Final Report for Sounding Rocket
This paper presents a final report on sounding rocket flight activity. The period of performance is from 11 February 1997 to 31 January 2000.
Clarke, John T.↗
Engineering topics
Clarke, John T.
Publications and source records attributed to Clarke, John T..
Analysis of Midlatitude Auroral Emissions Observed During the Impact of Comet Shoemaker-Levy 9 with Jupiter
During the impact of Comet Shoemaker-Levy 9 fragment K on Jupiter observers detected aurora-like emissions near the impact region as well as in the other hemisphere at approximately magnetic conjugate positions equatorward of aurorae latitudes. A number of generation mechanisms were suggested, but investigations of their significance have been hampered by a lack of knowledge about the jovian internal magnetic field, the exact timing, and the geometry of the impact and emission sites. We use the VIP 4 model of the internal magnetic field, high-time-resolution calculations of the fragment K trajectory, and images from the Hubble Space Telescope Wide Field Planetary Camera 2 with advanced processing to reanalyze the relationship between these emissions. The impact location is enclosed to the north and south by two regions of enhanced far-ultraviolet emissions reaching a maximum distance of 18,000 km south of the impact site roughly along the line of the incoming fragment's trajectory. The southern region can be further divided into two subregions, which partly overlap with magnetic projections of two brighter emission regions observed in the northern hemisphere close to the line of footprints of Amalthea. The area of the southern region approximates the area of these projections. No enhanced emissions are found conjugate to the impact site and the northward emission region. The magnetic projections suggest that the Gossamer ring scattered particles coming from the region southward of the impact site and prevented precipitation from the northward region into the northern hemisphere. Particle acceleration by upward accelerating shocks seems feasible to explain the geometry of the southern and northern hemispheric emission regions if we assume that a part of the plume bounced twice and provided enough energy at its second bounce to also generate shock waves.
Bauske, Rainer↗
Sounding Rocket
This project has resulted in 4 launches from the White Sands Missile Range of our payload in different stages of development, on: (1) 4 May 1991 Flight 36.062; (2) 16 June 1993 Flight 36.101; (3) 1 April 1995 Flight 36.104; and (5) 28 October 1996 Flight 36.149. For each flight, the following activities were accomplished: (1) prepare the experiment, including replacement and upgrading of critical components, at the University of Michigan; (2) integrate the payload either at the Wallops Flight Facility; (3) perform final far-UV calibration of instrument; (4) perform final alignment, integration, and electrical checks at WSMR (5) launch payload from WSMR (6) check condition of recovered payload (7) perform post-flight calibration, if necessary and applicable; and (8) reduce and analyze flight data. A description of the instrument is presented, along with overviews of the results of the four flights. We are presently preparing the payload for the next flight in 1998/99.
Clarke, John T.↗
Detection of the Tip of Red Giant Branc in NGC 5128
We present a color-magnitude diagram of more than 10,000 stars in the halo of galaxy NGC 5128 (Centaurus A), based on WFPC2 images through the V and I filters. The position of the red-giant branch stars is compared with the loci of the RGB in six well-studied globular clusters and in the dwarf elliptical galaxy NGC 185;...
NGC 185 galaxy NGC 5128 Centaurus A WFPC2↗
Line Profile of H Lyman (alpha) from Dissociative Excitation of H2 with Application to Jupiter
Observations of the H Lyman(alpha) (Ly-alpha) emission from Jupiter have shown pronounced emissions, exceeding solar fluorescence, in the polar aurora and equatorial "bulge" regions. The H Ly-alpha line profiles from these regions are broader than expected, indicating high-energy processes producing fast atoms as determined from the observed Doppler broadening. Toward understanding that process a high-resolution ultraviolet (UV) spectrometer was employed for the first measurement of the H Ly-alpha emission Doppler profile from dissociative excitation of H2 by electron impact. Analysis of the deconvolved line profile reveals the existence of a narrow central peak of 40 +/- 4 mA full width at half maximum and a broad pedestal base about 240 mA wide. Two distinct dissociation mechanisms account for this Doppler structure. Slow H(2p) atoms characterized by a distribution function with peak energy near 80 meV produce the peak profile, which is nearly independent of the electron impact energy. Slow H(2p) atoms arise from direct dissociation and predissociation of singly excited states which have a dissociation limit of 14.68 eV. The wings of H Ly-alpha arise from dissociative excitation of a series of doubly excited states which cross the Franck-Condon region between 23 and 40 eV. The profile of the wings is dependent on the electron impact energy, and the distribution function of fast H(2p) atoms is therefore dependent on the electron impact energy. The fast atom kinetic energy distribution at 100 eV electron impact energy spans the energy range from 1 to 10 eV with a peak near 4 eV. For impact energies above 23 eV the fast atoms contribute to a slightly asymmetric structure of the line profile. The absolute cross sections of the H Ly-alpha line peak and wings were measured over the range from 0 to 200 eV. Analytic model coefficients are given for the measured cross sections which can be applied to planetary atmosphere auroral and dayglow calculations. The dissociative excitation process, while one contributing process, appears insufficient by itself to explain the line broadening observed at Jupiter.
Ajello, Joseph M.↗
The performance and calibration of WFPC2 on the Huble Space Telescope
The Wide Field Planetary Camera 2 (WFPC2) was installed in the Hubble Space Telescope (HST) in 1993 December. Since then, the instrument has been providing high-quality images. A significant among of calibration data has been collected to aid in the understanding of the on-orbit performance of the instrument. Generally, the behavior of the camera is similar to its performance during the system-level thermal vacuum test at JPL in 1993 May. Surprises were a significant charge-transfer-efficiency (CTE) problem and a significant growth rate in hot pixels at the original operating temperature of the CCDs (-76 deg C). The operating temperature of the WFPC2 CCDs was changed to -88 deg C on 1994 April 23, and significant improvements in CTE and hot pixels are seen at this temperature. In this paper we describe the on-orbit performance of the WFPC2. We discuss the optical and thermal history, the instrument throughput and stability, the Point Spread Function (PSF), the effects of undersampling on photometry, the properties of cosmic rays observed on-orbit, and the geometric distortion in the camera. We present the best techniques for the reduction of WFPC2 data, and describe the construction of calibration products including superbiases, superdarks, and flat fields.
Holtzman, Jon↗
The on-orbit performance of WFPC2
The second Wide Field and Planetary Camera (WFPC2) was successfully installed in the Hubble Space Telescope during the STS-61 servicing mission in 1993 December. The primary objective of this new camera is to provide diffraction-limited photometric imaging over a wide field and a spectral range from 0.12 to 1.0 micrometer. Here we provide an overview of the characteristics of the new instrument and offer our perspectives based on the first 6 months of operations on-orbit.
Trauger, John T.↗
Far-ultraviolet imaging of the globular cluster NGC 6681 with WFPC2
We have imagined the globular cluster NGC 6681 in the far-UV and visible with Wide Field/Planetary Camera-2 (WFPC2) on the Hubble Space Telescope (HST). Our far-UV images show a sparsely populated and fully resolved central region, and we detect 122 stars. The far-UV to visible color-magnitude diagram shows a well-defined horizontal branch with no evidence for hot, more evolved descendants. We find one hot horizontal-branch star significantly below the model zero-age horizontal branch, but the rest are consistent with evolutionary models within uncertainties in calibration, distance, and reddening. The center of the cluster harbors two luminous blue stragglers. Our far-UV images graphically confirm that there is no steep density gradient at small radii among the horizontal-branch stars of this post-core-collapse cluster and show no evidence for significant color gradients.
Watson, Alan M.↗
Hubble Space Telescope Goddard high-resolution spectrograph H2 rotational spectra of Jupiter's aurora
We have observed the emission spectrum from Jupiter's north auroral atmosphere with 0.57 A spectral resolution over 1204-1241 A. Bright emissions have been detected from 50 deg to 60 deg latitude at locations consistent with 6 to 30 R (sub J) auroral ovals, with much fainter emissions away form the auroral ovals. The emission spectrum is well fitted by both laboratory spectra and theoretical models of optically thin electron excited H2, with added Doppler-broadened Lyman Alpha emission. The observed Lyman Alpha emission wings extend more than 1 A from line center and appear correlated in strength with the H2 brightness. Individual rotational lines in the H2 Werner band system are resolved, allowing a determination of the H2 rotational temperature at the altitude of the emission. We derive best-fit temperatures from 400-450 to 700-750 K, with the auroral emission layer temperature changing either across the auroral oval or over several days' time. These observations demonstrate for the first time the ability to measure the observed rapid H2 temperature variations across Jupiter's auroral atmosphere.
Clarke, John T.↗
Hubble Space Telescope UV spectral observations of Io passing into eclipse
Time-resolved spectra of Io have been obtained with the Faint Object Spectrograph on the Hubble Space Telescope in January 1992 at times centered on the passage of Io into Jupiter's shadow. Two different eclipse observations covered 1100-1600 A and 2250-3300 A. In the far ultraviolet(far-UV) range, emission lines of atomic sulfur and oxygen from Io's atmosphere (similar to those previously detected with the International Ultraviolet Explorer (IUE)) have been observed from Io in sunlight, and the spatial extent of the emitting region has been resolved for the first time: this is 0.5-1 Io radii (R(sub Io)) above the surface. The emission lines are typically 1kR in brightness while Io is in sunlight, and decrease to a few hundred Rayleighs within 20 min or less of Io's passing into shadow. If the emissions are produced in Io's ionosphere, the decrease in shadow appears consistent with the collisional slowing and recombination of photoelectrons in 100-1000 s, with recombination an important quenching process if the dominant ion is molecular (i.e., SO2(+)) condensation, with the residual emission in shadow due either to plasma impact of gas above the hot volcanic calderas or electron impact on S and O. In the near-UV range, we have not detected any airglow emissions from Io's atmosphere in shadow, with the main limitation being a high level of scattered light from Jupiter. We derive a 3 sigma upper limit to the 2560 A SO emission feature of 1 KR, which is close to what is expected from electron impact on SO2 based on the obs erved brightness of the FUV S and O lines in shadow. A high signal-to-noise spectrum of Io's albedo in sunlight reveals a spectral shape similar to laboratory spectra of SO2 frost reflectivity, and the relative albedo spectrum changed as Io passed into eclipse and part of the disk was in shadow. No specific SO2 gas absorption features appear in the albedo spectrum, although there could be substantial gas absorption near 2800 A if the individual lines are narrow and saturated. Finally, we present preliminary models for the near-UV spectrum of Io as functions of SO2 frost areal coverage and SO2 gas density.
Clarke, John T.↗
The Lyman alpha bulge of Jupiter - Effects of non-thermal velocity field
We outline for the first time the effect of such nonthermal line broadening processes as turbulence, random waves, convection, etc., on the shape and intensity of the H Ly-alpha line resonance scattered from the atmosphere of Jupiter. We show how a nonthermal velocity field confined to the bulge region, in the upper atmosphere of Jupiter, may account for most of the H Ly-alpha bulge features. Both the shape and the brightness of the Ly-alpha line from the bulge region as reported by the IUE instrument and the Voyager UV Spectrometer can be recovered assuming resonant scattering with a total atomic hydrogen of about 4 x 10 exp 17/sq cm, and a nonthermal component H of about 2 x 10 exp 15/sq cm above the thermopause.
Ben Jaffel, Lotfi↗
H I Lyman alpha emission from Saturn (1980-1990)
The characteristics of the H I Ly alpha dayglow and auroral emissions from Saturn are investigated by analyzing observations made aboard the IUE satellite on Saturn over the 1980-1990 decade (covering a complete solar activity cycle). It was found that, in this period, the dayglow emission from Saturn had significant long-term variability, with decreasing disk brightness between 1980 and 1984 and increasing brightness, as well as increased short-term (days to months) variability, between 1987 and 1990. Both the long-term and short-term variability in the Saturn Ly alpha dayglow were strongly correlated with several different solar indices. Neither the dayglow nor the auroral emission exhibited any dependence on magnetic longitude. Attempts to observe auroral activity associated with a major solar flare which produced unprecedented geomagnetic activity in March 1989 were not successful.
Mcgrath, Melissa A.↗
Variability in the outer planet aurorae
Multiwavelength observations of the aurorae of the outer planets are reviewed emphasizing the findings on physical processes derived from specific wavelengths. The review examines features of the auroral zones such as ionospheric currents, atmospheric heating, and compositional changes in the aurorae of Jupiter, Saturn, Uranus, Neptune, and the earth for comparison. Jupiter's multiwavelength aurora receives special attention since recent observations shed light on the distribution of the UV auroral ovals, the spectroscopy of the UV auroral emissions, auroral dynamics and ion upwelling, and IR emission from auroral latitudes. The observational data on Jupiter facilitate the modeling of variability and detailed thermospheric and magnetospheric processes. Saturn can be studied by extending findings fron Jupiter's aurora, and deficiencies are found in the observational datasets for Neptune and Uranus.
Clarke, John T.↗
Pluto's extended atmosphere - An escape model and initial observations
The observational determination of the extent, escape rate, and composition of Pluto's upper atmosphere is presently approached via calculations of the rates of production and hydrodynamic outflow of atomic H generated by methane photodissociation. Observations have been conducted with the IUE in order to detect atomic H's Lyman-alpha emission in the conjectured cloud; these results are used to derive cloud-property upper limits as a function of the extent of the cloud. It is concluded that the extent and density of the hydrogen atmosphere is more dependent on the escape process than on the atmospheric fraction of methane.
Clarke, John T.↗
Doppler shifted H Ly alpha emission from Jupiter's aurora
High-spectral-resolution IUE observations of the aurora on Jupiter were obtained in order to search for Doppler shifted H-Ly-alpha emission produced through charge exchange by fast precipitating protons. Although no emission has been observed corresponding to proton energies greater than 200 eV, a large fraction of the H-Ly-alpha emission has appeared Doppler shifted, mainly toward the blue, by about 50 km/s. These results show that the acceleration of ionospheric plasma in an H2 atmosphere can lead to bright Ly-alpha emission, setting constraints on the production of the outer planet airglow emissions.
Clarke, John T.↗
Deuterium content of the Venus atmosphere
The abundance of deuterium in the atmosphere of Venus is an important clue to the role of water in the planet's history, because ordinary and deuterated water escape the atmosphere at different rates. The high-resolution mode of the IUE was used to measure hydrogen Lyman-alpha emission from Venus, but only an upper limit on deuterium Lyman-alpha emission was found, from which was inferred a D/H ratio of less than 0.002-0.005. This is smaller by a factor of 3-8 than the D/H ratio derived from measurements by the Pioneer Venus Large Probe, and may indicate either a stratification of D/H ratio with altitude or a smaller overall ratio than previously thought.
Bertaux, Jean-Loup↗
IUE observations of Neptune for H Lyman-alpha emission
Study of seven IUE observations has not resulted in the detection of H Ly-alpha emission from Neptune, with 1-sigma upper limits to the planet-averaged surface brightness as low as 180 Rayleighs. The intrinsic brightness is found to be less than the 400-1500 R that would be expected from scaling arguments, and it is noted that this upper limit excludes neither the possibility of scattered solar H Ly-alpha emission from a Jupiter-like atmosphere, nor auroral emission from an active magnetosphere. The results suggest that Neptune has a lower upper atmospheric temperature than Uranus.
Clarke, John T.↗
Line formation in the hot spot region of cataclysmic variable accretion disks
The paper presents a theoretical analysis of the emission lines observed in the cataclysmic variable A0 Psc (=H2252-035), including detailed modeling of the hydrogen Balmer line emission. The analysis makes it possible to deduce the physical conditions in the so called 'hot spot', or 'bulge' region where the accretion column hits the rim of the accretion disk. It is concluded that the bulge is optically thick to the ionizing disk radiation. Consequently, its disk illuminated face is fully ionized whereas the side facing away from the disk is neutral, resulting in modulation of the observed emission lines with the orbital period. The density in the hot spot is about 5 x 10 to the 12th to 10 to the 13th/cu cm.
Elitzur, Moshe↗