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 F_10.7 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.     

Corotating interplanetary streams and associated ionospheric disturbances at Venus and Earth

During the summer of 1979, solar coronal structure was such that a sequence of recurrent regions produced a corresponding sequence of corotating solar wind streams, with pronounced downstream signatures. One of these stream events passed Earth on July 3, and was observed later at Venus late on July 11th, with similar characteristics. Corresponding in-situ measurements at Earth from the Atmospheric Explorer-E satellite and at Venus from the Pioneer Venus Orbiter are examined for evidence of comparable perturbations of the planetary ionospheres. The passage of the stream shock front is found to be associated with pronounced fluctuations in n(0⁺) which appear as pronounced local depletion of ion concentrations in both ionospheres. The ionosphere disturbances appear to be closely associated with large variations in the solar wind momentum flux. The implied local ionospheric depletions observed at each planet are interpreted to be the consequence of plasma redistribution, rather than actual depletions of plasma.     

Empirical model of the composition of the Venus ionosphere: Repeatable characteristics and key features not modeled

In-situ measurements of positive ion composition of the ionosphere of Venus are combined in an empirical model which is a key element for the Venus International Reference Atmosphere (VIRA) model. The ion data are obtained from the Pioneer Venus Orbiter Ion Mass Spectrometer (OIMS) which obtained daily measurements beginning in December 1978 and extending to July 1980 when the uncontrolled rise of satellite periapsis height precluded further measurements in the main body of the ionosphere. For this period, measurements of 12 ion species are sorted into altitude and local time bins with altitude extending from 150 to 1000 km. The model results exhibit the appreciable nightside ionosphere found at Venus, the dominance of atomic oxygen ions in the dayside upper ionosphere and the increase in prominence of atomic oxygen and deuterium ions on the nightside. Short term variations, such as the abrupt changes observed in the ionopause, cannot be represented in the model.     

Modulation of dayside ion and neutral distributions at Venus: Evidence of direct and indirect solar energy inputs

In-situ measurements of ion and neutral composition and temperature across the dayside of Venus during 1979–1980 exhibit long and short-term changes attributed to solar variations. Following solar maximum, dayside concentrations of CO⁺ and the neutral gas temperature are relatively smoothly modulated with a 28-day cycle reasonably matching that of the solar F_10.7 and EUV fluxes. Measurements some 6–8 months earlier show less pronounced and more irregular modulation, and short-term day-to-day fluctuations in the ions and neutrals are relatively more conspicuous than in the later period. During the earlier period, the solar wind at Venu exhibits relatively large velocity enhancements, which appear to be consistent with differences in solar coronal behavior during the two periods. It is suggested that through the solar wind variations and associated changes in the draping of the interplanetary magnetic field about the dayside, fluctuating patterns of joule heating may occur, producing the observed short term ion and neutral variations. This indirect energy effect, if verified, presents a complication for quantitatively analyzing the modulation in neutral temperature and ion concentration produced by changes in direct EUV radiation.     

The pioneer venus orbiter ultraviolet spectrometer experiment: Analysis of hydrogen lyman alpha data

Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS) HI 1216Å data from six (6) orbits are analyzed. Analysis of subsolar region periapsis data show that for an exobase temperature of 305K, the exobase density is $\text{5 ± 2(4)}{}^{\mathrm{@}}\text{cm}{}^{\mathrm{?3}}$ and the column abundance of atomic hydrogen between 110 and 200 km is 2.4 ± 0.8(13) cm^?2. The upward flux through the exobase is determined to be 7.5 ± 2.5(7)/cm²s. Apoapsis data were analyzed for both evening and morning geometries. We conclude: (1) the observed limb profiles show a diurnal variation consistent with Brinton et al.; (2) the model temperature field provides a good fit to the morning data, but the morning temperature field must be used to match the evening data; and (3) the theoretical Ly α limb intensity profiles are sensitive to small changes in the shape and magnitude of the variation of exobase hydrogen with solar zenith angle. The solar Ly α flux at line center required to fit the magnitude of the data is 8(11) photons/cm²s Å at Venus.     

Hybrid simulations of the O ion escape from Venus: Influence of the solar wind density and the IMF x component

As an initial effort to study the evolution of the Venus atmosphere, the influence of the solar wind density and the interplanetary magnetic field (IMF) x component (the x-axis points from Venus towards the Sun) on the O⁺ ion escape rate from Venus is investigated using a three-dimensional quasi-neutral hybrid (HYB-Venus) model. The HYB-Venus model is first applied to a case of the high-density (100 cm⁻³) solar wind interaction with Venus selected from the Pioneer Venus Orbiter observations to demonstrate its capability for the study. Two sets of simulations with a wide range of solar wind densities and different IMF x components are then performed. It is found that the O⁺ ion escape rate increases with increasing solar wind density. The O⁺ ion escape rate saturates when the solar wind density becomes high (above 100 cm⁻³). The results also suggest that the IMF x component enhances the O⁺ ion escape rate, given a fixed IMF component perpendicular to the x-axis. Finally, the results imply a higher ion loss rate for early-Venus, when solar conditions were dramatically different.     

Evidence for mass-loading of the Venus magnetosheath

The observed magnetic field configuration in the Venus magnetosheath contains information about the solar wind mass-loading processes occurring as a result of the extension of the neutral atmosphere into the magnetosheath. In this paper, magnetic field signatures of various mass-loading processes are discussed and experimental results from the Pioneer Venus Orbiter magnetometer experiment are examined for evidence of these signatures. The data suggest that the ?$\text{V}$X$\text{B}$ acceleration process, stochastic pickup of ionospheric ions, and $\text{J}$X$\text{B}$ force “scavenging” at the ionopause all occur at various times.     

EUV imaging of near-Venus space

Japan’s Venus Climate Orbiter (the Planet-C spacecraft) will be launched in 2008 and will reach an orbit in the ecliptic plane around Venus in 2009. We propose two eXtreme UltraViolet (XUV) imagers to take global two-dimensional snapshots of near-Venus space, including the Venus ionosphere and the interaction region between the solar wind plasma and the Venus ionospheric plasma. The imagers detect the resonantly scattering emissions of oxygen ions (O II 83.4 nm) and atoms (O I 130.3 nm), neutral helium (He I 58.4 nm), and hydrogen (H Ly-α 121.6 nm). Scientific goals are to investigate mechanisms of momentum and mass transfer across the ionopause, of convection in the upper atmosphere and ionosphere, and of atmospheric escape. Especially, we emphasize that sequential images of the O II 83.4-nm emission will enable us to understand temporal evolution of the vortex produced by the Kelvin–Helmholtz (K–H) instability. Though the wave structure due to the K–H instability is generated also at the terrestrial magnetopause, oxygen ions are too tenuous to detect the emission. On the other hand, at the Venus ionopause oxygen ions have enough density to image the resonance emission, i.e., the Venus ionosphere plays a role as a space laboratory for plasma physics.     

Magnetic flux ropes in the Venus ionosphere: In situ observations of force-free structures?

Force-free magnetic structures with cylindrical geometry appear under a variety of conditions in nature. Filamentary helical magnetic structures are observed to be associated with prominences and flares in the solar atmosphere, and can arise in superconductors and laboratory plasmas. Another example of cylindrical quasi-force-free configurations appears to exist in the Venus ionosphere. Magnetic flux ropes with diameters of ～20 – 30 km have been observed by the Pioneer Venus Orbiter to be a nearly ubiquitous feature of the dayside Venus ionosphere. Models of flux ropes suggest that many of these structures tend to be quasi-force-free, i.e., $\text{J}$×$\text{B}$～0, while others are correlated with pressure variations in the ambient thermal plasma, $\text{J}$×$\text{B}$=-?(nkT).     

Solar wind governing the response of Venusian atmosphere and ionosphere

The ionosphere of Venus is primarily formed by photoionization of a gaseous blanket around Venus. The impact ionization by energetic solar charged particles also plays an important role in the variability of Venusian ionospheric ion, electron density and their temperature profiles. The microscopic variations in the solar wind velocity, particle flux and orientations of frozen-in interplanetary magnetic field determine the solar wind interaction with the Venusian ionosphere. The ion and electron density profiles obtained by Pioneer Venus Orbiter and Pioneer Venus Entry Probes have been analysed in the light of simultaneous solar wind velocity and particle flux. Marked changes in height profiles of ion, electron densities and their temperatures have been found to correlate with the simultaneous changes in the solar wind velocity and particle flux. It is shown that the solar wind plays a more important role in controlling the physical properties and behavior of daytime as well as nighttime ionosphere of Venus, whereas the solar xuv sustains the primary ionization process.     

On the diurnal variations in the temperature and composition: a three-dimensional model with superrotation

Based on a simplified theoretical interpretation of the composition measurements with the ONMS and OIMS experiments on Pioneer Venus, the conclusion was drawn that the rotation rate of the thermosphere should be close (within a factor of two) to that of the lower atmosphere. A more realistic three-dimensional model of the thermosphere dynamics is now being developed, considering non-linear processes, higher order modes and collisional momentum exchange between the major species CO₂, CO and O, which describes the diurnal variations in temperature and composition (Niemann et al., $\text{JGR}$, 1980). The computed horizontal winds are about 300 m/sec near the terminators and poles. Results are also presented from a two-dimensional (quasi-axisymmetric) spectral model which describes the four day superrotation in the lower atmosphere of Venus.     

Cosmic ray H/H ratio measured from BESS in 2000 during solar maximum

The Balloon-borne Experiment with a Superconducting Spectrometer (BESS) was flown from Lynn Lake, Manitoba, Canada in August, 2000, during the maximum solar modulation period, with an average residual atmospheric overburden of 4.3 g/cm². Precise spectral measurements of cosmic ray hydrogen isotopes from 0.178 GeV/n to 1.334 GeV/n were made during the 28.7 h of flight. This paper presents the measured energy spectra and their ratio, ²H/¹H. The results are also compared with previous measurements and theoretical predictions.     

Neutral upper atmospheres of Venus and Mars

Numerous measurements of the neutral upper atmosphere above 100 km have been made from spacecraft over Venus and over Mars. The Venus exospheric temperatures are unexpectedly low (less than 300°K near noon and less than 130°K near midnight). These very low temperatures may be partially caused by collisional excitation of CO₂ vibrational states by atomic oxygen and partially by eddy cooling. The Venus atmosphere is unexpectedly insensitive to solar EUV variability. On the other hand, the Martian dayside exospheric temperature varies from 150°K to 400°K over the 11-year solar cycle, where CO₂ 15-μm cooling may be less effective because of lower atomic oxygen mixing ratios. On Venus, temperature increases with altitude on the dayside (thermosphere), but decreases with altitude from 100 to 150 km on the nightside (cryosphere). However, dayside Martian temperatures near solar minimum for maximum planet-sun distance and low solar activity are essentially isothermal from 40 km to 200 km. During high solar activity, the thermospheric temperatures of Mars sharply increase. The Venus neutral upper atmosphere contains CO₂, O, CO, C, N₂, N, He, H, D and hot nonthermal H, O, C, and N, while the dayside Mars neutral upper atmosphere contains CO₂, O, O₂, CO, C, N₂, He, H, and Ar. There is evidence on Venus for inhibited day-to-night transport as well as superrotation of the upper atmosphere. Both atmospheres have substantial wave activity. Various theoretical models used to interpret the planetary atmospheric data are discussed.     

Evolution of solar wind structures from 0.72 to 1 AU

Understanding the evolution of solar wind structures in the inner heliosphere as they approach the Earth is important to space weather prediction. From the in situ solar wind plasma and magnetic field measurements of Pioneer Venus Orbiter (PVO) at 0.72 AU (1979–1988), and of Wind/Advanced Composition Explorer (ACE) missions at 1 AU (1995–2004), we identify and characterize two major solar wind structures, stream interaction regions (SIRs) and interplanetary coronal mass ejections (ICMEs). The average percentage of SIRs occurring with shocks increases significantly from 3% to 24% as they evolve from 0.72 to 1 AU. The average occurrence rate, radial extent, and bulk velocity variation of SIRs do not change from 0.72 to 1 AU, while peak pressure and magnetic field strength both decrease with the radial evolution of SIRs. Within the 0.28 AU distance from the orbit of Venus to that of Earth, the average fraction of ICMEs with shocks increases from 49% to 66%, and the typical radial extent of ICMEs expands by about a fraction of 1.4, with peak pressure and magnetic field strength decreasing significantly. The mean occurrence rate and expansion velocity of ICMEs do not change from 0.72 to 1 AU.     

Absorbers seen near the Venus cloud tops from pioneer Venus

Spin-scan images from the Pioneer Venus Orbiter UV Spectrometer and the Cloud Photopolarimeter provide a set of planetary contrast measurements in the wavelength range 1990A to 3650A and phase angles from 33°–130°. The planet is darkest at the point where the UVS line of sight penetrates perpendicular to the cloud tops: thus the absorbing material responsible must be deep in the atmosphere. Sulfur dioxide absorption can explain the amount of contrast seen between 2000A and 3200A. At the longer wavelengths, the persistence of contrast requires another absorber which is deeper in the atmosphere and strongly associated with the location of the SO₂. Part of the observed contrast is due to the high-lying haze discovered from Pioneer Venus polarimetry. The correlation between planetary contrast and polarization does not support large scale clearing or major vertical motions of the cloud tops as the sole cause of the observed contrast. However, a scheme in which absorbers subject to photochemical destruction are mixed upward into the cloud top region provides a consistent explanation for the origin of these markings.     

Dependence of Venus ionopause altitude and ionospheric magnetic field on solar wind dynamic pressure

The shape of the dayside Venus ionopause, and its dependence on solar wind parameters, is examined using Pioneer Venus Orbiter field and particle data. The ionopause is defined here as the altitude of pressure equality between magnetosheath magnetic pressure and ionospheric thermal pressure; its typical altitudes range from ～300 km near the subsolar point to ～900 km near the terminator. A strong correlation between ionopause altitude and magnetosheath magnetic pressure is demonstrated; correlation between magnetic pressure and the normally incident component of solar wind dynamic pressure is also evident. The data support the hypothesis of control of the ionopause altitude by solar wind dynamic pressure, manifested in the sheath as magnetic pressure. The presence of large scale magnetic fields in the ionosphere is observed primarily when dynamic pressure is high and the ionopause is low.     

北京地区电离层电子含量的某些特性

从对1980年至今的北京电离层电子含量的常规观测资料分析发现:电离层电子含量(IEC)的周日变化曲线除了绝大多数呈单峰变化外, 还有非扰动状态下的周日双峰变化、夜增长现象以及日出后不久的次极大正午之后的主极大等.本文着意于报道北京地区的IEC双峰特性和夜增现象及其随太阳活动的周期变化, 并与其它报道进行比较.      

Interannual and seasonal variations of diurnal tide,gravity wave,ozone, and water vapor as observed by MLS during 1991–1994

The diurnal tide in the mesosphere and lower thermosphere (MLT) shows large seasonal and interannual variations. Despite recent modeling investigations, the underlying physical mechanisms for causing these variations remain unclear. This paper provides further observational constraints to tide-sensitive variables (H₂O, O₃, and gravity wave variances) used by the models, which are obtained simultaneously by upper atmosphere research satellite microwave limb sounder at altitudes below the MLT region. The strong quasi biannual oscillation and semiannual oscillation variations in these measurements reveal good correlations between the diurnal tide with other tide-sensitive variables, which should be taken into account for further modeling studies.     

The investigation of powerful solar flares characteristics by analysis of excited states of C and various neutrons capture lines

Accelerated energetic particles in solar flares produced nuclear γ-lines in interactions with ambient solar atmosphere. Analysis of intensity of ratios between various γ-lines allows us to make estimations of abundance of elements, parameters of surrounding media and other solar characteristics. In this article we discuss the flux ratio between two lines from excited states of ¹²C (f_15.11/f_4.44) and our results of preliminary calculation of intensity ratio between two neutron capture lines at ³He and ¹H (f_20.58/f_2.223). In particular we consider the opportunity to obtain n(³He)/n(¹H) ratio during solar flares and using high-energy gamma-emission studying, based on the satellite data. Possible interpretation of spectral features observed during the January 20, 2005 solar flare is discussed. Preliminary analysis of energy spectrum in the band of 2–21 MeV gives n(³He)/n(¹H) ∼ 8 × 10⁻⁴ for January 20, 2005 solar flare.     

Models of the structure of the atmosphere of Venus from the surface to 100 kilometers altitude

From a critical comparison and synthesis of data from the four Pioneer Venus Probes, the Pioneer Venus Orbiter, and the Venera 10, 12, and 13 landers, models of the lower and middle atmosphere of Venus are derived. The models are consistent with the data sets within the measurement uncertainties and established variability of the atmosphere. The models represent the observed variations of state properties with latitude, and preserve the observed static stability. The rationale and the approach used to derive the models are discussed, and the remaining uncertainties are estimated.