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NASA NTRS1980

The High Visible Resolution (HVR) instrument of the spot ground observation satellite

Two identical high resolution cameras, capable of attaining a track width of 116 km in an almost vertical line of sight from the two 60 km images of each instrument, will be carried on the initial mission of the space observation of Earth satellite (SPOT). Specifications for the instrument, including the telescope and CCD devices are summarized. The present status of development is described including the optical characteristics, structure and thermal control, detector assembly, electronic equipment, and calibration. SPOT mission objectives include the developments relating to soil use, the exploration of EART Earth resources, the discrimination of plant species, and cartography.

Otrio, G.↗
NASA NTRS1988

Physical studies of small asteroids and cometary cores

The main goal of the research is to carry on an extensive study of physical properties (colors and variability) of asteroids in the 1 to 10 km diameter range and of cometary cores, with the use of a charge coupled device (CCD) camera, PMT photometer, or both. Particular attention is paid to asteroids being observed by radar because the greatest gain is found from the combination of radar results with the data obtained by optical techniques. To satisfy the goal, 100 nights/year have been scheduled on the 2.3 m and 1.5 m telescopes. During the past 2 years small asteroids were observed. Out of 22 asteroids for which periods of rotation could be precisely measured, 17 have periods of rotation less than 5 hours. This indicates that small objects rotate faster. By now researchers know 14 asteroids with rotation periods in the 2 hour range. At the same time they confirmed the existence of a number of exceptionally slow rotators e.g., 1367 Nongoma with P = 5.65 days. Taxonomic observations lead to a conclusion that Apollo, Amor, and Aten asteroids represent a variety of classes and are not predominantly of class S. Apollo asteroid 3361 Orpheus was found to belong to the rare class V. In collaboration with Dr A. Harris of JPL the opposition effect was studied for 30 Urania and 64 Angelina. Seven comets: P/Helin, P/Brooks 2, P/Klemola, P/Borrelly, Wilson (1986 1) and Shoemaker (1987 o) were monitored for variability. The results were negative with the exception of P/Brooks 2 for which 0.35 mag amplitude was detected.

Wisniewski, Wieslaw Z.↗
NASA NTRS1993

Optical properties of orbital debris

This paper discusses the albedoes and phase angle effects found for 400 members of eight debris swarms using observations obtained in 1990 and 1991 with the NASA Johnson Space Center CCD-equipped portable Schmidt telescope. The mean albedo derived by comparing optical brightnesses with averaged radar cross sections (RCS's) is 10 percent. The standard deviation in log albedo is 0.40 if the RCS's are assumed to contribute equally to the scatter. The RCS's used in this work are the median values obtained by comparing data from 47 separate RCS catalog updates. Forty objects are sufficiently free of tumble or aspect angle effects to yield consistent phase function slopes. These range from specular in nature (zero slope) through Lambertian (moderate slope) to lunar (steep slope) with the mean lying midway between specular and Lambertian.

Henize, Karl G.↗
NASA NTRS2009

OP09O-OP404-9 Wide Field Camera 3 CCD Quantum Efficiency Hysteresis

The HST/Wide Field Camera (WFC) 3 UV/visible channel CCD detectors have exhibited an unanticipated quantum efficiency hysteresis (QEH) behavior. At the nominal operating temperature of -83C, the QEH feature contrast was typically 0.1-0.2% or less. The behavior was replicated using flight spare detectors. A visible light flat-field (540nm) with a several times full-well signal level can pin the detectors at both optical (600nm) and near-UV (230nm) wavelengths, suppressing the QEH behavior. We are characterizing the timescale for the detectors to become unpinned and developing a protocol for flashing the WFC3 CCDs with the instrument's internal calibration system in flight. The HST/Wide Field Camera 3 UV/visible channel CCD detectors have exhibited an unanticipated quantum efficiency hysteresis (QEH) behavior. The first observed manifestation of QEH was the presence in a small percentage of flat-field images of a bowtie-shaped contrast that spanned the width of each chip. At the nominal operating temperature of -83C, the contrast observed for this feature was typically 0.1-0.2% or less, though at warmer temperatures contrasts up to 5% (at -50C) have been observed. The bowtie morphology was replicated using flight spare detectors in tests at the GSFC Detector Characterization Laboratory by power cycling the detector while cold. Continued investigation revealed that a clearly-related global QE suppression at the approximately 5% level can be produced by cooling the detector in the dark; subsequent flat-field exposures at a constant illumination show asymptotically increasing response. This QE "pinning" can be achieved with a single high signal flat-field or a series of lower signal flats; a visible light (500-580nm) flat-field with a signal level of several hundred thousand electrons per pixel is sufficient for QE pinning at both optical (600nm) and near-UV (230nm) wavelengths. We are characterizing the timescale for the detectors to become unpinned and developing a protocol for flashing the WFC3 CCDs with the instrument's internal calibration system in flight. A preliminary estimate of the decay timescale for one detector is that a drop of 0.1-0.2% occurs over a ten day period, indicating that relatively infrequent cal lamp exposures can mitigate the behavior to extremely low levels.

Collins, Nick↗
NASA NTRS1993

Searching for Lyman alpha emission from a possible Zel'dovich pancake

The detection of 2 x 10(exp 14) solar mass of neutral hydrogen at a redshift of 3.397 has been reported. Top-down theories of structure formation predict such a mass of hydrogen collapsing to form a protocluster of galaxies. We sought to observe this object in Lyman-alpha, which could be produced through ionization by the metagalactic ionizing radiation field or through internal ionization processes. On 29 Apr. 1992, the region of the reported HI emission for 1800 seconds with the 1.3 meter McGraw-Hill reflector at Michigan-Dartmouth-M.I.T. Observatory was observed. Because the H1 emission reported has a transverse scale of 300 sec, a 1/3.06 reducing camera and a Thomson CCD were used to obtain a field of view of about 600 sec by 840 sec. A filter 88 A wide, centered at 5354 A was used; Lyman-alpha emission at z = 3.4 is redshifted to 5347 A. In order to avoid saturating the CCD with a bright star in the field, nine 200 second exposures were taken. The combination of these images shows no obvious extended Lyman-alpha emission at a level of about 28 magnitudes per square arcsecond. The field observed also shows a distant cluster of galaxies. In order to determine if the cluster could in any way be associated with the cloud of neutral hydrogen at z = 3.4, we sought to estimate its redshift from the size and magnitude of the galaxies and of the cluster as a whole. Omega = 1 and H(sub O) = 50 km s(sup -1) Mpc(sup -1) were adopted; our redshift estimates range from z = 0.2 to z = 0.6. The cluster is clearly not associated with the HI cloud at z = 3.4.

Cox, Caroline↗
NASA NTRS1999

Seasonal and Interannual Variabilities in Tropical Tropospheric Ozone

This paper presents a detailed characterization of seasonal and interannual variability in tropical tropospheric column ozone (TCO). TCO time series are derived from 20 years (1979-1998) of total ozone mapping spectrometer (TOMS) data using the convective cloud differential (CCD) method. Our study identifies three regions in the tropics with distinctly different zonal characteristics related to seasonal and interannual variability. These three regions are the eastern Pacific, Atlantic, and western Pacific. Results show that in both the eastern and western Pacific seasonal-cycle variability of northern hemisphere (NH) TCO exhibits maximum amount during NH spring whereas largest amount in southern hemisphere (SH) TCO occurs during SH spring. In the Atlantic, maximum TCO in both hemispheres occurs in SH spring. These seasonal cycles are shown to be comparable to seasonal cycles present in ground-based ozonesonde measurements. Interannual variability in the Atlantic region indicates a quasi-biennial oscillation (QBO) signal that is out of phase with the QBO present in stratospheric column ozone (SCO). This is consistent with high pollution and high concentrations of mid-to-upper tropospheric O3-producing precursors in this region. The out of phase relation suggests a UV modulation of tropospheric photochemistry caused by the QBO in stratospheric O3. During El Nino events there is anomalously low TCO in the eastern Pacific and high values in the western Pacific, indicating the effects of convectively-driven transport of low-value boundary layer O3 (reducing TCO) and O3 precursors including H2O and OH. A simplified technique is proposed to derive high-resolution maps of TCO in the tropics even in the absence of tropopause-level clouds. This promising approach requires only total ozone gridded measurements and utilizes the small variability observed in TCO near the dateline. This technique has an advantage compared to the CCD method because the latter requires high-resolution footprint measurements of both reflectivity and total ozone in the presence of tropopause-level cloud tops.

Ziemke, J. R.↗
NASA NTRS2005

XMM-Newton Observations of GX 339-4

Observations of GX 339-4 were completed in March, 2004, but processed data were only sent to the PI in the late fall of 2004. Thus, serious work on the observations only began in the winter and spring of 2005. Data from each of the instruments - the EPIC CCD cameras, the Reflection Grating Spectrometer, and the Optical Monitor have been reduced and a preliminary analysis is underway. Four days of simultaneous optical radio, and X-ray data was obtained at SAAO, the Australia Telescope, and RXTE, respectively, and preliminary work on the broad-band multi-wavelength lightcurves is also underway. This observational effort is the most extensive ever undertaken to observe a black hole in the "low/hard" state, and will provide new insights into accretion onto black holes at low mass accretion rates.

Mushotzky, Richard F.↗
NASA NTRS1986

The ionospheric extent of P/Halley from ground based operations during the 1985-1986 apparition

Spatial profiles of H2O(+) emission in the inner coma of P/Halley on 8 dates during the 1985 to 1986 apparition are presented. The profiles are derived from two dimensional CCD spectrograms taken with the spectrograph slit oriented along the cometary radius vector. The observations cover the range from 16 Oct. 1985 (r = 2.14 AU preperihelion) to 6 June 1986 (r = 2.14 AU postperihelion). The surface brightness profile of the 6198A H2O(+) emission line is calibrated in terms of the column density of H2O(+) ions. The profile from 14.4 Mar. 1986 is compared with the Giotto Ion Composition Experiment measurement of the shape of the H2O(+) space density profile. The ionosphere becomes larger and steeper in the sunward direction with an increasing water production rate.

Mccarthy, Patrick J.↗
NASA NTRS1988

Observations of F- and P-Mode Oscillations of High Degree (500 Is Less Than L Is Less Than 2500) in Quiet and Active Sun

Spectra (l - v diagrams) from high resolution observations taken at the Vacuum Tower Telescope (NSO/Sunspot) are presented. The raw data are CCD images taken through the SOUP narrowband filter in Fe I 5576 A. Four filtergrams spaced through the spectral line are combined to form velocity movies. Spectra for 80 min of data with 0.5 to 1.5 arcsec resolution are presented for the entire field-of-view and for quiet and magnetic (plage) subregions. Ridges f and p1 to p5 are evident in velocity spectra, extending to l = 2500 (f), l = 1800 (p1), and l = 1200 (p2). Much less power is seen in the magnetic region than in the quiet Sun. Three-dimensional Fourier filtering shows that oscillation velocity amplitude drops sharply at the boundary of the active region for each family of modes considered.

Tarbell, T. D.↗
NASA NTRS1990

Camcorders in Space Shuttle earth observations

A project to evaluate the use of commercially available camcorder systems during Shuttle flights is discussed, focusing on the use of an 8-mm camcorder for earth observations during the STS-30 mission in May, 1989. The camcorder with a 2/3-inch CCD is described, noting the modifications to prepare the camcorder for use on the Shuttle. The results of the camcorder project are summarized, listing the types of earth-viewing video images that were aquired with the camcorder.

Lulla, Kamlesh↗
NASA NTRS1990

An atlas of mixed-morphology pairs

Current theories of galaxy formation imply that environment is the most important factor in deciding whether a collapsing region becomes a spiral galaxy or an elliptical galaxy. If this is the case, then isolated pairs comprising a galaxy of each morphology provide an especially interesting test, since, strictly speaking, they should not exist. The first phase of studying E-S pairs is to define a sample which is genuinely isolated and genuinely E-S. The catalog compiled by Karachentsev is the most useful Northern Hemisphere starting point, containing 603 pairs which obeyed certain well-defined angular isolation criteria. All of these pairs now have velocities, enabling further removal of at least some of the non-physical binaries. The conventional cutoff at 500 km s(exp -1) was used for this purpose. However, the morphological types were estimated from Palomar Sky Survey prints, and are consequently not very accurate. It was therefore decided to obtain good charge coupled device (CCD) images in the B-band of all pairs from the Karachentsev sample which were classified as either E-S (73 pairs) or E-SO (91 pairs), for a total of 164 pairs. This includes the 24 pairs with known redshift differences greater than 500 km s(exp -1) (9 E-S and 15 E-SO), which were also imaged in this survey, not only for completeness but also in case the criteria were later changed, or the redshifts were found to be in error. A total of 198 CCD frames were obtained between February 1986 and the present, with most of the observations being completed by the summer of 1987. A number of missing stragglers and replacements for poor quality images were taken more recently. Most observations used the number 1-0.9m telescope at Kitt Peak, with a few frames from the 2.1m. Some pairs required more than one image due to their separation. The morphological study did not require photometric conditions, but calibrations were made for almost all images anyway, and the weather was probably photometric at least half of the time, at least to an accuracy of around 0.1 magnitudes. The results of this first phase will be published in the form of an Atlas, from which any interested readers will be able to select their own sub-sample using their preferred morphological criteria.

Sharp, N. A.↗
NASA NTRS1992

Production rates for comet P/Temple 2 from long slit CCD spectroscopy

Since comet P/Temple-2 is one of the potential targets for the CRAF (Comet Rendezvous Asteroid Flyby) mission, we obtained long slit spectroscopic data with our CCD spectrograph during its 1988 apparition. As the same spectrograph was extensively used for observations of P/Halley, this allowed a direct spectroscopic comparison between the two objects. Furthermore we could choose a P/Halley spectrum which was taken at a heliocentric distance very close to that of P/Temple-2. Finally, we could adjust the integration windows along the slit to compensate for the different geocentric distances, so that roughly the same projected distance of the comets' comae was observed. The parameters for our observations are given.

Fink, Uwe↗
NASA NTRS1993

Distribution and geologic history of materials excavated by the lunar crater Bullialdus

The crater Bullialdus is a 61 km, Eratosthenian-age impact crater located on the western edge of Mare Nubium. Previous analysis of the spatial distribution of materials in the area using nine telescopic near-infrared spectra suggested a possible three-layer structure prior to the impact event: two shallow gabbroic layers and one deeper noritic layer (from a potential depth of 5.5 km). The initial interpretation of this stratigraphy was that Bullialdus may have tapped a layered mafic pluton, such as have been invoked to explain the existence of Mg-suite rocks. High-spatial resolution CCD images of Bullialdus were analyzed to better map the spatial distribution of the observed lithologies, and to assess the plausibility of the pluton interpretation.

Tompkins, Stefanie↗
NASA NTRS1994

Optical observations of NGC 2915: A nearby blue compact dwarf galaxy

This paper presents B and R band Charge Coupled Device (CCD) images and medium resolution spectroscopy of NGC 2915, a relatively isolated BCD (blue compact dwarf) galaxy at a distance of approximately 5 Mpc. NGC 2915 contains two stellar populations: a high surface brightness blue core population and a red diffuse population. The core population contains all of the H II, and numerous embedded objects. It is the locus of current high mass star formation. The brightest embedded objects are likely to be young ionizing clusters, while many of the fainter objects are likely to be individual supergiant stars with masses up to approximately 25 solar mass, or blends of a few such stars. Curious aligned structures on the SE side of the galaxy are seen and their nature discussed. The spectrum of the core is dominated by bright narrow emission lines like that of a high excitation and low metallicity (less than half solar) H II region. The continuum is flat, with Balmer and Ca II features seen in absorption. The velocity of the Ca II features suggest contamination by galactic interstellar absorption. There is a significant velocity gradient in the spectra, probably indicative of rotation. Outside of its core, NGC 2915 resembles a dE (dwarf elliptical) galaxy, in that it has an exponential surface brightness profile, is red ((B-R)(sub 0) = 1.65), and has a low extrapolated central surface brightness (B(0)(sub c) = 22.44). NGC 2915's properties are compared with other BCDs, concentrating on two morphologically similar BCDs that are near enough to resolve into stars: NGC 1705 and NGC 5253. It is noted that the presence of winds in BCDs invalidates closed box chemical evolution models and the remaining constraints on star formation duration are relatively weak. Some BCDs, including NGC 2915, may be able to maintain their present star formation rate for Gyr time scales. This suggests that the overall evolution of these BCDs may be much slower than the approximately 10 Myr burst time scales commonly quoted. However, shortly after the formation of a massive (10(exp 6) solar mass) cluster a BCD will have all the properties of strong starburst galaxy).

Meurer, G. R.↗
NASA NTRS2002

A Chandra X-ray Study of Cygnus A: The Nucleus - 2

We report Chandra Advanced CCD Imaging Spectrometer and quasi-simultaneous Rossi X-Ray Timing Explorer (RXTE) observations of the nearby, powerful radio galaxy Cygnus A, with the present paper focusing on the properties of the active nucleus. In the Chandra observation, the hard (less than a few keV) X-ray emission is spatially unresolved with a size is approximately 1" (1.5 kpc, H(sub 0) = 50 km/s/Mpc) and coincides with the radio and near-infrared nuclei. In contrast, the soft (less than 2 keV) emission exhibits a bipolar nebulosity that aligns with the optical bipolar continuum and emission-line structures and approximately with the radio jet. In particular, the soft X-ray emission corresponds very well with the [O III] (lambda)5007 and H(alpha) + [N II] lambda(lambda)6548, 6583 nebulosity imaged with Hubble Space Telescope. At the location of the nucleus, there is only weak soft X-ray emission, an effect that may be intrinsic or result from a dust lane that crosses the nucleus perpendicular to the source axis. The spectra of the various X-ray components have been obtained by simultaneous fits to the six detectors. The compact nucleus is detected to 100 keV and is well described by a heavily absorbed power-law spectrum with Gamma(sub h) = 1.52(sup + 0.12, sub -0.12) (similar to other 0.12 narrow-line radio galaxies) and equivalent hydrogen column N(sub H)(nuc) = 2.0(sup +0.1, sub -0.1) x 10(exp 23)/sq cm. This 0.2 column is compatible with the dust obscuration to the near-infrared source for a normal gas-to-dust ratio. The soft (less than 2 keV) emission from the nucleus may be described by a power-law spectrum with the same index (i.e., Gamma(sub l) = Gamma(sub h), although direct fits suggest a slightly larger value for Gamma(sub l). Narrow emission lines from highly ionized neon and silicon, as well as a "neutral" Fe K(alpha) line, are detected in the nucleus and its vicinity (r approximately less than 2 kpc). The equivalent width (EW) of the Fe K(alpha) line (182(sup +40, sub -54) eV) is in good agreement with theoretical predictions for the EW versus N(sub H)(nuc) relationship in various geometries. An Fe K edge is also seen. The RXTE observations indicate a temperature of kT = 6.9(sup +0., sub -1.0) keV for the cluster gas (discussed in Paper III of this series) and cluster emission lines of Fe K(alpha) and Fe K(beta) and/or Ni K(alpha). We consider the possibility that the extended soft X-ray emission is electron-scattered nuclear radiation. Given that 1% of the unabsorbed 2 - 10 keV nuclear radiation would have to be scattered, the necessary gas column [N(sub H)(Scattering) approx. = 3.5 x 10(exp 22)/sq cm] would absorb the X-rays rather than scatter them if the gas is cold. Thus, the scattering plasma must be highly ionized. If this ionization is achieved through photoionization by the nucleus, the ionization parameter zeta greater than 1 ergs cm/s and the electron density n(sub e) approx. = 6 cc given the observed distance of the soft X-ray emission from the nucleus. The electron column density inferred from the X-ray observations is much too low to account for the extended optical scattered light, strongly suggesting that the polarized optical light is scattered by dust. The presence of highly ionized Ne lines in the soft X-ray spectrum requires 20 ergs cm/s approximately less than zeta approximately less than 300 ergs cm/s these lines may originate closer to the nucleus than the extended soft continuum or in a lower density gas. A collisionally ionized thermal model of the extended soft X-rays cannot be ruled out but is unattractive in view of the low metal abundance required (Z = 0.03 Z(mass)). The hard X-ray to far-infrared ratio for the nucleus of Cygnus A is similar to that seen in Seyfert 1 and unobscured radio galaxies. By means of the correlation between hard X-ray luminosity and nuclear optical absolute magnitude for these classes of object, we estimate M(sub B) = -22.4 for Cygnus A, near the .borderline between Seyfert galaxies and QSOs.

Young, Andrew J.↗
NASA NTRS1992

Numerical simulations of cometary dust

Most observations of comets are done photometrically or spectrophotometrically. The interpretation of the aperture-averaged flux is relatively simple for an isotropic, radially expanding coma of infinite extent - the canonical model. However, the interpretation of the observations is not so clear when the motion of the dust is affected by radiation pressure, or when the emission is time-varying and anisotropic. For example, in a sample of CCD images of 10 comets, Jewitt and Meech (1987, Ap.J. 317, 992) found that the photometric profiles of only three comets were consistent, within the observational errors, with the profiles predicted form the canonical model. Photometric observations with large apertures, however, seem to suggest that the canonical model may be quite adequate (c.f. Osip, Schleicher, and Millis, 1992, Icarus 98, 115). The dust itself is characterized by a size distribution, with size dependencies on the expansion velocity, the scattered and thermal radiation, the response to radiation pressure, and probably the density. How good then are the approximations normally used in determining the production rates of the dust when these effects are present? As part of a program to better understand the dynamics of cometary dust and gas, a computer program has been developed which numerically simulates the emission of both dust and gas from a tilted rotating nucleus.

Lien, David J.↗
NASA NTRS2009

An Assessment of GEO Orbital Debris Photometric Properties Derived from Laboratory-Based Measurements

Optical observations of orbital debris offer insights that differ from radar measurements (specifically the size parameter and wavelength regime). For example, time-dependent photometric data yield lightcurves in multiple bandpasses that aid in material identification and possible periodic orientations. This data can also be used to help identify shapes and optical properties at multiple phase angles. Capitalizing on optical data products and applying them to generate a more complete understanding of orbital space objects, is a key objective of NASA s Optical Measurement Program, and a primary driver for creation of the Optical Measurements Center (OMC). The OMC attempts to emulate space-based illumination conditions using equipment and techniques that parallel telescopic observations and source-target-sensor orientations. The OMC uses a 300 Watt Xenon arc lamp as a solar simulator, a CCD camera with Johnson/Bessel colored filters, and a robotic arm to orientate/rotate objects to simulate an object's orbit/rotational period. A high-resolution, high bandwidth (350nm-2500nm) Analytical Spectral Devices (ASD) spectrometer is also employed to baseline various material types. Since observation of GEO targets are generally restricted to the optical regime (due to radar range limitations), analysis of their properties is tailored to those revealed by optical data products. In this connection, much attention has been directed towards understanding the lightcurves of orbital debris with high area-to-mass (A/m) ratios (greater than 0.9 square meters per kilogram). A small population of GEO debris was recently identified, which exhibits the properties of high A/m objects, such as variable eccentricities and inclinations a dynamical characteristic generally resulting from varying solar radiation pressure on high A/m objects. Materials such as multi-layered insulation (MLI) and solar panels are two examples of materials with high area-to mass ratios. Lightcurves for such objects can vary greatly (even for the same object under different illumination conditions). For example, specular reflections from multiple facets of the target surface (e.g. Mylar or Aluminized Kapton) can lead to erratic, orientation-dependent light curves. This paper will investigate published color photometric data for a series of orbital debris targets and compare it to the empirical photometric measurements generated in the OMC. The specific materials investigated (known to exist in GEO) are: an intact piece of MLI, separated layers of MLI, and multiple solar cells materials. Using the data acquired over specific rotational angles through different filters (B, V, R, I), a color index is acquired (B-R, R-I). As a secondary check, the spectrometer is used to define color indexes for the same material. Using these values and their associated lightcurves, this laboratory data is compared to observational data obtained on the 1m telescope of the Astronomical Institute of the University of Bern (AUIB) and the 0.9 m Small and Moderate Aperture Research Telescope System (SMARTS) telescope at Cerro Tololo Inter-American Observatory (CTIO). We will present laboratory generated lightcurves with color indexes of the high A/m materials alongside telescopic data of targets with high A/m values. We will discuss the relationship of laboratory to telescope data in the context of classification of GEO debris objects.

Rodriquez-Cowardin, H.↗
NASA NTRS2001

Discovery of Soft X-Ray Emission from Io, Europa and the Io Plasma Torus

The Chandra X-ray Observatory observed the Jovian system for about 24 hours on 25-26 Nov 1999 with the Advanced CCD Imaging Spectrometer (ACIS), in support of the Galileo flyby of Io, and for about 10 hours on 18 Dec 2000 with the imaging array of the High Resolution Camera (HRC-I), in support of the Cassini flyby of Jupiter. Analysis of these data have revealed soft (0.25--2 keV) x-ray emission from the moons Io and Europa, probably Ganymede, and from the Io Plasma Torus (IPT). Bombardment by energetic (greater than 10 keV) H, O, and S ions from the region of the IPT seems the likely source of the x-ray emission from the Galilean moons. According to our estimates, fluorescent x-ray emission excited by solar x-rays is about an order of magnitude too weak even during flares from the active Sun to account for the observed x-ray flux from the IPT. Charge-exchange processes, previously invoked to explain Jupiter's x-ray aurora and cometary x-ray emission, and ion stripping by dust grains both fall by orders of magnitude. On the other hand, we calculate that bremsstrahlung emission of soft X-rays from non-thermal electrons in the few hundred to few thousand eV range accounts for roughly one third of the observed x-ray flux from the IPT. Extension of the far ultraviolet (FUV) IPT spectrum likely also contributes.

Elsner, R. F.↗