Joseph M. Ajello

Extreme Ultraviolet Observations from Voyager 1 Encounter with Jupiter

Observations of the optical extreme ultraviolet spectrum of the Jupiter planetary system during the Voyager 1 encounter have revealed previously undetected physical processes of significant proportions. Bright emission lines of S III, S IV, and O III indicating an electron temperature of 105 K have been identified in preliminary analyses of the Io plasma torus spectrum. Strong auroral atomic and molecular hydrogen emissions have been observed in the polar regions of Jupiter near magnetic field lines that map the torus into the atmosphere of Jupiter. The observed resonance scattering of solar hydrogen Lyman α by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (≥ 1000 K) wvith a large atomic hydrogen abundance. A stellar occultation by Ganymede indicates that its atmosphere is at most an exosphere.

Ultraviolet emissions from the magnetic footprints of Io, Ganymede and Europa on Jupiter

Io leaves a magnetic footprint on Jupiters upper atmosphere that appears as a spot of ultraviolet emission that remains fixed underneath Io as Jupiter rotates. The specific physical mechanisms responsible for generating those emissions are not well understood, but in general the spot seems to arise because of an electromagnetic interaction between Jupiters magnetic field and the plasma surrounding Io, driving currents of around 1 million amperes down through Jupiters ionosphere. The other galilean satellites may also leave footprints, and the presence or absence of such footprints should illuminate the underlying physical mechanism by revealing the strengths of the currents linking the satellites to Jupiter. Here we report persistent, faint, far-ultraviolet emission from the jovian footprints of Ganymede and Europa. We also show that Ios magnetic footprint extends well beyond the immediate vicinity of Ios flux-tube interaction with Jupiter, and much farther than predicted theoretically; the emission persists for several hours downstream. We infer from these data that Ganymede and Europa have persistent interactions with Jupiters magnetic field despite their thin atmospheres.

H_2 fluorescence spectrum from 1200 to 1700 Å by electron impact: Laboratory study and application to Jovian aurora

A combined experimental study of the fluorescence spectrum of H_2 at wavelengths of 1200-1700 A by electron impact and its application to modeling the Jovian aurora have been carried out. Our laboratory data suggest that at 100 eV the relative cross sections for direct excitation of Lyɑ, Lyman bands (B^1Σ_u^+-X^1Σ_g^+), and Werner bands (C^1π_u-X^1Σ_g^+) are 1, 2.3±0.6, and 2.6±0.5, respectively, in conflict with Stone and Zipfs (1972) results for the Werner bands. Cascade from E,F^1Σ_g^+ states contributes an additional 31% to the B^1Σ_u^+ state population. It is shown that the most likely fate for the metastable H(2^2S) atoms produced in the Jovian aurora is collisional quenching to H(2^2P), and this could add as much as 60% to the predicted Lyɑ emission. On the basis of detailed atmospheric and radiative transfer modeling, we conclude that the recent IUE and Voyager observations are consistent with precipitation of electrons with energy in the range of 1-30 keV or other energetic particles that penetrate to number densities of 4 X 10^(10)-5 X 10^(13) cm^(-3) or column densities of 5 X 10^(17)-2 X 10^(20) cm^(-2) in the atmosphere. The globally averaged energy flux and production of hydrogen atoms are 0.5-2 ergs cm^(-2) s^(-1) and 1-4 X 10^(10) atoms cm^(-2) s^(-1), respectively.

Ultraviolet spectrometer experiment for the Voyager mission

The Voyager Ultraviolet Spectrometer (UVS) is an objective grating spectrometer covering the wavelength range of 500–1700 Å with 10 Å resolution. Its primary goal is the determination of the composition and structure of the atmospheres of Jupiter, Saturn, Uranus and several of their satellites. The capability for two very different observational modes have been combined in a single instrument. Observations in the airglow mode measure radiation from the atmosphere due to resonant scattering of the solar flux or energetic particle bombardment, and the occultation mode provides measurements of the atmospheric extinction of solar or stellar radiation as the spacecraft enters the shadow zone behind the target. In addition to the primary goal of the solar system atmospheric measurements, the UVS is expected to make valuable contributions to stellar astronomy at wavelengths below 1000 Å.

Solar cycle study of interplanetary Lyman-alpha variations - Pioneer Venus Orbiter sky background results

PVO observations of the interplanetary Ly-alpha (IPL) background, obtained over an entire solar cycle (SC) from 1979 to 1985, are compiled and analyzed statistically, along with data from other instruments and earlier solar cycles. The results are presented in extensive tables and graphs and characterized in detail. Findings reported include SC variation of 1.8 for the longitudinally averaged IPL intensity (in agreement with the variation of the 27-d disk-averaged integrated solar Ly-alpha flux), yearly averaged ecliptic H-atom lifetime at 1 AU equal to 1.0 Ms at solar minimum and 1.5 Ms at solar maximum, interplanetary H density equal to 0.07 + or - 0.01/cu cm, and interplanetary H/He within the heliopause but far from the sun of 7 + or - 3. 74 references.

Electron impact excitation of H2 - Rydberg band systems and the benchmark dissociative cross section for H Lyman-alpha

The cross sections sigma R 1 (2p) for excitation of H Ly-alpha emission produced by electron impact on H2 is reexamined. A more accurate estimate for sigma R 1 (2p) is obtained based on Born approximation estimates of the H2 Rydberg system cross sections using measured relative excitation functions. The obtained value is (8.18 + or -1.2) x 10 to the -18th sq cm at 100 eV, a factor of 0.69 below the value universally applied to cross section measurements over the past decade. Cross sections for the H2 Rydberg systems fixed in magnitude by the Born approximation have also been obtained using experimentally determined excitation functions. Accurate analytic expressions for these cross sections allow the direct calculation of rate coefficients. 30 references.

Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Atomic hydrogen Lyman alpha radiation (121.6 nm) has been measured in emission from the atmosphere of Ganymede with the Galileo ultraviolet spectrometer. An exospheric model with the following parameters has been fit to the observational data: atomic hydrogen density directly above the surface (radius 2634 km) equal to 1.5 × 104 atoms cm−3 scale height 2634 km, exospheric temperature 450 K. A model calculation of the photodissociation of water vapor from surface ice at 146 K is used to obtain the photodissociation rate necessary to supply the hydrogen atoms that are escaping from the exosphere of Ganymede. The calculated escape flux of atomic hydrogen is 7 × 108 atoms/cm² sec. Two alternate but speculative sources of the atomic hydrogen escaping from Ganymede are photodesorption of water ice by ultraviolet photons in the wavelength range 120.5–186.0 nm and sputtering of water ice by Jupiters magnetospheric ion plasma.

The Composition of Phobos: Evidence for Carbonaceous Chondrite Surface from Spectral Analysis

A reflectance spectrum of Phobos (from 200 to 1100 nanometers) has been compiled from the Mariner 9 ultraviolet spectrometer, Viking lander imaging, and ground-based photometric data. The reflectance of the martian satellite is approximately constant at 5 percent from 1100 to 400 nanometers but drops sharply below 400 nanometers, reaching a value of 1 percent at 200 nanometers. The spectral albedo of Phobos bears a striking resemblance to that of asteroids (1) Ceres and (2) Pallas. Comparison of the reflectance spectra of asteroids with those of meteorites has shown that the spectral signature of Ceres is indicative of a carbonaceous chondritic composition. A physical explanation of how the compositional information is imposed on the reflectance spectrum is given. On the basis of a good match between the reflectance spectra of Phobos and Ceres and the extensive research that has been done to infer the composition of Ceres, it seems reasonable to believe that the surface composition of Phobos is similar to that of carbonaceous chondrites. This suggestion is consistent with the recently determined low density of Marss inner satellite. Our result and recent Viking noble gas measurements suggest different modes of origin for Mars and Phobos.

Simple ultraviolet calibration source with reference spectra and its use with the Galileo orbiter ultraviolet spectrometer

We have developed a simple compact electron impact laboratory source of UV radiation whose relative intensity as a function of wavelength has an accuracy traceable to the fundamental physical constants (transitions probabilities and excitation cross sections) for an atomic or molecular system. Using this laboratory source, calibrated optically thin vacuum ultraviolet (VUV) spectra have been obtained and synthetic spectral models developed for important molecular band systems of H(2) and N(2) and the n(1)P(0) Itydberg series of He. The model spectrum from H(2) represents an extension of the molecular branching ratio technique to include spectral line intensities from more than one electronic upper state. The accuracy of the model fit to the VUV spectra of H(2) and N(2) is sufficient to predict the relative spectral intensity of the electron impact source and to serve as a primary calibration standard for VUV instrumentation in the 80-230-nm wavelength range. The model is applicable to VUV instrumentation with full width at half-maximum >/= 0.4 nm. The present accuracy is 10% in the far ultraviolet (120-230 nm), 10% in the extreme ultraviolet (EUV) (90-120 nm), and 20% in the EUV (80-90 nm). The n(1)P(0) Rydberg series of He has been modeled to 10% accuracy and can be considered a primary calibration standard in the EUV (52.2-58.4 nm). A calibrated optically thin spectrum of Ar has been obtained at 0.5-nm resolution and 200-eV electron impact energy to 35% accuracy without benefit of models over the EUV spectral range of 50-95 nm. The Ar spectrum expands the ultimate range of the VUV relative calibration using this source with the four gases, He, Ar, H(2), and N(2), to 50-230 nm. The calibration of the Galileo orbiter ultraviolet spectrometer for the upcoming Jupiter mission has been demonstrated and compared to results from other methods.

A photoionization mass spectrometer study of CFCl3, CF2Cl2, and CF3Cl

A photoionization mass spectrometer has been used over the wavelength range 650–1100 A (19.07–11.27 eV) to determine the appearance potentials, heats of formation, and photoionization efficiencies of the following ions: CFCl+2, CCl+3,CFCl+, and CCl+2 from CFCl3; CF2Cl+2, CFCl+2, CF2Cl+, CFCl+, and CF+2 from CF2Cl2; and CF3Cl+, CF3+, and CF+2 from CF3Cl. The parent ion CFCl+3 of CFCl3 was not observed.