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Impact expansions and interference patterns in atomic scattering theory for cases of forward scattering, backscatter, inversion problem and screened Coulomb potential approximation
It is noted that negative ions may be formed in the ionospheres of Mercury, the moon, and Jupiter's satellites with densities of a few percent of the ionospheric electron density. Negative ions result from three mechanisms at the planetary surface: charge inversion during energetic proton scattering, with simultaneous secondary negative ion emission, and micrometeorite impacts. The density and distribution of negative ions around planetary bodies depends primarily on the negative ion lifetimes determined through photodetachment by solar radiation.
The goal of this research grant was to calculate accurate oscillator strengths and electron collisional excitation strengths for inelastic transitions in atomic species of relevance to Planetary Atmospheres. Large scale configuration-interaction atomic structure calculations have been performed to obtain oscillator strengths and transition probabilities for transitions among the fine-structure levels and R-matrix method has been used in the calculations of electron-ion collision cross sections of C II, S I, S II, S III, and Ar II. A number of strong features due to ions of sulfur have been detected in the spectra of Jupiter satellite Io. The electron excitation cross sections for the C II and S II transitions are studied in collaboration with the experimental atomic physics group at the Jet Propulsion Laboratory. There is excellent agreement between experiment and theory which provide an accurate and broad-base test of the ability of theoretical methods used in the calculation of atomic processes. Specifically, research problems have been investigated for: electron impact excitation cross sections of C II: electron impact excitation cross sections of S III; energy levels and oscillator strengths for transitions in S III; collision strengths for electron collisional excitation of S II; electron impact excitation of inelastic transitions in Ar II; oscillator strengths of fine-structure transitions in neutral sulfur; cross sections for inelastic scattering of electrons from atomic nitrogen; and excitation of atomic ions by electron impact.
Dissociative recombination of O2+ and CO2+ molecular ions in the upper atmosphere of Mars is a major source of suprathermal (hot) oxygen atoms. A significant fraction of hot O atoms are sufficiently fast to overcome Mars' gravitational potential and escape to space. This mechanism is known as photochemical escape (PE) and found to be one of the major escape mechanisms presently active on Mars. The Mars Atmosphere and Volatile Evolution Mission (MAVEN) can constrain the PE rate from measurements of electron temperature and density and ion temperature and density. Escape probabilities of hot O can then be constructed from in-situ neutral densities and escaping O flux loss due to collisions with atmospheric background gases. Due to CO2 abundance in the upper atmosphere, O+CO2 scattering is the most important sink of the escaping hot O flux that strongly affects estimates of total atomic oxygen escape rate obtained from MAVEN data, as outlined above. New O+CO2 elastic and inelastic cross sections were constructed from first principles for collision energies from 0.01-5 eV. O(3P)+CO2 collisions were described using new 4D electronic potential energy surfaces (PESs) for restricted geometries. Three lowest triplet states of CO2 were considered for symmetric, bending, and asymmetric vibrational modes. The cross sections were determined from close-coupling calculations performed on individual surfaces using vibrating rotor approximations for CO2 molecule. Differential and momentum transfer cross sections, of use in energy transfer atmospheric models, were also constructed. The obtained cross sections are in excellent agreement with published results. Our new elastic cross sections are significantly smaller than assumed in earlier atmospheric escape models, resulting in larger photochemical escape rate of O from Mars, as estimated from a 1D escape model. Our results agree well with estimated hot O flux (about 9 x 10(sup 25) s(sup -1)) derived from altitude profiles of 130.4 nm O emission measured up to ~4,000 km by the Imaging Ultraviolet Spectrograph on MAVEN orbiter.
Approximate variation principle for calculating energies and wave functions in electron impact experiments
To study the complex interactions between the space environment surrounding the International Space Station (ISS) and the ISS space vehicle, we are exploring a specialized suite of plasma sensors, manipulated by the Space Station Remote Manipulator System (SSRMS) to probe the near‐ISS mesosonic plasma ionosphere moving past the ISS. It is proposed that SASSI consists of the NASA Marshall Space Flight Center's (MSFC's) Thermal Ion Capped Hemispherical Spectrometer (TICHS), Thermal Electron Capped Hemispherical Spectrometer (TECHS), Charge Analyzer Responsive to Local Oscillations (CARLO), the Collimated PhotoElectron Gun (CPEG), and the University of Michigan Advanced Langmuir Probe (ALP). There are multiple expected applications for SASSI. Here, we will discuss the study of fundamental plasma physics questions associated with how an emitted plasma plume (such as from the ISS Plasma Contactor Unit (PCU)) responds and expands in a mesosonic magnetoplasma as well as emit and collect current. The ISS PCU Xe plasma plume drifts through the ionosphere and across the Earth's magnetic field, resulting in complex dynamics. This is of practical and theoretical interest pertaining to contamination concerns (e.g. energetic ion scattering) and the ability to collect and emit current between the spacecraft and the ambient plasma ionosphere. This impacts, for example, predictions of electrodynamic tether current performance using plasma contactors as well as decisions about placing high‐energy electric propulsion thrusters on ISS. We will discuss the required measurements and connection to proposed instruments for this study.
The shifts of the Ly-alpha, Ly-beta, and H-alpha lines of He(+) in a plasma, produced by electron scattering from radiating ions are calculated. Electron densities in the neighborhood of 10 to the 17th/cu cm and plasma temperatures from 1 to 6 eV are considered. The calculation is made in the impact approximation, and is based on a six-state close-coupling computation of the scattering matrices, into which an optical potential has been inserted. Small red shifts of the lines are obtained. The contribution of electron scattering to the linewidths is also considered.
Energy-loss spectra for excitation of the 2S to 2PO transitions in ZNII and CdII by electrons have been observed for the first time. Differential scattering cross sections are reported at 75 eV electron energy. Comparisons are made to close-coupling theory for the 4,2S to 4,2P transition in ZnII. In CdII comparisons are made to the 5,2S(1/2) to 5,2S(3/2) integral cross section from absolute emission measurements, to theoretical results for the analogous 2S to 2P transition in ZnII, and to Gaunt-factor calculations. A description of the crossed beam apparatus is given, and conversion of scattering intensity to absolute cross section discussed.
Current interest in clusters stems from their role as novel materials as well as a possible extension of cluster results to bulk systems. Experimental investigations on clusters have been carried out using laser spectroscopy, microwave spectroscopy, heavy-particle collisions, as well as electron collisions with earlier experimental work on electron attachment and ionization having been reviewed previously. Recently, Mark and coworkers studied the decay channels of cluster ions following electron impact ionization. Rauth et al. reported the formation of the superhalogen ion SF7(-) and other nonstoichiometric cluster ions in their study of electron attachment to SF6 clusters. Kresin et al. measured the absolute electron-impact depletion cross section of metal clusters Na8, Na(20), and Na(40). They found that the inelastic scattering cross section increased with cluster size and was considerably greater than the hard sphere collision cross sections. They hypothesized that electron attachment and collision-induced fragmentation were the dominant physical processes responsible for this effect. For the two smaller clusters, they also found a sharp increase in the cross section near threshold. Most theoretical studies of clusters have been devoted to their electronic structures, vibrational relaxation, and predissociation while investigations of electron scattering from clusters has been lacking. In view of this, we recently undertook an ab initio study of electron scattering from small Be clusters and BeCO. Beryllium was chosen because it is readily amenable to ab t'nitio calculations. Moreover, the electronic structure of Be clusters has been studied extensively, showing that the Be-Be bond is relatively weak in comparison with a normal chemical bond. Our investigation focuses on how the cross sections change with cluster size and geometry. The range of energy studied, 0.05 - 5.0 eV, is chosen because of the ubiquitous resonance in the low-energy scattering of Be. Hence it can be determined if it is possible, as a consequence of the weaker bonds in the Be clusters, to identify the atomic origin of the cluster resonance.
Physical properties such as temperature and electron density of solar plasma and other astrophysical objects can be inferred from EUV and X-ray emission lines observed from space. These lines are emitted when the higher states of an ion are excited by electron impact and then decay by photon emission. Excitation cross sections are required for the spectroscopic analyses of the observations and various approximations have been used to calculate the scattering functions. One of them which has been widely used is a distorted wave approximation. This approximation, along with its applications to solar observations, is discussed. The Bowen fluorescence mechanism and optical depth effects are also discussed. It is concluded that such calculations are reliable for highly charged ions and for high electron temperatures.
Physical properties such as temperature and electron density of solar plasma and other astrophysical objects can be inferred from EUV and X-ray emission lines observed from space. These lines are emitted when the higher states of an ion are excited by electron impact and then decay by photon emission. Excitation cross sections are required for the spectroscopic analyses of the observations and various approximations have been used to calculate the scattering functions. One of them which has been widely used is a distorted wave approximation. This approximation, along with its applications to solar observations, is discussed. The Bowen fluorescence mechanism and optical depth effects are also discussed. It is concluded that such calculations are reliable for highly charged ions and for high electron temperatures.
This first analysis of Pioneer Venus Orbiter (PVO) plasma analyzer electron measurements obtained in early 1992 during teh PVO entry phase of the mission indicates the presence downstream from the terminator of a depletion or 'bite out' of energetic ionosheath electrons similar to that observed on Mariner 10. There is more than one possible explanation for this energetic electron depletion. If it is due to atmospheric scattering, the electrons traveling along draped magnetic flux tubes that thread through the Venus neutral atmosphere would lose energy from impact ionization with oxygen. The cross-section for such electron impact ionization of oxygen has a peak near 100 eV, and it remains high above this energy, so atmospheric loss could provde a natural process for electrons at these energies to be selectively removed. In this case, our results are consistent with the Kar et al. (1994) study of PVO atmospheric entry ion mass spectrometer data, which indicates that electron impact plays a significant role in maintaining the nightside ionosphere. Although it is appealing to interpret the energetic electron depletion in terms of direct atmospheric scattering, alternatively it could result from strong draping which connects the depletion region magnetically to the weak downstream bow shock and thereby reduces the electron source strength.
This first analysis of Pioneer Venus Orbiter (PVO) plasma analyzer electron measurements obtained in early 1992 during the PVO entry phase of the mission indicates the presence downstream from the terminator of a depletion or "bite out" of energetic ionosheath electrons similar to that observed on Mariner 10. There is more than one possible explanation for this energetic electron depletion. If it is due to atmospheric scattering, the electrons traveling along draped magnetic flux tubes that thread through the Venus neutral atmosphere would lose energy from impact ionization with oxygen. The cross-section for such electron impact ionization of oxygen has a peak near 100 eV, and it remains high above this energy, so atmospheric loss could provide a natural process for electrons at these energies to be selectively removed. In this case, our results are consistent with the Kar et al. (1994) study of PVO atmospheric entry ion mass spectrometer data which indicates that electron impact plays a significant role in maintaining the nightside ionosphere. Although it is appealing to interpret the energetic electron depletion in terms of direct atmospheric scattering, alternatively it could result from strong draping which connects the depletion region magnetically to the weak downstream bow shock and thereby reduces the electron source strength.
The new R-matrix package for comprehensive close-coupling calculations for electron scattering with the first three ions in the boron isoelectronic sequence, the astrophysically significant C(+), N(2+), and O(3+), is presented. The collision strengths are calculated in the LS coupling approximation, as well as in pair-coupling scheme, for the transitions among the fine-structure sublevels. Calculations are carried out at a large number of energies in order to study the detailed effects of autoionizing resonances.
Voyager 1 and 2 sensor data are analyzed in order to derive the composition, energy spectra, and spatial distribution of energetic ions in the Saturn magnetosphere. In order of abundance, the major species are H, H2(+), He, H3(+), C, and O. The fluxes of all species decreased inside the orbit of Dione, and nearly vanished in the 'slot' region within the orbit of Tethys. Both satellite absorption and precipitation due to pitch angle scattering may be important loss processes in that region. In the outer magnetosphere, photodissociation rapidly destroys a large fraction of the H2(+) ions, but dissociation by impact with neutral H atoms is faster for H2(+) ions in the lowest vibrational state. The ground state lifetime of about 23 days places a limit of about 10-100 days on the mean overall residence time for energetic ions in Saturn's magnetosphere.
We use measurements of aerosol ion composition and size made from the DC8 aircraft during the 2006 INTEX-B airborne campaign to identify mineral dust signatures, and look for evidence for interaction of dust with acidic components. Coating of dust with sulfate or nitrate favors the role of dust particles as cloud condensation nucleii, can promote further uptake of SO2 and N2O5, can impact NOx/HNO3 partitioning, and can shift sulfate or nitrate towards larger sizes, affecting atmospheric lifetimes for both aerosol and gas components. Mineral dust had a pervasive presence on flights made during the Northern Pacific deployment of the INTEX-B mission. We use scatter plots of ion mixing ratios with Na+ and Ca(2+) to distinguish sea salt and mineral components of the aerosol distribution, respectively. Positive correlations of non-sea-salt sulfate and nitrate with calcium indicate that the dusty air stream is associated with polluted air masses. Sulfate-ammonium scatter plots indicate sulfate to be primarily in the form of (NH4)2SO4. A positive correlation between Ca(2+) and NO-, but little evidence of NH4NO3, suggests that NO3- may be associated with mineral dust surfaces. 3-d model simulations conducted with the GEOS-Chem chemical transport model indicate that transpacific transport from East Asia was principally responsible for the dust observed from the aircraft over the Pacific. We compare the aerosol component relationships in the model with those observed. Uptake of sulfate and nitrate on the dust is not yet represented in the model.
Using plane waves to describe the incident and scattered particles, and screened hydrogenic and Coulomb functions to describe the atomic electrons before and after ejections, we have calculated the differential and total ionization cross sections of 11 atoms and one ion by electron impact, and ionization of helium by proton impact. The effective charges of the screened hydrogenic functions are fixed by the Hartree-Fock calculations. Calculations have been carried out for the atomic s, p, and d electrons. For low atomic numbers, we find reasonable agreement with the experimental data. For intermediate atomic numbers, we expect our results to overestimate the actual cross sections, since our choice of a unit charge for the Coulomb function of the ejected electrons will overestimate the atomic dipole potential strength, and in turn the high-energy cross sections. The advantage of the method presented here is that the ionization amplitude is given in analytic form. This may allow further analysis on this amplitude, and facilitates extension of the numerical integration for the cross section to high impact energies.
We describe a new imager suitable for measurements of magnetospheric neutrals with energies from about 100 eV to about 10 keV; an energy range adequate for imaging the plasmasheet neutral atoms out to about 10 R(E). The instrument, an outgrowth of a study of atom-surface collisions in support of satellite drag calculations, separates incident photons from neutral atoms by surface scattering and conversion of the neutrals to ions. Subsequently, the ions formed on the first surface are accelerated through a light rejection section which also disperses the ions according to energy. The dispersed ion beam is then allowed to impact a second surface where a start pulse is generated to obtain ion velocity and energy/charge. The second surface is chosen to give large secondary electron emission without regard to charge state of the particles reflected from it. The data supporting the proposed ILENA design is presented in the first part of the paper.