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Paul D. Feldman,
Dana M. Hurley,
Kurt D. Retherford,
G. Randall Gladstone,
S. Alan Stern,
Wayne Pryor,
Joel Wm. Parker,
David E. Kaufmann,
Michael W. Davis,
Maarten Versteeg,
LAMP team
[show abstract]
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ABSTRACT: We report observations of the lunar helium exosphere made between December
29, 2011, and January 26, 2012, with the Lyman Alpha Mapping Project (LAMP)
ultraviolet spectrograph on NASA's Lunar Reconnaissance Orbiter Mission (LRO).
The observations were made of resonantly scattered He I 584 from illuminated
atmosphere against the dark lunar surface on the dawn side of the terminator.
We find no or little variation of the derived surface He density with latitude
but day-to-day variations that likely reflect variations in the solar wind
alpha flux. The 5-day passage of the Moon through the Earth's magnetotail
results in a factor of two decrease in surface density, which is well explained
by model simulations.
09/2012;
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Stephen Smee,
James E. Gunn,
Alan Uomoto,
Natalie Roe,
David Schlegel,
Constance M. Rockosi,
Michael A. Carr,
French Leger,
Kyle S. Dawson,
Matthew D. Olmstead, [......],
Walter Siegmund,
Audrey Simmons,
D. Smith,
Stephanie Snedden,
Donald P. Schneider,
Michael Strauss,
Mark SubbaRao,
Christy Tremonti,
Patrick Waddell,
Donald G. York
[show abstract]
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ABSTRACT: We present the design and performance of the multi-object fiber spectrographs
for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon
Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999
on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the
spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II
surveys, enabling a wide variety of Galactic and extra-galactic science
including the first observation of baryon acoustic oscillations in 2005. The
spectrographs were upgraded in 2009 and are currently in use for BOSS, the
flagship survey of the third-generation SDSS-III project. BOSS will measure
redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-alpha
absorption of 160,000 high redshift quasars over 10,000 square degrees of sky,
making percent level measurements of the absolute cosmic distance scale of the
Universe and placing tight constraints on the equation of state of dark energy.
The twin multi-object fiber spectrographs utilize a simple optical layout
with reflective collimators, gratings, all-refractive cameras, and
state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in
two channels over a bandpass covering the near ultraviolet to the near
infrared, with a resolving power R = \lambda/FWHM ~ 2000. Building on proven
heritage, the spectrographs were upgraded for BOSS with volume-phase
holographic gratings and modern CCD detectors, improving the peak throughput by
nearly a factor of two, extending the bandpass to cover 360 < \lambda < 1000
nm, and increasing the number of fibers from 640 to 1000 per exposure. In this
paper we describe the original SDSS spectrograph design and the upgrades
implemented for BOSS, and document the predicted and measured performances.
08/2012;
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G. Randall Gladstone,
S. Alan Stern,
Kurt D. Retherford,
Ronald K. Black,
David C. Slater,
Michael W. Davis,
Maarten H. Versteeg,
Kristian B. Persson,
Joel W. Parker,
David E. Kaufmann,
Anthony F. Egan,
Thomas K. Greathouse, Paul D. Feldman,
Dana Hurley,
Wayne R. Pryor,
Amanda R. Hendrix
[show abstract]
[hide abstract]
ABSTRACT: The Lyman Alpha Mapping Project (LAMP) is a far-ultraviolet (FUV) imaging spectrograph on NASA’s Lunar Reconnaissance Orbiter (LRO) mission. Its main objectives are to (i) identify and localize exposed water frost in permanently shadowed regions (PSRs),
(ii) characterize landforms and albedos in PSRs, (iii) demonstrate the feasibility of using natural starlight and sky-glow
illumination for future lunar surface mission applications, and (iv) characterize the lunar atmosphere and its variability.
As a byproduct, LAMP will map a large fraction of the Moon at FUV wavelengths, allowing new studies of the microphysical and
reflectance properties of the regolith. The LAMP FUV spectrograph will accomplish these objectives by measuring the signal
reflected from the night-side lunar surface and in PSRs using both the interplanetary HI Lyman-α sky-glow and FUV starlight as light sources. Both these light sources provide fairly uniform, but faint, illumination. With
the expected LAMP sensitivity, by the end of the primary 1-year LRO mission, the SNR for a Lyman-α albedo map should be >100 in polar regions >1km2, providing useful FUV constraints to help characterize subtle compositional and structural features. The LAMP instrument
is based on the flight-proven Alice series of spectrographs flying on the Rosetta comet mission and the New Horizons Pluto mission. A general description of the LAMP instrument and its initial ground calibration results are presented here.
Lunar-Ultraviolet-LRO-Lyman-α
Space Science Reviews 04/2012; 150(1):161-181. · 3.61 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report results of a Hubble Space Telescope (HST) campaign with the Advanced Camera for Surveys to observe Europa at eastern elongation, i.e., Europa's leading side, on 2008 June 29. With five consecutive HST orbits, we constrain Europa's atmospheric O I 1304 Å and O I 1356 Å emissions using the prism PR130L. The total emissions of both oxygen multiplets range between 132 ± 14 and 226 ± 14 Rayleigh. An additional systematic error with values on the same order as the statistical errors may be due to uncertainties in modeling the reflected light from Europa's surface. The total emission also shows a clear dependence of Europa's position with respect to Jupiter's magnetospheric plasma sheet. We derive a lower limit for the O2 column density of 6 × 1018 m–2. Previous observations of Europa's atmosphere with the Space Telescope Imaging Spectrograph in 1999 of Europa's trailing side show an enigmatic surplus of radiation on the anti-Jovian side within the disk of Europa. With emission from a radially symmetric atmosphere as a reference, we searched for an anti-Jovian versus sub-Jovian asymmetry with respect to the central meridian on the leading side and found none. Likewise, we searched for departures from a radially symmetric atmospheric emission and found an emission surplus centered around 90° west longitude, for which plausible mechanisms exist. Previous work about the possibility of plumes on Europa due to tidally driven shear heating found longitudes with strongest local strain rates which might be consistent with the longitudes of maximum UV emissions. Alternatively, asymmetries in Europa's UV emission can also be caused by inhomogeneous surface properties, an optically thick atmospheric contribution of atomic oxygen, and/or by Europa's complex plasma interaction with Jupiter's magnetosphere.
The Astrophysical Journal 08/2011; 738(2):153. · 6.02 Impact Factor
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Paul D. Feldman,
Andrew J. Steffl,
Joel Wm. Parker,
Michael F. A'Hearn,
Jean-Loup Bertaux,
S. Alan Stern,
Harold A. Weaver,
David C. Slater,
Maarten Versteeg,
Henry B. Throop,
Nathaniel J. Cunningham,
Lori M. Feaga
[show abstract]
[hide abstract]
ABSTRACT: The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter
with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on
25 February 2007, closest approach being at 01:54UT. The Alice instrument on
board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for
in situ cometary spectroscopy in the 750-2000 A spectral band, was used to
study the daytime Mars upper atmosphere including emissions from exospheric
hydrogen and oxygen. Offset pointing, obtained five hours before closest
approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta
emissions from exospheric hydrogen out beyond 30,000 km from the planet's
center. These data are fit with a Chamberlain exospheric model from which we
derive the hydrogen density at the 200 km exobase and the H escape flux. The
results are comparable to those found from the the Ultraviolet Spectrometer
experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen
emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb
during limb scans shortly after closest approach. However, the derived oxygen
scale height is not consistent with recent models of oxygen escape based on the
production of suprathermal oxygen atoms by the dissociative recombination of
O2+.
06/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We report results of a Hubble Space Telescope (HST) campaign with the
Advanced Camera for Surveys to observe Europa at eastern elongation, i.e.
Europa's leading side, on 2008 June 29. With five consecutive HST orbits, we
constrain Europa's atmospheric \ion{O}{1} 1304 \A and \ion{O}{1} 1356 \A
emissions using the prism PR130L. The total emissions of both oxygen multiplets
range between 132 $\pm$ 14 and 226 $\pm$ 14 Rayleigh. An additional systematic
error with values on the same order as the statistical errors may be due to
uncertainties in modelling the reflected light from Europa's surface. The total
emission also shows a clear dependence of Europa's position with respect to
Jupiter's magnetospheric plasma sheet. We derive a lower limit for the O$_2$
column density of 6 $\times$ 10$^{18}$ m$^{-2}$. Previous observations of
Europa's atmosphere with STIS in 1999 of Europa's trailing side show an
enigmatic surplus of radiation on the anti-Jovian side within the disk of
Europa. With emission from a radially symmetric atmosphere as a reference, we
searched for an anti-Jovian vs sub-Jovian asymmetry with respect to the central
meridian on the leading side, and found none. Likewise, we searched for
departures from a radially symmetric atmospheric emission and found an emission
surplus centered around 90 degree west longitude, for which plausible
mechanisms exist. Previous work about the possibility of plumes on Europa due
to tidally-driven shear heating found longitudes with strongest local strain
rates which might be consistent with the longitudes of maximum UV emissions.
Alternatively, asymmetries in Europa's UV emission can also be caused by
inhomogeneous surface properties, inhomogeneous solar illuminations, and/or by
Europa's complex plasma interaction with Jupiter's magnetosphere.
06/2011;
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[show abstract]
[hide abstract]
ABSTRACT: Partial frequency redistribution (PRD), describing the formation of the line
profile, has negligible observational effects for optical depths smaller than
~10^3, at the resolving power of most current instruments. However, when the
spectral resolution is sufficiently high, PRD modeling becomes essential in
interpreting the line shapes and determining the total line fluxes. We
demonstrate the effects of PRD on the H_2 line profiles observed at high
spectral resolution by the Far-Ultraviolet Spectroscopic Explorer (FUSE) in the
atmospheres of Jupiter and Saturn. In these spectra, the asymmetric shapes of
the lines in the Lyman (v"- 6) progression pumped by the solar Ly-beta are
explained by coherent scattering of the photons in the line wings. We introduce
a simple computational approximation to mitigate the numerical difficulties of
radiative transfer with PRD, and show that it reproduces the exact radiative
transfer solution to better than 10%. The lines predicted by our radiative
transfer model with PRD, including the H_2 density and temperature distribution
as a function of height in the atmosphere, are in agreement with the line
profiles observed by FUSE. We discuss the observational consequences of PRD,
and show that this computational method also allows us to include PRD in
modeling the continuum pumped H_2 fluorescence, treating about 4000 lines
simultaneously.
02/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We present a measurement of the lifetime of ground state atomic carbon, C(^3P), against ionization processes in interplanetary space and compare it to the lifetime expected from the dominant physical processes likely to occur in this medium. Our measurement is based on analysis of a far ultraviolet (FUV) image of comet C/2004 Q2 (Machholz) recorded by the Galaxy Evolution Explorer (GALEX) on 2005 March 1. The bright CI 1561 A and 1657 A multiplets dominate the GALEX FUV band. We used the image to create high S/N radial profiles that extended beyond one million km from the comet nucleus. Our measurements yielded a total carbon lifetime of 7.1 -- 9.6 x 10^5 s (scaled to 1 AU). Which compares favorably to calculations assuming solar photoionization, solar wind proton change exchange and solar wind electron impact ionization are the dominant processes occurring in this medium and that comet Machholz was embedded in the slow solar wind. The shape of the CI profiles inside 3x10^5 km suggests that either the CO lifetime is shorter than previously thought and/or a shorter-lived carbon-bearing parent molecule, such as CH_4 is providing the majority of the carbon in this region of the coma of comet Machholz. Comment: 26 pages, 6 figures, accepted for publication in the Astrophysical Journal
11/2010;
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G Randall Gladstone,
Dana M Hurley,
Kurt D Retherford, Paul D Feldman,
Wayne R Pryor,
Jean-Yves Chaufray,
Maarten Versteeg,
Thomas K Greathouse,
Andrew J Steffl,
Henry Throop,
Joel Wm Parker,
David E Kaufmann,
Anthony F Egan,
Michael W Davis,
David C Slater,
Joey Mukherjee,
Paul F Miles,
Amanda R Hendrix,
Anthony Colaprete,
S Alan Stern
[show abstract]
[hide abstract]
ABSTRACT: On 9 October 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) sent a kinetic impactor to strike Cabeus crater, on a mission to search for water ice and other volatiles expected to be trapped in lunar polar soils. The Lyman Alpha Mapping Project (LAMP) ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO) observed the plume generated by the LCROSS impact as far-ultraviolet emissions from the fluorescence of sunlight by molecular hydrogen and carbon monoxide, plus resonantly scattered sunlight from atomic mercury, with contributions from calcium and magnesium. The observed light curve is well simulated by the expansion of a vapor cloud at a temperature of ~1000 kelvin, containing ~570 kilograms (kg) of carbon monoxide, ~140 kg of molecular hydrogen, ~160 kg of calcium, ~120 kg of mercury, and ~40 kg of magnesium.
Science 10/2010; 330(6003):472-6. · 31.20 Impact Factor
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Stephan R. McCandliss,
Brian Fleming,
Mary Elizabeth Kaiser,
Jeffrey Kruk, Paul D. Feldman,
Alexander S. Kutyrev,
Mary J. Li,
Phillip A. Goodwin,
David Rapchun,
Eric Lyness,
Ari D. Brown,
Harvey Moseley,
Oswald Siegmund,
John Vallerga
[show abstract]
[hide abstract]
ABSTRACT: The Johns Hopkins University sounding rocket group is building the Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy (FORTIS), which is a Gregorian telescope with rulings on the secondary mirror. FORTIS will be launched on a sounding rocket from White Sand Missile Range to study the relationship between Lyman alpha escape and the local gas-to-dust ratio in star forming galaxies with non-zero redshifts. It is designed to acquire images of a 30' x 30' field and provide fully redundant "on-the-fly" spectral acquisition of 43 separate targets in the field with a bandpass of 900 - 1800 Angstroms. FORTIS is an enabling scientific and technical activity for future cutting edge far- and near-uv survey missions seeking to: search for Lyman continuum radiation leaking from star forming galaxies, determine the epoch of He II reionization and characterize baryon acoustic oscillations using the Lyman forest. In addition to the high efficiency "two bounce" dual-order spectro-telescope design, FORTIS incorporates a number of innovative technologies including: an image dissecting microshutter array developed by GSFC; a large area (~ 45 mm x 170 mm) microchannel plate detector with central imaging and "outrigger" spectral channels provided by Sensor Sciences; and an autonomous targeting microprocessor incorporating commercially available field programable gate arrays. We discuss progress to date in developing our pathfinder instrument.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
07/2010;
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[show abstract]
[hide abstract]
ABSTRACT: Galaxy Evolution Explorer (GALEX) observations of comet 9P/Tempel 1 using the near-ultraviolet (NUV) objective grism were made before, during and after the Deep Impact event that occurred on 2005 July 4 at 05:52:03 UT when a 370 kg NASA spacecraft was maneuvered into the path of the comet. The NUV channel provides usable spectral information in a bandpass covering 2000-3400 Å with a point source spectral resolving power of R 100. The primary spectral features in this range include solar continuum scattered from cometary dust and emissions from OH and CS molecular bands centered near 3085 and 2575 Å, respectively. In particular, we report the only cometary CS emission detected during this event. The observations allow the evolution of these spectral features to be tracked over the period of the encounter. In general, the NUV emissions observed from Tempel 1 are much fainter than those that have been observed by GALEX from other comets. However, it is possible to derive production rates for the parent molecules of the species detected by GALEX in Tempel 1 and to determine the number of these molecules liberated by the impact. The derived quiescent production rates are Q(H2O) = 6.4 × 1027 molecules s–1 and Q(CS2) = 6.7 × 1024 molecules s–1, while the impact produced an additional 1.6 × 1032 H2O molecules and 1.3 × 1029 CS2 molecules, a similar ratio as in quiescent outgassing.
The Astrophysical Journal 02/2010; 711(2):1051. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: GALEX observations of comet 9P/Tempel 1 using the near ultraviolet (NUV) objective grism were made before, during and after the Deep Impact event that occurred on 2005 July 4 at 05:52:03 UT when a 370 kg NASA spacecraft was maneuvered into the path of the comet. The NUV channel provides usable spectral information in a bandpass covering 2000 - 3400 A with a point source spectral resolving power of approximately 100. The primary spectral features in this range include solar continuum scattered from cometary dust and emissions from OH and CS molecular bands centered near 3085 and 2575 A, respectively. In particular, we report the only cometary CS emission detected during this event. The observations allow the evolution of these spectral features to be tracked over the period of the encounter. In general, the NUV emissions observed from Tempel 1 are much fainter than those that have been observed by GALEX from other comets. However, it is possible to derive production rates for the parent molecules of the species detected by GALEX in Tempel 1 and to determine the number of these molecules liberated by the impact. The derived quiescent production rates are Q(H2O) = 6.4e27 molecules/s and Q(CS2) = 6.7e24 molecules/s, while the impact produced an additional 1.6e32 H2O molecules and 1.3e29 CS2 molecules, a similar ratio as in quiescent outgassing. Comment: 15 pages, 4 figures, accepted for publication in the Astrophysical Journal
01/2010;
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Mary Elizabeth Kaiser,
Jeffrey W. Kruk,
Stephan R. McCandliss,
David J. Sahnow,
Robert H. Barkhouser,
W. Van Dixon, Paul D. Feldman,
H. Warren Moos,
Joseph Orndorff,
Russell Pelton, [......],
Dominic J. Benford,
Jonathan P. Gardner,
Robert J. Hill,
Bruce E. Woodgate,
Ralph C. Bohlin,
Susana E. Deustua,
Robert Kurucz,
Michael Lampton,
Saul Perlmutter,
Edward L. Wright
[show abstract]
[hide abstract]
ABSTRACT: Improvements in the precision of the astrophysical flux scale are needed to answer fundamental scientific questions ranging from cosmology to stellar physics. The unexpected discovery that the expansion of the universe is accelerating was based upon the measurement of astrophysical standard candles that appeared fainter than expected. To characterize the underlying physical mechanism of the "Dark Energy" responsible for this phenomenon requires an improvement in the visible-NIR flux calibration of astrophysical sources to 1% precision. These improvements will also enable large surveys of white dwarf stars, e.g. GAIA, to advance stellar astrophysics by testing and providing constraints for the mass-radius relationship of these stars. ACCESS (Absolute Color Calibration Experiment for Standard Stars) is a rocket-borne payload that will enable the transfer of absolute laboratory detector standards from NIST to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of R = 500 across the 0.35-1.7 micron bandpass. Among the strategies being employed to minimize calibration uncertainties are: (1) judicious selection of standard stars (previous calibration heritage, minimal spectral features, robust stellar atmosphere models), (2) execution of observations above the Earth's atmosphere (eliminates atmospheric contamination of the stellar spectrum), (3) a single optical path and detector (to minimize visible to NIR cross-calibration uncertainties), (4) establishment of an a priori error budget, (5) on-board monitoring of instrument performance, and (6) fitting stellar atmosphere models to the data to search for discrepancies and confirm performance. Comment: 14 pages, 13 figures
01/2010;
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[show abstract]
[hide abstract]
ABSTRACT: Observations of four comets made with the Far Ultraviolet Spectroscopic
Explorer show the rotational envelope of the (0,0) band of the CO
Hopfield-Birge system (C - X) at 1088 A to consist of both "cold" and "hot"
components, the "cold" component accounting for ~75% of the flux and with a
rotational temperature in the range 55-75 K. We identify the "hot" component as
coming from the dissociation of CO2 into rotationally "hot" CO, with electron
impact dissociation probably dominant over photodissociation near the nucleus.
An additional weak, broad satellite band is seen centered near the position of
the P(40) line that we attribute to CO fluorescence from a non-thermal high J
rotational population produced by photodissociation of formaldehyde into CO and
H2. This process also leaves the H2 preferentially populated in excited
vibrational levels which are identified by fluorescent H2 lines in the spectrum
excited by solar OVI 1031.9 and solar Lyman-alpha. The amount of H2 produced by
H2CO dissociation is comparable to the amount produced by photodissociation of
H2O. Electron impact excitation of CO, rather than resonance fluorescence,
appears to be the primary source of the observed (B - X) (0,0) band at 1151 A.
05/2009; 38.
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[show abstract]
[hide abstract]
ABSTRACT: During the 1995 March flight of the Hopkins Ultraviolet Telescope, as part of the Astro-2 Space Shuttle mission, we observed the Moon over the wavelength range 820-1840 Å, with a resolution of 3-4 Å, finding the ultraviolet albedo of the Moon to be 0.038 ± 0.0038 at 1700 Å. We also find a gentle increase in the albedo toward shorter wavelengths. Ultraviolet albedo measurements may represent a useful tool for selenographic exploration.
The Astrophysical Journal 01/2009; 454(1):L69. · 6.02 Impact Factor
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[hide abstract]
ABSTRACT: We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space Telescope and compare them to similar phenomena on Earth. We find that the tenuous nature of Ganymede's atmosphere precludes excitation of the aurora by high-energy electrons and requires a local acceleration mechanism. We propose the following as plausible mechanisms for generating both the continuous background emission and the intense auroral bright spots:
The Astrophysical Journal 12/2008; 555(2):1013. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Hubble Space Telescope observations of Callisto with the Space Telescope Imaging Spectrograph were performed at both eastern and western elongations to search for the UV emissions diagnostic of the presence of O2, CO2, and/or CO atmospheres. We report upper limits of 5 × 10-5 photons cm-2 s-1 or 15 R for a uniform disk the diameter of Callisto on emissions of O I λ1304, O I λ1356, C I λ1561, C II λ1335, and CO fourth positive bands. These upper limits yield upper bounds on O2, CO2, and CO atmospheres far in excess of the detected CO2 atmosphere by R. W. Carlson. Our results are interpreted in terms of a strong electrodynamic interaction with the Jovian magnetosphere, which drives ~1.5 × 105 A through Callisto's highly conducting (~104 mho) ionosphere and generates a highly reduced ionospheric electric field, severely retarded ionospheric convection (~0.1 km s-1), and a factor of ~1500 reduction in the net electron impact emission rate. Callisto's highly conducting ionosphere renders it the most tenable unipolar inductor of the Galilean satellites.
The Astrophysical Journal 12/2008; 581(1):L51. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report the detection of atomic chlorine emissions in the atmosphere of Io using Hubble Space Telescope observations with the Goddard High Resolution Spectrograph (GHRS). The Cl I λ1349 dipole allowed and Cl I] λ1386 forbidden transition multiplets are detected at a signal-to-noise ratio of 6 and 10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996. Oxygen and sulfur emissions are simultaneously detected with the chlorine, which allows for self-consistent abundance ratios of chlorine to these other atmospheric species. The disk-averaged ratios are Cl/O = 0.017 ± 0.008, Cl/S = 0.10 ± 0.05, and S/O = 0.18 ± 0.08. We also derive a geometric albedo of 1.0% ± 0.4% for Io at 1335 Å, assuming an SO2 atmospheric column density of 1 × 1016 cm-2.
The Astrophysical Journal 12/2008; 610(2):1191. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report new observations of the spectrum of Ganymede in the spectral range 1160-1720 Å made with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) on 1998 October 30. The observations were undertaken to locate the regions of the atomic oxygen emissions at 1304 and 1356 Å, previously observed with the Goddard High Resolution Spectrograph on HST, that Hall et al. claimed indicated the presence of polar aurorae on Ganymede. The use of the 2'' wide STIS slit, slightly wider than the disk diameter of Ganymede, produced objective spectra with images of the two oxygen emissions clearly separated. The O I emissions appear in both hemispheres, at latitudes above |40|°, in accordance with recent Galileo magnetometer data that indicate the presence of an intrinsic magnetic field such that Jovian magnetic field lines are linked to the surface of Ganymede only at high latitudes. Both the brightness and relative north-south intensity of the emissions varied considerably over the four contiguous orbits (5.5 hr) of observation, presumably because of the changing Jovian plasma environment at Ganymede. However, the observed longitudinal nonuniformity in the emission brightness at high latitudes, particularly in the southern hemisphere, and the lack of pronounced limb brightening near the poles are difficult to understand with current models. In addition to observed solar H I Lyα reflected from the disk, extended Lyα emission resonantly scattered from a hydrogen exosphere is detected out to beyond two Ganymede radii from the limb, and its brightness is consistent with the Galileo UVS measurements of Barth et al.
The Astrophysical Journal 12/2008; 535(2):1085. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Observations of comet C/2001 A2 (LINEAR) were made with the Far Ultraviolet Spectroscopic Explorer beginning 2001 July 12.58 and coinciding with a photometric increase of ~1.5 mag. Spectra were obtained in the 905-1180 Å range at 0.25 Å spectral resolution using the 30'' × 30'' aperture. Several new cometary emissions were identified, particularly the (0, 0) bands of the CO Hopfield-Birge systems C-X and B-X at 1088 and 1151 Å, respectively, O I (1D-1D) at 1152 Å, and three lines of the H2 Lyman system at 1071.6, 1118.6, and 1166.8 Å, pumped by solar Lyβ fluorescence. Also detected were O I multiplets at 989, 1027, and 1040 Å and several lines of the H I Lyman series. The rotational envelopes of the CO bands are resolved and appear to consist of both "cold" and "hot" components, the cold component accounting for 70% of the flux and with a rotational temperature of 55 ± 5 K. The hot component may be indicative of a CO2 source. The CO bands, H2 lines, and O I λ1152 all decreased by a factor of 2 over the 7.5 hr observation. The derived time-averaged production rates are Q(CO) = 1.3 × 1027 molecules s-1 and Q(H2O) = 2.1 × 1029 molecules s-1. These values may be uncertain by as much as a factor of 2 because of uncertainties in the solar flux and the electron impact contribution to the excitation.
The Astrophysical Journal 12/2008; 576(1):L91. · 6.02 Impact Factor
