摘要:

The arrival of the Pioneer Venus spacecraft in December of 1978 marked the initiation of a new phase of planetary exploration. For the first time a mission had been designed to investigate the global scale properties of another planet's atmosphere. The four entry probes and their bus spacecraft were largely new innovations. The orbiter instruments drew heavily upon the successful Atmosphere Explorer satellites which had been used for studies of earth's atmosphere since 1973.

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07573

年代:1980

数据来源: WILEY

摘要:

The nation's most extensive and intensive study of the planet Venus and its environment occurred during and subsequent to December 1978: Pioneer Venus. This introductory paper to a special issue of theJournal of Geophysical Researchdevoted to the scientific findings of both the orbiter and multiprobe missions documents the major aspects of the missions and provides the reader with necessary background information to cope intelligently with the myriad of scientific findings found elsewhere in the issue. Specific topics covered include program history, key scientific questions addressed by Pioneer Venus, scientific payloads, science management, spacecraft descriptions, launch and interplanetary cruise features, encounter features, nominal mission descriptions, instrument anomalies, and the orbiter extended mission.

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07575

年代:1980

数据来源: WILEY

摘要:

We briefly describe the Pioneer Venus plasma wave instrument and continue with a discussion of wave observations throughout the typical. near‐noon and near‐midnight orbits. This is followed by a detailed comparison of the bow shock turbulence characteristics at earth and at Venus. We then turn to analyze the wave‐particle interactions detected near the dayside ionopause, and we show that the whistler mode Landau damping develops when theBfield direction changes so that the Whistler becomes ob

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07599

年代:1980

数据来源: WILEY

摘要:

Collection of data from the Ames plasma analyzer on the Pioneer Venus orbiter has permitted long‐term measurements of the interaction of the solar wind with Venus. This paper presents a mapping of the ionosheath flow field, as well as plasma measurements in the distant ionosheath and near the distant plasma cavity (∼10 Venus radii downstream), a summary of observations of jumps in the solar wind proton parameters across Venus' bow shock, and also the apparent detection of ionospheric O+accelerated up to solar wind speeds downstream in Venus' ionoshe

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07613

年代:1980

数据来源: WILEY

摘要:

Pioneer Venus observations are used in conducting a study of the location and structure of the Venus bow shock. The trace of the shock in the solar wind aberrated terminator plane is nearly circular at an altitude of 1.38RVindependent of interplanetary magnetic field orientation with an extrapolated subsolar height of 0.38RV. Gas dynamic relations and scaling of the terrestrial analogue are used to determine the effective impenetrable obstacle altitude from the mean shock surface with the conclusion that it lies beneath the observed height of the ionopause. The short‐term variability in shock position is similar to that found at the earth, while over the long‐term bow shock, altitude varies by up to ∼35% in phase with the solar cycle owing to causes other than changing solar wind Mach number. In contrast to ionopause position, which is shown to be well determined by external pressure measurements, bow shock altitude is found to be only weakly dependent upon ionopause height and solar wind dynamic pressure. These results are interpreted in terms of interactions with exospheric neutrals and/or lack of complete deflection of the incident solar wind by currents induced in the ionosphere modifying the flow about Venus from that associated with a tangential discontinuity obstacle of nearly constant radius. The downstream bow shock is smaller in diameter than that of terrestrial case despite the larger Mach cone angle at 0.72 AU most probably due to the smaller relative size of the Venus magnetotail. A brief survey of shock structure with Pioneer Venus instrumentation shows general agreement as to the time and location of the shock crossings with a transition layer thickness of the order of the ion inertial length scale. The observed variation in bow shock structure and the foreshock with upstream parameters was similar to that seen at the

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07625

年代:1980

数据来源: WILEY

摘要:

Solar wind plasma and magnetic field data and ionospheric data obtained from the Pioneer Venus orbiter are considered. It is shown that the variation of the magnetic field pressure within the magnetic barrier is similar to that expected for the solar wind pressure variations along an obstacle's boundary when a more realistic approximation of the shape of the ionosphere is included. Simultaneous solar wind pressure, ionospheric pressure, and magnetic barrier pressure data show that the ionospheric pressure below the ionopause is approximately equal to the solar wind pressure. The magnetic barrier pressure is equal to approximately 2/3 to 3/4 of both the solar wind pressure and the ionopsheric pressure. Estimates of the hot plasma pressure contribution to the total pressure within the magnetic barrier vary from 1/4 to 1/3. Just below the ionopause the ionospheric pressure deviations from the mean ionospheric pressure are significant especially below ∼400 km and appear to be indicative of the adjustment of the ionospheric structure to changing solar wind conditions. A first‐order model of the ionopause pressure variations as a function of height and solar‐zenith angle is suggested. This model provides an estimate of an ‘instant’ ionopause profile for the given solar wind conditions. The mean and ‘instant’ shapes of the ionopause do not appear to correspond to the estimates obtained from ionospheric pressure equilibrium fo

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07642

年代:1980

数据来源: WILEY

摘要:

Venera 9 and Venera 10 bow shock crossings are analyzed for solar zenith angles in the range from ∼25° to ∼153°. The best fit to 62 observed crossings gives the location of the subsolar point of the bow shock as ∼7600 km from the center of Venus or 1.27Rv. Comparison of Venera 9 and Venera 10 bow shock crossings with those observed by the Pioneer Venus orbiter indicate that both the secular variation and the latitudinal asymmetry may be responsible for the closer shock crossings of the Venera 9 and Venera 10 spa

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07651

年代:1980

数据来源: WILEY

摘要:

For three orbit paths of the Pioneer Venus orbiter the interaction between the solar wind and the Venusian ionosphere has been studied. Results of the retarding potential analyzer and the magnetometer are described for the boundary region between the solar wind and the planetary ionosphere. These are the first measurements that show that a transition region exists between the two plasmas of different origin. The observed magnetic field and current system producing it appear strong enough to stop the solar wind flow in front of the ionosphere and to separate the shocked solar wind from the ionosphere. The transition region between the ionosheath and the ionosphere is called the ‘mantle.’ The observed mantle electron energy spectra close to the ionopause show ionospheric character. With increasing height the number of electrons that have ionospheric energies decreases, and the number of electrons that have solar wind energies gradually increases toward the ionosheath boundary, where only solar wind energy spectra are observed. The mantle surrounds the frontside of the ionosphere and extends probably more than eight Venus radii downstr

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07655

年代:1980

数据来源: WILEY

摘要:

Measurements of electron density and temperature by the Pioneer Venus orbiter electron temperature probe (OETP) are used to describe the dynamic behavior of the Venus ionosphere and to begin to relate this complex behavior to variations in the solar wind and the ionosheath magnetic field, parameters that are also measured by orbiter instruments. The average ionopause height rises from about 330 km at the subsolar point to 700 km at the dusk terminator and 1000 km at the dawn terminator, in both cases exhibiting a stronger dependence upon solar zenith angle than that reported from Venera 9 and 10 occultation data. The ionopause on the dayside tends to expand and contract with changes in solar wind pressure, becoming asymptotic to about 290 km at pressures above 4 × 10−8dyn/cm² and rising to over 1000 km for pressures below 5 × 10−9dyn/cm². The solar wind pressure, after correction for solar zenith angle, agrees approximately with the magnetic field pressure applied at the ionopause, confirming earlier suggestions that the pressure is conveyed to the ionosphere primarily by the magnetic field rather than by the shocked solar wind plasma. On the nightside the ionopause is much more highly variable, sometimes falling below 200 km or rising above 3500 km. The present Pioneer Venus orbit does not permit the true configuration to be measured. Within the nightside ionosphere itself, we find extreme spatial irregularities in the form of holes, horizontally stratified layers, detached plasma clouds, and dual temperature plasma in regions of low electron density. A scenario is developed to describe the process of ion pickup on the dayside in terms of solar wind pressure discontinuities inducing wavelike structure at the ionopause, which then is penetrated by ionosheath plasma and magnetic fields that remove ionospheric plasma impulsively in the form of detached plasma clouds. The energy released in this process may be responsible for the elevated electron temperatures observed in both the dayside and nightside of the Venus ion

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07663

年代:1980

数据来源: WILEY

摘要:

Some of the principal features of the dayside solar wind ionosphere interaction at Venus are presented. The dayside ionopause and ionosphere are observed to respond dramatically to solar wind pressure variations. The ram pressure of the solar wind is manifested mainly as magnetic pressure just external to the subsolar ionopause, and the ionopause location is controlled principally by this pressure. The ionosheath field is observed to drape across the dayside ionosphere, and ionopause currents over most of the dayside usually act to exclude the high ionosheath field from the generally low‐field ionosphere. Tenuous, warm ionospheric plasma is sometimes observed on field lines outside the ionopause current sheet, suggesting that this dayside ionospheric plasma can be transported to the nightside and into the Venus wake. Some of the magnetic and thermal plasma features of the dayside ionosphere are shown, and modeling and distribution of flux ropes, small scale helical magnetic structures, are discussed in the context of thermal plasma observation

ISSN:0148-0227    

DOI:10.1029/JA085iA13p07679

年代:1980

数据来源: WILEY