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Cassini/UVIS FUV observations of stellar occultations at Titan are well suited for probing its atmospheric composition and structure. However, due to instrument pointing motion, only five out of tens of observations have been analyzed. We present an innovative retrieval method that corrects for the effect of pointing motion by forward modeling the Cassini/UVIS instrument response function with the pointing motion value obtained from the SPICE C-kernel along the spectral dimension. To illustrate the methodology, an occultation observation made during flyby T52 is analyzed, when the Cassini spacecraft had insufficient attitude control. A high-resolution stellar model and an instrument response simulator that includes the position of the point source on the detector are used for the analysis of the pointing motion. The Markov Chain Monte-Carlo method is used to retrieve the line-of-sight abundance profiles of eleven species (CH4, C2H2, C2H4, C2H6, C4H2, C6H6, HCN, C2N2, HC3N, C6N2 and haze particles) in the spectral vector fitting process. We obtain tight constraints on all of the species aside from C2H6, C2N2 and C6N2, for which we only retrieved upper limits. This is the first time that the T52 occultation was used to derive abundances of major hydrocarbon and nitrile species in Titan's upper and middle atmosphere, as pointing motion prohibited prior analysis. With this new method, nearly all of the occultations obtained over the entire Cassini mission could yield reliable profiles of atmospheric composition, allowing exploration of Titan's upper atmosphere over seasons, latitudes, and longitudes., Comment: Accepted by Earth and Space Science, 8 pages, 6 figures

Seasonal variation is significant in Titan's atmosphere due to the large change of solar insolation resulting from Titan's 26.7{\deg} axial tilt relative to the plane of Saturn's orbit. Here we present an investigation of hydrocarbon and nitrile species in Titan's upper atmosphere at 400-1200 km, which includes the mesosphere and the lower thermosphere, over more than one fourth of Titan's year (2006-2014, LS=318{\deg}-60{\deg}), using eighteen stellar occultation observations obtained by Cassini/UVIS. Vertical profiles of eight chemical species (CH4, C2H2, C2H4, C2H6, C4H2, C6H6, HCN, HC3N) and haze particles are retrieved from these observations using an instrument forward model, which considers the technical issue of pointing motion. The Markov-chain Monte Carlo (MCMC) algorithm is used to obtain the posterior probability distributions of parameters in the retrieval, which inherently tests the extent to which species profiles can be constrained. The results show that no change of the species profiles is noticeable before the equinox, while the decrease of atmospheric temperature and significant upwelling in the summer hemisphere are found five terrestrial years afterwards. Altitude of the detached haze layer decreases towards the vernal equinox then it disappears, and no reappearance is identified within the time range of our data, which is consistent with observations from Cassini/ISS. This study provides observational constraints on the seasonal change of Titan's upper atmosphere, and suggests further investigations of the atmospheric chemistry and dynamics therein., Comment: 11 pages, 12 figures, accepted by PSJ

Dissociative excitation of molecular hydrogen plays an important role in the heating of outer planet upper thermospheres. This paper addresses the role of one of the triplet states involved in the process. H2 excited to the a 3?+ g state, or higher triplet-ungerade states, is dissociated via the a 3?+ g ?b 3?+ u continuum. The kinetic energy distribution of H(1s) produced from direct X 1?+ g -a 3?+ g (v, J) excitation by electrons is investigated by an accurate theoretical evaluation of spontaneous transition probabilities of the a 3?+ g (v, J)?b 3?+ u continuum transition. It is shown that the X 1?+ g (0)-a 3?+ g (v, J) excitation primarily produces H(1s) atoms with kinetic energies lower than 2 eV. In addition to the continuum a 3?+ g (v, J)?b 3?+ u transition probabilities, spontaneous emission lifetimes of the a 3?+ g (v, J) (v = 0-20, J ? 14) levels have been calculated by considering both the a 3?+ g ?b 3?+ u and a 3?+ g -c 3? u transitions. The calculated lifetimes show a moderately strong rotational dependence, and the lifetimes for the J = 0 rotational level of the low v levels agree well with previous calculations and experimental measurements. Calculations of the a 3?+ g ?b 3?+ u continuum emission spectra from electron impact X 1?+ g -a 3?+ g excitation are included.

Calculated and recently measured photoabsorption transition probabilities of the H2 np?1?+ u and np?1? u ? X 1?+ g band systems have been examined with high-resolution (?? = 95-115 m?) electron-impact induced emission spectra obtained previously by Jonin et al. and Liu et al. When localized rovibronic coupling is insignificant, transition probabilities calculated with the adiabatic approximation are found to be generally consistent with experiment. However, in the presence of significant coupling, the transition probabilities obtained from a nonadiabatic calculation of B'1?+ u , D 1?+ u , , D'1?+ u , and 5p?1?+ u state coupling give better agreement with the experimental spectra. Emission yields obtained by comparison of the calculated and experimental spectra are also consistent with the measured predissociation and autoionization yields. In addition, more accurate excitation and emission cross sections and nonradiative yields have been obtained for a number of the np?1?+ u and np?1? u states. The results obtained in the present investigation lead to a significantly more accurate calibration of the Cassini UVIS instrument and laboratory spectrometers in the range 790-920 ?. They are also an important step toward an accurate model of extreme ultraviolet H2 auroral and dayglow emissions in the outer planet atmospheres.

Rotationally resolved molecular hydrogen transitions originating from excitation of highly excited rovibrational levels of the X 1Σ state have been systematically identified for the first time in the Far Ultraviolet Spectroscopic Explorer (FUSE) observation of comets C/2000 WM1 (LINEAR) and C/2001 A2 (LINEAR). Spectral assignments for the observed lines of H2 and other atomic and molecular species are given. All observed H2 transitions belong to the Lyman (B 1Σ-X 1Σ) and Werner (C 1Πu-X 1Σ) band systems. Solar Lyα fluorescence excitation of highly rovibrationally excited H2 X 1Σ is found to be almost solely responsible for the observed H 2 emission. Resonant excitation of H2 by Lyβ and other solar lines is very limited.

A simple model, based on fitting experimental energy term values, has been developed to calculate the line oscillator strengths (f) of N 2 b' 1Σ(1), c 1Σ(0), b 1Π(4), b 1Π(5), and c3 1Π(0)-X 1Σ(0) bands. The strong rovibronic coupling among these states leads to quantum interference in the transition dipole matrix elements and results in anomalous P/R branch oscillator strength ratios. The calculated line oscillator strengths are in very good agreement with experimental values. The calculated values can be used for modeling atmospheric emission. Calculated line oscillator strengths for J' up to 30 are provided.

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H-2 band systems and H Lyman-alpha from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2-4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5-16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassim Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325-326 (November 20-21). The global brightening events on November 20-21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for similar to 2 min is associated with aurora] flares inside the main auroral ovals. On January 8, 2001, from 18:45-19:35 UT UVIS H-2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to amoral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6-5 kHz Galileo PWS data. (c) 2005 Elsevier Inc. All rights reserved.

Observations of the Europa environment using the Cassini UltraViolet Imaging Spectrograph (UVIS) show the presence of an extended atomic oxygen atmosphere in addition to the bound molecular oxygen atmosphere first detected by Hubble Space Telescope in 1994 [D.T. Hall, D.F. Strobel, P.D. Feldman, M.A. McGrath, H.A. Weaver, 1995, Detection of an oxygen atmosphere on Jupiter's moon Europa, Nature 373, 677–679]. The atomic oxygen measurement provides a direct constraint on the sputtering and loss of Europa's water ice surface and the interaction of Europa's atmosphere with Jupiter's magnetosphere. We derive a loss rate for O 2 based on the emission rate of the OI 1356 A multiplet. UVIS detected substantial variability in the oxygen emission from Europa's oxygen atmosphere that we attribute to the viewing geometry. B.H. Mauk, D.G. Mitchell, S.M. Krimigis, E.C. Roelof, C.P. Paranicas [2003, Energetic neutral atoms from a trans-Europa gas torus at Jupiter, Nature 421, 920–922] inferred the presence of a torus of neutral gas at Europa's orbit based on Cassini's energetic neutral atom (ENA) image of the Jupiter system acquired with the Magnetospheric Imaging Instrument (MIMI), with the most likely torus constituents being hydrogen and oxygen species sputtered from Europa. Cassini UVIS data rule out O and O 2 as the possible torus species reported by Mauk et al. however, unless the torus density is so low that it is undetectable by UVIS (less than 8 atoms / cm 3 ). The UVIS observations indicate the presence of atomic hydrogen and possibly other species, but a full analysis is deferred to a following paper. The hydrogen in the present observations shows a local-time asymmetry and complex spatial distribution.

The Pioneer 10 (P10) and Voyager 2 (V2) calibration difference of 4.4 at Lyα has made it difficult to interpret the Lyα data and also to resolve the outer planetary upper atmosphere excess Lyα glow problem. We have carried out radiative transfer calculations using an improved radiative transfer code and six heliosphere neutral-plasma density models to study the calibration of P10 and V2 at Lyα and found that both P10 and V2 intensity measurements are in need of revision. The intercalibration difference is discussed using our model calculations, recent large-distance neutral hydrogen density determinations obtained from pickup-ion and solar wind slow-down data, the recent change in the estimate of the solar Lyα flux values, and Voyager 1 energetic particle measurements. These recent heliospheric measurements and Lyα glow model calculations support the need for an upward revision of P10 and a downward revision of V2 Lyα intensity. It is not yet possible to give a definitive estimate of the required revision because of lack of knowledge of the very local interstellar medium neutral hydrogen density. The calibration revision is found to reduce the range of variation of Jovian dayglow.

The Cassini Ultraviolet Imaging Spectrometer (UVIS) observed the extinction of photons from two stars by the atmosphere of Titan during the Titan flyby. Six species were identified and measured: methane, acetylene, ethylene, ethane, diacetylene, and hydrogen cyanide. The observations cover altitudes from 450 to 1600 kilometers above the surface. A mesopause is inferred from extraction of the temperature structure of methane, located at 615 km with a temperature minimum of 114 kelvin. The asymptotic kinetic temperature at the top of the atmosphere determined from this experiment is 151 kelvin. The higher order hydrocarbons and hydrogen cyanide peak sharply in abundance and are undetectable below altitudes ranging from 750 to 600 km, leaving methane as the only identifiable carbonaceous molecule in this experiment below 600 km.

A physical chemistry model has been developed to predict the non-LTE state of hydrogen-dominated astrophysical objects under electron and photon forcing. The model is composed of five constituents, H2, H, H, H, and H+, and is unique in the application of physical chemistry at the rotational level. The range of behavior is explored for electron and solar deposition. Application of solar forcing includes a prediction of the steady state photoelectron energy distribution, calculated from the interaction of solar photons and photoelectrons with the fine structure. The model is applied to a wide range of physical conditions, including those appropriate to the outer-planet upper atmospheres, from the exobase to the hydrocarbon homopause. Steady state partitioning is found to vary by multiple orders of magnitude in response to variation of neutral diffusion, ambient electron density, and gas kinetic temperature. The model is particularly sensitive to neutral diffusion. The non-LTE H2 X 1Σ (v : J) partitioning exhibited for the range of explored physical conditions and forcing is critical to the prediction of the states of the outer-planet ionospheres. The determination of rotational-level H2 partitioning allows the prediction of discrete and continuum emission features running the entire spectral range for comparison to observations of astrophysical phenomena, including outer-planet aurora and dayglow, comet coma environments, and stellar atmospheres.

A high-resolution, 33 mA (FWHM), optically thin emission study of the N2 c' 1Σ(0)-X 1Σ(0) band transition has been performed with electron impact at 100 eV. Recently measured line oscillator strengths and lifetimes have been examined by comparing model spectra with the observed emission. Good agreement between model and data indicates consistency between the measured lifetimes and oscillator strengths. Predissociation yields for various Jj levels are estimated. The calculated predissociation yield is in the range 0.2-0.5 for Jj = 4-23 and negligible for Jj = 0-2.

The Cassini Ultraviolet Imaging Spectrograph (UVIS) is part of the remote sensing payload of the Cassini orbiter spacecraft. UVIS has two spectrographic channels that provide images and spectra covering the ranges from 56 to 118 nm and 110 to 190 nm. A third optical path with a solar blind CsI photocathode is used for high signal-to-noise-ratio stellar occultations by rings and atmospheres. A separate Hydrogen Deuterium Absorption Cell measures the relative abundance of deuterium and hydrogen from their Lyman-α emission. The UVIS science objectives include investigation of the chemistry, aerosols, clouds, and energy balance of the Titan and Saturn atmospheres; neutrals in the Saturn magnetosphere; the deuterium-to-hydrogen (D/H) ratio for Titan and Saturn; icy satellite surface properties; and the structure and evolution of Saturn’s rings.

A general model, based on a theoretically calculated ionization oscillator strength and an experimentally determined excitation shape function, has been obtained for calculating the molecular hydrogen electron-impact ionization cross section of a transition between any discrete vibrational levels of the neutral X 1Σ state and ionic X 2Σ state. Specifically, the excitation shape function and ionization oscillator strength for transitions from the vi = 0 level of the X 1Σ neutral state to the discrete levels of the X 2Σ ionic state are derived from analyzing several experimental measurements. The derived oscillator strength is found to be consistent with the 1994 photoabsorption measurements of Samson & Haddad and the 1977 theoretical cross sections of Flannery and coworkers. The derived excitation function, along with the oscillator strengths for transitions involving the vi > 0 level calculated from the data of Flannery and coworkers, permits an accurate calculation of the nondissociative cross sections of H2 between any discrete vibrational levels over a wide energy range.

We report electron-impact-induced emission cross sections for the 2p4 3P–3s 3So (130.4 nm), 2p4 3P–3d 3Do (102.7 nm), 2p4 3P–3s' 3Do (98.9 nm) and 2p4 3P–3s'' 3Po (87.8 nm) transitions of atomic oxygen. Measurements were made using a magnetically confined electron beam in collision with atomic oxygen produced by an extended microwave discharge source in a crossed-beam arrangement. A 0.2 m vacuum ultraviolet diffraction spectrometer was used to record the emission. The absolute excitation functions of these transitions were determined by normalizing to the O I (130.4 nm) cross section produced by dissociative excitation of O2 at 100 eV impact energy (Noren et al 2001a Geophys. Res. Lett. 28 1379). A 60-state model of the O I electron reaction structure has been used to extract the excitation cross sections from the experimental measurements. The model process establishes analytic collision strengths to the asymptotic limit.

The electron impact emission function of the P(3) branch for the (0, 4) band of the H2 B 1? u+?X 1? g+ band system has been measured from threshold to 1800 eV. The emission function exhibits structure indicating strong contributions from both resonance and non-resonance excitation. The non-resonance component contains direct and cascade contributions. A combination of experimental and theoretical considerations permits separation of resonance, dipole-allowed direct, dipole-allowed indirect, and dipole-forbidden excitation components for the Jj = 2, vj = 0 level of the B 1? u+ state. An effective excitation function for the EF 1? g+?X 1? g+ band system has been obtained from a nonlinear least-squares analysis of the dipole-forbidden component of the B 1? u+ state emission function. The absolute value of EF 1? g+?X 1? g+ cross section is established on the basis of earlier experimental results of Liu et al 1995 Astrophys. J. Suppl. 101 375?99 and 2002 Astrophys J. Suppl. 138 229?45 and Abgrall et al 1997 Astrophys. J. 481 557?66 and 1999 J. Phys. B: At. Mol. Opt. Phys. 32 3813?38. A near-threshold apparent resonance excitation cross section of (8.1 ? 3.2) ? 10?18 cm2 is obtained for the B 1? u+ (Jj = 2 and vj = 0). An EF 1? g+?X 1? g+ Born cross section has been calculated from the electronic form factor of Kolos et al 1982a J. Chem. Phys. 77 1335?44. Analysis shows that the Born asymptotic shape function of the EF 1? g+?X 1? g+ band system starts at ~400 eV, a significantly higher energy than previously expected. The excitation function is especially important for interpreting outer planet atmospheric dayglow and auroral activity and can be used to infer energy deposition and heating rates.

We conducted a sensitive search for the resonance lines of Ar I (λλ1048.22, 1066.66) and O VI (λλ1031.93, 1037.62) in the spectra of three long-period comets observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). Argon emission was not detected from C/1999 T1 (McNaught-Hartley), C/2001 A2 (LINEAR), or C/2000 WM1 (LINEAR). Compared with the solar value, the [Ar/O] ratio is depleted in C/A2 by at least a factor of 10 and in C/WM1 by at least a factor of 13. The [Ar/O] upper limit for C/T1 is essentially the solar value, as our measurement was much less sensitive for that case. We also detected CO, which has a volatility similar to that of argon, during the FUSE observations. C/T1 was CO-rich ([CO/H2O] ≈ 13%), while both C/A2 and C/WM1 were CO-poor ([CO/H2O] ≈ 0.7% for C/A2 and ≈0.4% for C/WM1). The argon and CO depletions in C/A2 and C/WM1 suggest formation temperatures 60 K for both of these comets. The high CO abundance and upper limit on the argon abundance in C/T1 suggest that its formation temperature was in the range of ~40-50 K. No O VI emission was detected from comets C/T1 or C/A2, but the stronger line was marginally detected in C/WM1.

Medium-resolution (full width at half-maximum Δλ = 0.5 A) vacuum ultraviolet (VUV) emission spectra of H2 from forbidden excitation to the excited singlet-gerade states and subsequent cascade to the singlet-ungerade states have been studied with a time-resolved electron-impact technique. Emissions from the B 1Σ, C 1Πu, B' 1Σ, and D 1Πu states that are entirely due to cascade from the singlet-gerade states have been observed. Lyman band emission intensities dominate over the entire 940-1650 A wavelength region, though Werner band emission intensities are also significant. In addition, very weak cascade emissions from the vj = 0 and 1 levels of the B' 1Σ and D 1Πu states are also evident. In contrast to previous assumption, the cascade excitation is observed to take place over all the vibrational levels of the B 1Σ and C 1Πu states, while contributing heavily to low vj levels of the B 1Σ state. Synthetic spectra based on the calculated transition probabilities of the EF 1Σ, GK 1Σ, H 1Σ, I 1Πg, J 1Δg, O 1Σ and P 1ΣB 1Σ, C 1Πu, B' 1Σ, and D 1Πu band systems and Franck-Condon factors of the EF 1Σ-X 1Σ, GK 1Σ-X 1Σ, H 1Σ-X 1Σ, I 1Πg-X 1Σ, J 1Δg-X 1Σ, O 1Σ-X 1Σ, and P 1Σ-X 1Σ are in good agreement with the observed Lyman and Werner band intensities. The present study shows that a number of singlet-gerade and singlet-ungerade states are involved in the cascade excitation process of H2. The cascade from these states in the VUV provide diagnostic information on astrophysical sources for H2 emission.

The Galileo Extreme Ultraviolet Spectrometer (EUVS) and the Hopkins Ultraviolet Telescope (HUT) acquired UV spectra of Jupiter Aurora in the period from 1995 through 1997. The EUVS spectra spanned the wavelength range 540–1280 A and the HUT spectra measured the extreme ultraviolet and far ultraviolet (EUV+FUV) wavelength range 830–1850 A. Both sets of spectra present evidence of high-altitude, optically thin H 2 band emissions from the exobase region. The analysis of the UV spectra with a two-stream electron transport model and a jovian model auroral atmosphere indicates that the primary electron flux is composed of both soft and hard electrons with characteristic energies in the soft electron energy range of 20–200 eV and the hard electron range of 5–100 keV. The soft electron flux causes enhanced EUV emission intensities below 1100 A. The soft electron flux may explain the high temperature of the upper atmosphere above the homopause as measured from H 3 + rovibrational temperatures in the IR. For the deep aurora, a high primary characteristic energy above 5 keV is known to be present. The Galileo Energetic Particle Detector (EPD) has measured the electron distribution functions for energies above 15 keV in the middle magnetosphere. The high-energy distribution functions can be modeled by a combination of Maxwellian and kappa distributions. However, the EUV (800–1200 A) portion of the HUT spectrum cannot be modeled with a single distribution of hard electrons as was possible in the past for the FUV (1200–1650 A) spectrum measured by itself. The combination of EUV and FUV spectral observations by HUT serves to identify the amount of soft electron flux relative to the hard primary flux required to produce the high-altitude aurora in the neighborhood of the exobase.

A high-resolution (Δλ ~ 95 and 240 mA, FWHM) electron-impact-induced emission spectrum of molecular hydrogen has been obtained in the wavelength range 760-900 A. Analysis of the spectrum results in the assignment of some previously unobserved as well as observed but unassigned emission lines. Emissions observed in the 760-900 A range and a few selected regions above 900 A at room temperature (300 K) are tabulated, together with their spectral assignments. The tabulation can be considered as a supplement to the H2 atlas of Roncin & Launay and can be used for the analysis of EUV emission spectra observed in the interstellar medium and planetary atmospheres where H2 is the major constituent. Emission, predissociation, and autoionization yields for some transitions below 800 A are discussed. A Δv ≠ 0 perturbation between the Jj = 2 levels for vj = 5 of the D' 1Π state and vj = 2 of the 10pπ 1Π state is inferred.

The high-resolution (full width at half maximum = 0.11 A) emission spectrum of the deuterium molecule produced by electron-impact excitation at 100 eV has been measured in the wavelength range 1200 - 1660 A. In conjunction with the experimental measurement, transition probabilities of and D band systems are calculated. Synthetic spectra based on the calculated transition probabilities are in very good agreement with the optically thin experimental spectra. While centrifugal effects on the overall Lyman band emission intensity in D2 and H2 are similar, the effect of rotation-vibrational perturbations between the B 1u+ and C 1u+ states on spectral intensities is found to be less significant in D2. Excitations via isotropic ( J = 0) and anisotropic ( J = ±2, and 0) interactions for the dipole forbidden E, F 1g+-X 1g+ transition are inferred from the measured spectra. Excitation cross sections of the B 1u+-X 1g+ and C 1u-X 1g+ bands, along with the estimated E, F 1g+-B 1u+ cascade cross section, for D2 are obtained.

In 1996 and 1997 the Galileo Ultraviolet Spectrometer (UVS) obtained the first measurements of Jupiter's nightside midultraviolet (MUV) polar auroral spectrum from 1620 to 3231 A at 13 A resolution. The reduced polar spectra, after removal of off-axis scattered radiation from the sunlit dayside of Jupiter, contain a spectrum that matches laboratory spectra of the H 2 continuum in the a-b dissociative emission transition. This is the first direct identification of the H 2 a-b transition in astronomy. The a-b emission is excited by electron impact exchange reactions with H 2 that peak in cross section near 15 eV. The emission threshold is at 1216 A, and the continuum peaks in intensity in the 2000-2500 A range. Jupiter's observed wavelength-integrated MUV H 2 a-b emissions (1620-3231 A) have a photon flux ∼8 times smaller than simultaneously observed wavelength-integrated far-ultraviolet (FUV) H 2 band emissions (1230-1650 A). Because the FUV H 2 emissions have an emission cross section that peaks at higher energies near 50 eV, this FUV/MUV brightness ratio is diagnostic of the secondary electron energy distribution and is consistent with a warm distribution of electrons.

In 1996 during the first four orbits of the satellite tour the Galileo ultraviolet spectrometer (UVS) (11304320 A) and extreme ultraviolet spectrometer (EUVS) (540-1280 A) performed near-simultaneous observations of the Jupiter aurora in both the north and south polar regions. These observations are modeled to provide the absolute surface brightness of the aurora from the H 2 Rydberg Systems (B, B', B, C, D, D' → X band systems). The spectral distribution and brightness of the EUV aurora are sensitive to H 2 abundance, H 2 temperature, and CH 4 abundance. Analysis of the emission spectra indicates that the EUV aurora (800-1200 A) are produced over a range of altitudes corresponding to slant column abundances of H 2 from 10 16 to 10 20 cm -2 or greater. The UVS spectra of the far ultraviolet (FUV) from 1130 to 1700 A are optically thin in H 2 , but highly sensitive to the CH 4 column abundance and to the secondary electron energy distribution. The slant column abundance of CH 4 absorbers found from models of the FUV spectra varied in the range 0 - 10 x 10 16 cm -2 , indicating the presence of both high altitude aurora, at or above the homopause, and deep aurora. The FUV spectra show C 2 H 2 absorption bands near 1520 A. The surface brightness of the aurora from the H 2 Rydberg Systems ranged from 100 to 600 kR and of H Lyman α was 60 to 130 kR for a 2000 km wide oval. The total power input to the atmosphere from particle deposition is estimated to be ∼ 1 X 10 14 W.

The emission continua of the dissociative transitions B 1Σ+u-X 1Σ+g, B 1Σ+u-X 1Σ+g, C 1Πu-X 1Σ+g, and D 1Πu-X 1Σ+g of molecular hydrogen have been calculated. Synthetic spectra of H2 are in excellent agreement with high-resolution electron-impact-induced emission spectra obtained at 100 eV. An improved calibration standard is obtained, and the dissociation yields of the Lyman and Werner continua produced by electron-impact excitation are presented.

The optical excitation function of prompt Lyman-\ensuremath{\alpha} radiation, produced by electron impact on atomic hydrogen, has been measured over the extended energy range from threshold to 1.8 keV. Measurements were obtained in a crossed-beams experiment using both magnetically confined and electrostatically focused electrons in collision with atomic hydrogen produced by an intense discharge source. A vacuum-ultraviolet monochromator system was used to measure the emitted Lyman-\ensuremath{\alpha} radiation. The absolute H(1s-2p) electron impact excitation cross section was obtained from the experimental optical excitation function by normalizing to the accepted optical oscillator strength, with corrections for polarization and cascade. Our data are significantly different from the earlier experimental results of R. L. Long et al., J. Res. Natl. Bur. Stand. Sect. A 72A, 521 (1968) and J. F. Williams, J. Phys. B 9, 1519 (1976); 14, 1197 (1981), which are limited to energies below 200 eV. Statistical and known systematic uncertainties in our data range from \ifmmode\pm\else\textpm\fi{}4% near threshold to \ifmmode\pm\else\textpm\fi{}2% at 1.8 keV. Multistate coupling affecting the shape of the excitation function up to 1 keV impact energy is apparent in both the present experimental data and present theoretical results obtained with convergent close-coupling (CCC) theory. This shape function effect leads to an uncertainty in absolute cross sections at the 10% level in the analysis of the experimental data. The derived optimized absolute cross sections are within 7% of the CCC calculations over the 14 eV--1.8 keV range. The present CCC calculations converge on the Bethe-Fano profile for H(1s-2p) excitation at high energy. For this reason agreement with the CCC values to within 3% is achieved in a nonoptimal normalization of the experimental data to the Bethe-Fano profile. The fundamental H(1s-2p) electron impact cross section is thereby determined to an unprecedented accuracy over the 14 eV -- 1.8 keV energy range.

The next generation of high resolution UV imaging spacecraft are being prepared for studying the airglow and aurora of the Earth, the other terrestrial planets and the Jovian planets. To keep pace with these technological improvements we have developed a laboratory program to provide electron impact collision cross sections of the major molecular planetary gases (H2, N2, CO2, O2, and CO). Spectra under optically thin conditions have been measured with a high resolution (lambda/delta(lambda) = 50000) UV spectrometer in tandem with electron impact collision chamber. High resolution spectra of the Lyman and Wemer band systems of H2 have been obtained and modeled. Synthetic spectral intensities based on the J-dependent transition probabilities that include ro-vibronic perturbations are in very good agreement with experimental intensities. The kinetic energy distribution of H(2p,3p) atoms resulting from electron impact dissociation of H2 has been measured. The distribution is based on the first measurement of the H Lyman-alpha (H L(alpha)) and H Lyman-beta (H L(beta)) emission line Doppler profiles. Electron impact dissociation of H2 is believed to be one of the major mechanisms leading to the observed wide profile of H L-alpha from Jupiter aurora by the Hubble Space Telescope (HST). Analysis of the deconvolved line profile of H L-alpha reveals the existence of a narrow line peak (40 mA FWHM) and a broad pedestal base (240 mA FWHM). The band strengths of the electron excited N2 (C(sup 3) Pi(sub(upsilon) - B(sup 3)Pi(sub g)) second positive system have been measured in the middle ultraviolet. We report a quantitative measurement of the predissociation fraction 0.15 +/- 01(sup .045, sub .01) at 300 K in the N2 c'(sub )4 (1)sigma(sup +, sub g) - x(1)sigma(sup +, sub g)(00) band, with an experimental determination of rotational line strengths to be used to understand N2 EUV emission from Titan, Triton and the Earth.

[1] The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn's moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H2O gas, with a nominal line-of-sight column density of 0.90 ± 0.23 × 1016 cm−2 (upper limit of 1.0 × 1016 cm−2). The upper limit for N2 is 5 × 1013 cm−2, or

The importance of measuring electron-ion excitation cross sections in singly and multiply charged positive ions is outlined, and recent results for Mg II and O II ions are given using the JPL's electron energy-loss merged-beams apparatus. Theoretical comparisons are given with two five-state close-coupling calculations. The energy variation of the collision strength is fitted with a semiempirical analytic function which includes approximations to polarization, resonance, and exchange contributions. In O II, first spectra anywhere of electron excitation of the optically allowed transitions are presented. In addition, excitations of two low lying, optically forbidden transitions are detected for the first time.

[1] Terrestrial thermospheric dayglow emission from the coupled and overlapping c ' 4 1 E + u (0) and b' 1 Σ + u (1) levels of molecular nitrogen, observed by the Far Ultraviolet Spectroscopic Explorer, is analyzed with the aid of a coupled channels quantum mechanical model of N 2 spectroscopy and predissociation dynamics. Model emission spectra for the mixed c ' 4 1 Σ + u (0) ∼ b' E + u (1) ― X 1 Σ + g (v i = 2,6―9) transitions, calculated for the case of excitation by photoelectron impact, are in excellent agreement with the observations. While the principal excitation mechanism for N 2 in the thermosphere is photoelectron impact, evidence is also found in other transitions of resonant fluorescence, induced by lines in the solar atomic hydrogen Lyman series, atomic oxygen transitions, and other N 2 bands. The observed emission rate of the c ' 4 1 Σ + u (0) ∼ b' Σ + u (1) ― X 1 Σ + g (0) band is ∼1% of that inferred from the emission rates to X 1 Σ + g (v i > 2) levels. A qualitative explanation is given for the drastically reduced intensity and band shape distortion observed in the c ' 4 1 Σ + u (0) ― X 1 Σ + g (0) emission band. Estimates of the total electron excitation rates for the nominal b' 1 Σ + u (1) and c ' 4 1 Σ + u (0) levels are determined from the spectrum by extrapolating the model through regions containing unmeasured and/or resonantly absorbed bands.

[1] The emission properties of the N2c′41Σu+ − X1Σg+ band system have been investigated in a joint experimental and coupled-channels theoretical study. Relative intensities of the c′41Σu+(0) − X1Σg+(vi) transitions, measured via electron-impact-induced emission spectroscopy, are combined with a coupled-channel Schrodinger equation (CSE) model of the N2 molecule, enabling determination of the diabatic electronic transition moment for the c′41Σu+ − X1Σg+ system as a function of internuclear distance. The CSE rotational transition probabilities are further verified by comparison with a high-resolution experimental spectrum. Spontaneous transition probabilities of the c′41Σu+ − X1Σg+ band system, required for modeling atmospheric emission, can now be calculated reliably.

Analysis of the Cassini Ultraviolet Imaging Spectrometer (UVIS) stellar and solar occultations at Titan to date include 12 species: N$_{2}$ (nitrogen), CH$_{4}$ (methane), C$_{2}$H$_{2}$ (acetylene), C$_{2}$H$_{4}$ (ethylene), C$_{2}$H$_{6}$ (ethane), C$_{4}$H$_{2}$ (diacetylene), C$_{6}$H$_{6}$ (benzene), C$_{6}$N$_{2}$ (dicyanodiacetylene), C$_{2}$N$_{2}$ (cyanogen), HCN (hydrogen cyanide), HC$_{3}$N (cyanoacetylene), and aerosols distinguished by a structureless continuum extinction (absorption plus scattering) of photons in the EUV. The introduction of aerosol particles, retaining the same refractive index properties as tholin with radius $\sim$125 \AA and using Mie theory, provides a satisfactory fit to the spectra. The derived vertical profile of aerosol density shows distinct structure, implying a reactive generation process reaching altitudes more than 1000 km above the surface. A photochemical model presented here provides a reference basis for examining the chemical and physical processes leading to the distinctive atmospheric opacity at Titan. We find that dicyanodiacetylene is condensable at $\sim$650 km, where the atmospheric temperature minimum is located. This species is the simplest molecule identified to be condensable. Observations are needed to confirm the existence and production rates of dicyanodiacetylene., Comment: A typo in Table 1 was made in the previous version. The corrected tholin abundance is 4.6x10^11. ApJL in press. Will be published on June 1st, or May 21 online

The Cassini spacecraft flew close to Saturn's small moon Enceladus three times in 2005. Cassini's UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturn's E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system.

[1] Published experimental photon and electron impact ionization cross sections of CH4 have been reviewed and analyzed. Absolute partial ionization oscillator strengths (fij) for CH4+, CH3+, CH2+, CH+, H+, and H2+ have been obtained. Electron impact ionization cross sections are recommended based on agreement between the oscillator strength values derived from photoionization and electron impact measurements. Analytic functions for cross sections of CH4+, CH3+, CH2+, CH+, C+, H2+, and H+ produced by electron impact ionization of CH4 are established. The derived excitation cross-section functions are accurate from threshold to impact energies limited by relativistic effects (i.e., E < 0.2 MeV). The cross sections examined here are important for modeling Titan ionospheric chemistry.

Neutral oxygen in the saturnian system shows variability, and the total number of oxygen atoms peaks at 4 × 10 34 . Saturn's aurora brightens in response to solar-wind forcing, and the auroral spectrum resembles Jupiter's. Phoebe's surface shows variable water-ice content, and the data indicate it originated in the outer solar system. Saturn's rings also show variable water abundance, with the purest ice in the outermost A ring. This radial variation is consistent with initially pure water ice bombarded by meteors, but smaller radial structures may indicate collisional transport and recent renewal events in the past 10 7 to 10 8 years.

[1] Published experimental integrated electric dipole photoionization oscillator strengths (fij) have been applied to the determination of absolute electron impact excitation cross sections of the N2 X 1Σg+(0) state into the N2+ X 2Σg+(v), A 2Πu(v), and B 2Σu+(v) states. The required relative electron impact cross section shape functions from threshold to energy values, limited by the imposition of relativistic effects at the high end, are established in this work by extracting accurate analytic functions from a critical review of extensive published experimental results. The cross sections examined here are important benchmarks used in establishing absolute values for critical rate processes in aeronomy. The determination of the absolute cross sections using the derived fij is accomplished through the physical relationship to a modified Born approximation analytic function that defines the electron impact shape functions. The zero order term of the analytic function is established in value relative to the remaining terms through shape analysis of the experimental electron impact results. The value of fij fixes the absolute value of the zero order term, determining the cross sections over the entire electron impact energy range. It is argued that this methodology is the most accurate approach at this time because of the existence of the intrinsically more accurate photometrically dermined fij. The critical quantities in atmospheric process applications are the branching ratios of excitation into the N2+ X 2Σg+, A 2Πu, and B 2Σu+ states. The N2+ (X 2Σg+, A 2Πu, B 2Σu+) partitioning within the total ionization cross section has been a source of disagreement in the literature, in spite of the fact that substantial effort has been expended in establishing acceptably accurate values. The present work establishes accurate cross sections for these states over the entire practical energy range required for research in atmospheric N2 physical chemistry. The accuracy of the results obtained here are verified by consistency with the most reliably established past experimental electron impact cross sections, in agreement with the independently established total N2+ cross section. We provide analytic collision strengths for the vibrational levels of the N2+ X 2Σg+, A 2Πu, and B 2Σu+ states, allowing establishment of rate coefficients for the system, at an estimated error ≤±8%.

[1] A new hydrogen physical chemistry model has been developed at the fine-structure level for application to the giant outer planet thermospheres. The model is applied to Uranus because observations of dayglow H2 X 1Σg+ (v) quadrupole and H3+ vibration-rotation emission made at NASA IRTF and UKIRT provide critical constraints for thermospheric modeling. The observed H3+ vibration-rotation emission infers an H3+ dominant ionosphere, predicted only for non-LTE H2 X (v : J). Excitation mechanisms explored are solar and non-solar electron energy deposition. Non-solar electron forcing is constrained by the EUV H2 Lyman and Werner band emission measured by Voyager UVS. Analysis indicates that non-solar electrons are dominant in the energy budget required to predict the observed thermospheric temperature profile. The modeled H2 X quadrupole emission infers that an additional mechanism is required to excite the H2 X (v = 1) population. Non-thermal H produced in dissociative excitation of H2 X is a primary candidate.

[1] In this work, we report the OI(135.6 nm) absolute emission cross section resulting from the long-lived (180 μs) OI(5S → 3P) transition from dissociative excitation of O2. From the ratio of the integrated intensities of the OI(135.6 nm) and OI(130.4 nm) features and from the absolute emission cross section for the OI(130.4 nm) emission feature from electron impact dissociative excitation of O2 at 100 eV, the absolute emission cross section for the OI(135.6 nm) feature was determined to be 6.4 × 10−18 cm2 at 100 eV. Electron impact-induced optical excitation functions for optically allowed transitions at 115.2 nm and 130.4 nm and for an optically forbidden transition at 135.6 nm were also obtained over the electron impact energy range 0–600 eV. The OI(135.6 nm) emission cross section was measured in the laboratory utilizing a large collision chamber (1.5 m in diameter). Electrons were produced with an electrostatically focusing gun with a large focal length (50 cm). The OI(130.4 nm, 135.6 nm) excitation functions were put on an absolute scale as described in the text, and the OI(135.6 nm)/OI(130.4 nm) ratio was determined for the entire energy range (0–600 eV). The atomic O UV emission cross sections from dissociative excitation of O2 can be used to model the recent Hubble Space Telescope observations of OI(130.4 nm) and OI(135.6 nm) intensities from Ganymede [Feldman et al., 2000] and Europa [Hall et al., 1995, 1998].

Studies were initiated in 1995 at the Lockheed Martin Advanced Technology Center in Palo Alto to explore both the application of new technologies and the economical utilization of commercial products to the design of a new generation of scientific research satellites. A 2.4-meter Solar System Observatory (SSO) has been designed to carry out as its primary mission imaging and spectroscopy of comets and of the outer planets form geosynchronous orbit. Such a Hubble-class telescope with a science payload consisting of four UV/EUV spectrographs and a high- resolution imager having 0.06 arcsec spatial resolution can now be built and launched within the budget of a NASA Discovery Mission. Following a one-year science program under the direction of the principle investigator, the SSO would transition to a guest observer facility. Although optimized for cometary and planetary measurements, SSO would have outstanding capability for a variety of astrophysical measurements. SSO would also serve as a prototype for other similar low-cost space observatories that could be optimized for stellar, extragalactic and other applications.

We report on a design for a geosynchronous UV observatory optimized for imaging and spectrography of planets and comets. This solar system telescope (SST), based on a commercial developed spacecraft, was proposed to NASA as a Discovery mission. It can also serve as a low-cost orbiting observatory for other disciplines in space astronomy. The SST consists of a 140-cm-aperture telescope with an instrumentation section comprising four spectrographs and a wide-field UV imager. We use silicon carbide mirrors and a telescope structure provided by the Vavilov State Optical Institute in St. Petersburg, Russia. The spacecraft is derived from Lockheed Martin's commercial remote sensing satellite (CRSS), which provides attitude control, power, communications, and command and data handling, with minimal modifications. Using this approach, we were able to design an observatory with capabilities comparable to the Hubble Space Telescope at approximately 1/20th the cost.

State-to-state [X 1 � + (vi) → X 2 � + g (vj )] and quasi state-specific [X 1 � + (vi,Ji) → X 2 � + (vj )] photon and electron-impact ionization cross sections of hydrogen and deuterium molecules have been obtained with a combined analysis of experimental measurement and theoretical calculations. Rotation state ionization transitions are considered using the p-partial wave approximation. Previously calculated photoionization transition moments are refined and extended to ∼300 eV using accurate experimental photoionization cross sections. The transition moments are used to obtain the photoionization cross sections and oscillator strengths for the H2 and D2 X 1 � + (vi,Ji) → H + and D + X 2 � + g (vj,Jj ) transitions. The calculated ionization oscillator strengths, together with excitation functions derived from electron-impact ionization measurements of H2, enable accurate determination of electron-impact ionization cross sections for both H2 and D2 transitions to (vj,Jj) levels of the X 2 � + state at energies ranging from threshold to the non-relativistic limit. The autoionizing singlet-ungerade states contribute at least 0.6‐1% of the total e+H2 →H + cross section. The e+D2 →D + and e+H2 →H + cross sections are very similar above the threshold energy region. The D2 cross section is ∼1.6% larger than the H2 cross section. Thermally averaged nondissociative ionization cross sections of H2 and D2 at temperatures appropriate for plasma and planetary applications have been tabulated. (Some figures may appear in colour only in the online journal)

Photodissociation cross sections and oscillator strengths for H2 from the X?1?+g (vi, Ji) levels to the continuum levels of the B?1?+u, C?1?u, B, D'?1?u and 5p??1?+u states have been calculated. The (vi, Ji) state-specific electron impact dissociation cross sections to the continuum levels of these states have been obtained for the first time over a wide energy range using calculated continuum oscillator strengths along with previously published excitation functions of the Lyman and Werner bands. Estimated cross sections to the higher (n ? 5) np??1?+u and np??1?u continua are also provided. Both photon and electron impact excitation cross sections show strong dependences on the initial (vi, Ji) quantum numbers. Thermally averaged electron impact cross sections of all singlet ungerade states increase monotonically with temperature. While excitation to the B'?1?+u continuum is the dominant dissociation channel at room temperature, the C?1?u and B?1?+u continua become more important at high temperature (>5000 K). This work, along with the previous calculation of the B'?1?+u and D?1?u states by Liu et al (2009a J. Phys. B: At. Mol. Opt. Phys. 42 185203), provides the complete electron impact dissociation cross section of H2 through the singlet ungerade continua. Electron dissociation cross sections of the singlet ungerade continua are provided for the purpose of modelling atmospheric heating, analysis of occultation measurements and the hot atomic hydrogen plume observed at Saturn.