Theoretical model of HZE particle fragmentation by hydrogen targets.

The fragmenting of high energy, heavy ions (HZE particles) by hydrogen targets is an important, physical process in several areas of space radiation research. In this work quantum mechanical optical model methods for estimating cross sections for HZE particle fragmentation by hydrogen targets are presented. The cross sections are calculated using a modified abrasion-ablation collision formalism adapted from a nucleus-nucleus collision model. Elemental and isotopic production cross sections are estimated and compared with report measurements for the breakup of neon, sulphur, and iron, nuclei at incident energies between 400 and 910 MeV/nucleon. Good agreement between theory and experiment is obtained.     

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.     

Liquid drop model considerations in HZE particle fragmentation by hydrogen

The fragmenting of high energy, heavy ions by hydrogen targets is an important physical process in several areas of space radiation protection research. Quantum mechanical, optical model methods for calculating cross sections for particle fragmentation by hydrogen have been developed from a modified abrasion-ablation collision formalism. The abrasion stage is treated as a knockout process which leaves the residual prefragment in an excited state. In the ablation stage the prefragment deexcites to produce the final fragment. The prefragment excitation energies are estimated from a combination of liquid drop model considerations and frictional-spectator interaction processes. Estimates of elemental and isotopic production cross sections are in good agreement with published cross section measurements.     

K-ionization and biological effect.

Experimental inactivation cross sections are compared to vacancy production in inner shell (K-shell) of so-called strategic C, N, O atoms of the DNA: in a trial calculation, atoms on the DNA backbone or just contiguous to it are considered as strategic. Various processes are considered: ionization and electron capture by the primary particle, ionization by secondary electrons. The last phenomenon is shown to be important for highly charged and medium velocity ions. A strong similarity is observed between variations versus LET of inactivation and K-vacancy cross sections, especially with regards to LET values for which they maximize: for K-ionization these maxima occur when the ion impact velocity roughly matches the K electron velocity. It is shown that the "K-hypothesis" allows a quantitative fit of experimental inactivation cross sections if a 4% mean lethal efficiency is attributed to ion-induced K-vacancies in strategic C, N, O atoms.     

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.     

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.     

A program to measure new energetic particle nuclear interaction cross sections.

The Transport Collaboration, consisting of researchers from institutions in France, Germany, Italy and the USA, has established a program to make new measurements of nuclear interaction cross sections for heavy projectiles (Z > or = 2) in targets of liquid H2, He and heavier materials. Such cross sections directly affect calculations of galactic and solar cosmic ray transport through matter and are needed for accurate radiation hazard assessment. To date, the collaboration has obtained data using the LBL Bevalac HISS facility with 20 projectiles from 4He to 58Ni in the energy range 393-910 MeV/nucleon. Preliminary results from the analysis of these data are presented here and compared to other measurements and to cross section prediction formulae.     

The Venus ionosphere

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.     

Nuclear fragmentation database for GCR transport code development

A critical need for NASA is the ability to accurately model the transport of heavy ions in the Galactic Cosmic Rays (GCR) through matter, including spacecraft walls, equipment racks, etc. Nuclear interactions are of great importance in the GCR transport problem, as they can cause fragmentation of the incoming ion into lighter ions. Since the radiation dose delivered by a particle is proportional to the square of (charge/velocity), fragmentation reduces the dose delivered by incident ions. The other mechanism by which dose can be reduced is ionization energy loss, which can lead to some particles stopping in the shielding. This is the conventional notion of shielding, but it is not applicable to human spaceflight since the particles in the GCR tend to be too energetic to be stopped in the relatively thin shielding that is possible within payload mass constraints. Our group has measured a large number of fragmentation cross sections, intended to be used as input to, or for validation of, NASA’s radiation transport models. A database containing over 200 charge-changing cross sections and over 2000 fragment production cross sections has been compiled. In this report, we examine in detail the contrast between fragment measurements at large acceptance and small acceptance. We use output from the PHITS Monte Carlo code to test our assumptions using as an example ⁴⁰Ar data (and simulated data) at a beam energy of 650 MeV/nucleon. We also present preliminary analysis in which isotopic resolution was attained for beryllium fragments produced by beams of ¹⁰B and ¹¹B. Future work on the experimental data set will focus on extracting and interpreting production cross sections for light fragments.     

Models of Venus neutral upper atmosphere: Structure and composition

Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed. Tables are provided for density, temperature, composition (CO₂, O, CO, He, N, N₂, and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day- and nightsides.Estimates are made of other species, including O₂ and D. Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, O, C, and He) as well as the hot components (H, N, C, O) on the day- and nightsides.     

Overview of nuclear fragmentation models and needs.

It has been known for some time that adequate assessment of spacecraft shield requirements and concomitant estimates of astronauts radiation exposures from galactic cosmic radiation requires accurate, quantitative methods for characterizing these radiation fields as they pass through thick absorbers. The main nuclear interaction processes involved are (1) nuclear elastic and inelastic collisions, and (2) nuclear breakup (fragmentation) and electromagnetic dissociation (EMD). Nuclear fragmentation and EMD are important because they alter the elemental and isotopic composition of the transported radiation fields. At present, there is no suitably accurate theory for predicting nuclear fragmentation cross sections for all collision pairs and energies of interest in space radiation protection. Typical cross-section differences between theory and experiment range from about 25 percent to a factor of two. The resulting errors in transported flux, for high linear energy transfer (LET) particles, are comparble to these cross-section errors. In this overview, theoretical models of heavy ion fragmentation currently used to generate input data bases for cosmic-ray transport and shielding codes are reviewed. Their shortcomings are discussed. Further actions needed to improve their accuracy and generality are presented.     

Raman scattering as a probe of planetary atmospheres

The observable effects of Raman scattering on the spectra of the giant planets may provide new information on the composition and structure of these atmospheres. Satellite observations have already shown the influence of Raman scattering on the UV continuum albedo. A cross correlation technique is presented for detecting rotational and vibrational transitions of the Raman active gases in the atmosphere. This technique has been applied to ground-based visible spectra of Venus, Jupiter, Saturn and Uranus. Extension of this method into the UV would improve the detectability of the Raman lines because the ratio of Raman to Rayleigh cross section increases with decreasing wavelength. The technology currently exists to efficiently obtain high signal-to-noise ratio UV spectra through the use of silicon diode array detectors. Application of the cross-correlation technique to UV spectra obtained from space vehicles would give us a new important probe of the structure and composition of planetary atmospheres by enabling us to use the UV spectra of a planet to observe that would normally be an infrared molecular transition.     

Technology development for a long duration,mid-cloud level Venus balloon

This paper describes the results of ongoing technology development activities for a Venus spherical superpressure balloon capable of flying for long durations (30 days) in the middle cloud layer at an altitude of 55.5 km. Data is presented from a successful aerial deployment and inflation flight experiment on a 5.5 m diameter prototype balloon conducted at a 2.5 km altitude above the Earth. Although the balloon in that test was not released for free flight, all other steps in the deployment and inflation process were successfully executed. Experimental and computational results are also presented from an investigation of the stress concentration phenomenon at the junction of the metal end fitting and fabric end cap of the prototype Venus balloon. Good agreement was found between the simulation and experimental results and a stress concentration factor of 1.55 determined for this end cap design compared to the expectations of thin membrane theory. Finally, results are presented for a new, second-generation Venus balloon material utilizing Aclar™ film instead of Teflon. Optical property and sulfuric acid tolerance data are presented for this material based on laboratory testing of samples.     

A comparison of total reaction cross section models used in particle and heavy ion transport codes

Understanding the interactions and propagations of high energy protons and heavy ions are essential when trying to estimate the biological effects of Galactic Cosmic Rays (GCR) and Solar Particle Events (SPE) on personnel in space. To be able to calculate the shielding properties of different materials and radiation risks, particle and heavy ion transport codes are needed. In all particle and heavy ion transport codes, the probability function that a projectile particle will collide within a certain distance x in the matter depends on the total reaction cross sections, and the calculated partial fragmentation cross sections scale with the total reaction cross sections. It is therefore crucial that accurate total reaction cross section models are used in the transport calculations. In this paper, different models for calculating nucleon–nucleus and nucleus–nucleus total reaction cross sections are compared with each other and with measurements. The uncertainties in the calculations with the different models are discussed, as well as their overall performances with respect to the available experimental data. Finally, a new compilation of experimental data is briefly presented.     

Plasma measurements in the Venus near wake

A study of the plasma measurements conducted with the Mariner 5, Venera 9 and 10, and the Pioneer Venus spacecraft in the Venus ionosheath and near wake is presented. The data available indicate that in the inner ionosheath, downstream from the terminator, the density and the velocity of the plasma are significantly smaller than those measured further outside. The slower particle fluxes detected near the ionopause also exhibit higher plasma temperatures and show a tendency to move towards the nightside hemisphere. The observation of high plasma temperatures in the inner ionosheath indicates that the interaction of the solar wind with the Venus ionospheric/exospheric material is dominated by dissipative phenomena, and that its entry into the wake is due to local thermal expansion processes.     

Mass-loading and the formation of the Venus tail

Despite its lack of an intrinsic magnetic field Venus has a well defined magnetotail, containing about 3 megawebers of magnetic flux in a tail about 4 R_V across with perhaps a slightly elliptical cross section. This tail arises through the mass-loading of magnetic flux tubes passing by the planet. Mass-loading can occur due to charge exchange and photo-ionization as well as from the diffusion of magnetic field into the ionosphere. Various evidence exists for the mass-loading process, including the direct observation of the picked up ions with both the Venera and Pioneer Venus plasma analyzers.     

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.