modelling

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.

Variations in ion and neutral composition at Venus: Evidence of solar control of the formation of the predawn bulges in H+ and He1

Abstract 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 F 10.7 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.