UV Images Research Articles

On the magnetospheric source regions of substorm‐related field‐aligned currents and auroral precipitation

This paper presents a detailed analysis of field‐aligned currents and auroral UV emissions during an isolated substorm on January 9, 1997. The large‐scale upward field‐aligned currents derived from the assimilative mapping of ionospheric electrodynamics (AMIE) procedure are found to generally coincide with the relatively intense auroral emissions in the central auroral oval, and downward field‐aligned currents are mostly in the poleward edge (and with much weaker downward currents at the equatorward edge) of the auroral oval where auroral luminosity is considerably lower. However, the brightest, yet localized, discrete auroras shown in the UV images often lie at the boundary between upward and downward field‐aligned currents, indicating that the current AMIE spatial resolution is unable to resolve the fine‐scale filamentary field‐aligned currents that are expected to be associated with the localized auroral features. The initial auroral brightening at the substorm onset occurs near the midnight region; intense auroral precipitation then appears to shift toward dusk and to higher latitudes in the ionosphere as the substorm proceeds. Using an improved time‐dependent magnetic field model specified for this event, we find that the initial auroral intensification at the substorm onset maps to the inner central plasma sheet between −5 and −7 RE, with its earthward edge located just outside of the plasmapause. During the substorm expansion phase, the corresponding magnetospheric source region of auroral precipitation moves slightly tailward and toward the dusk flank near the low‐latitude boundary layer. At the late stage of the expansion phase, bright discrete auroras map to a narrow region of tailward plasma convection that is embedded in a wide and predominantly earthward convection zone. The mapping of substorm‐related field‐aligned currents, on the other hand, is more confined close to the Earth, with the current density peaks around geosynchronous altitude. The apparently different magnetospheric source regions imply that the processes that produce energetic precipitating particles are not the same ones that generate the strongest magnetic perturbations in the magnetosphere. The relative importance of electrostatic fields and the inductive electric fields during the substorm expansion phase is also investigated. It is shown that the inductive electric fields are comparable to or even larger in magnitude than the electrostatic fields at the early expansion phase, but they become less important at the late stage of the substorm expansion phase.

Global X-ray emission during an isolated substorm — a case study

The polar ionospheric X-ray imaging experiment (PIXIE) and the UV imager (UVI) onboard the Polar satellite have provided the first simultaneous global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral UV emissions. While the UV images in the Lyman–Birge–Hopfield-long band used in this study respond to the total electron energy flux which is usually dominated by low-energy electrons (<10 keV), the PIXIE images of X-ray bremsstrahlung above ∼2.7 keV respond to electrons of energy above ∼3 keV. Comparison of precipitation features seen by UVI and PIXIE provides information on essentially complementary energy ranges of the precipitating electrons. In this study an isolated substorm is examined using data from PIXIE, UVI, ground-based measurements, and in situ measurements from high- and low-altitude satellites to obtain information about the global characteristics during the event. Results from a statistical study of isolated substorms, which has reported a significant difference in the patterns of energetic electron precipitation compared to the less energetic precipitation are confirmed. A localized maximum of electron precipitation in the morning sector delayed with respect to substorm onset is clearly seen in the X-ray aurora, and the time delay of this morning precipitation relative to substorm onset strongly indicates that this intensification is caused by electrons injected in the midnight sector drifting into a region in the dawnside magnetosphere where some mechanism effectively scatter the electrons into the loss cone. In this study, we also present the results from two low-altitude satellite passes through the region of the localized maximum of X-ray emission in the morning sector. Measured X rays are compared with X-ray fluxes calculated from the electron spectral measurements. By fitting the electron spectra by a sum of two exponentials we obtain fairly good agreement between calculated and directly measured X-ray flux profiles.

Oscillations in a solar pore

Temporal variations of a solar pore were observed at the ground based Vacuum Tower Telescope (VTT) on Tenerife and with the satellite TRACE. At the VTT Stokes I and V of the iron line at 1.56μm originating in the deep photosphere, was measured. TRACE delivered UV images at 170 nm which show chromospheric continuum. In a part of the pore we find oscillations of the magnetic field in the 5 minute range. Velocities derived from shifts of the Stokes V profiles show 5 minutes everywhere in the pore, but the coherence of magnetic field and velocities is low. The intensity at 170 nm varies with 3 minutes, and for a part of the whole time series additionally with 4 minutes.

Intense growth phase events of substorms

Over the past few decades, several events of the intense growth phase preceding the expansion onset of substorms were reported. We reexamine these events using an extensive set of data, including ground‐based magnetograms at auroral latitudes, solar wind data from the Wind and IMP 8 satellites, and auroral UV images from the Polar and Viking satellites. Magnetograms show that the intense auroral electrojet is located in the early morning sector during these events. A common feature among the corresponding solar wind conditions for these events is the existence of a period of southward interplanetary magnetic field (IMF) preceded by a prolonged northward IMF. This intense growth phase, however, seems to be independent of the IMF By polarity. Enhanced auroras in the early morning sector appear during the growth phase. These auroras are consistent with relatively large geomagnetic disturbances during these events, indicating that enhanced conductances, combined with enhanced electric field caused by southward IMF, play an important role in the auroral electrojet during these intense growth‐phase events.

Hard X-rays and electric field in the subauroral afternoon sector

Abstract Very hard X-Rays involving MeV precipitating electrons appeared as a 30 mn huge burst recorded by a balloon-borne package of instruments during the INTERBOA balloon campaign. The geophysical context of these observations is presented and discussed with the support of UV images of POLAR and ground instruments.

HUBBLE SPACE TELESCOPE Imaging of the Host Galaxies of High‐Redshift Radio‐loud Quasars

We present rest-frame UV and Lyα images of spatially resolved structures (hosts) around five high-redshift radio-loud quasars obtained with the WFPC2 camera on the Hubble Space Telescope (HST). The quasars were imaged with the PC1 through the F555W (V-band) filter, which at the redshifts of the quasars (2.1 < z < 2.6) have central wavelengths of λrest ≈ 1500-1800 A, and at rest-frame Lyα using appropriately chosen narrowband filters with the WFC2. The objects were selected from ground-based imaging surveys. Those had shown that many radio-loud quasars at high redshift have prominent host galaxies that appeared to have properties similar to those of high-redshift radio galaxies. Our HST observations allow a more detailed investigation of quasar host morphologies and a comparison with similar HST studies of radio galaxies by others. Using several methods to measure and quantify the host properties we find that all five quasars are extended and that this fuzz contains ≈5%-40% of the total continuum flux and 15%-65% of the Lyα flux within a radius of about 15. The rest-frame UV luminosities of the hosts are log λPλ ≈ 11.9-12.5 L☉ (assuming no internal dust extinction), comparable to the luminous radio galaxies at similar redshifts and a factor 10 higher than both radio-quiet field galaxies at z ~ 2-3 and the most UV-luminous low-redshift starburst galaxies. The Lyα luminosities of the hosts are log LLyα ≈ 44.3-44.9 ergs s-1, which are also similar to the those of luminous high-redshift radio galaxies and considerably larger than the Lyα luminosities of high-redshift field galaxies. To generate the Lyα luminosities of the hosts would require roughly a few percent of the total observed ionizing luminosity of the quasar. The UV continuum morphologies of the hosts appear complex and knotty at the relatively high surface brightness levels of our exposures (about 24 V mag arcsec-2). In two quasars we find evidence for foreground galaxies that confuse the host galaxy morphologies and that are responsible for some of the perceived radio/optical misalignments observed in ground-based imaging data. We do find good alignment between the extended Lyα and the radio sources, strong evidence for jet-cloud interactions in two cases, again resembling radio galaxies, and what is possibly the most luminous radio-UV synchrotron jet in one of the hosts at z = 2.110. We discuss the significance of jet-cloud collisions in radio-loud quasars and their influence on radio morphologies in the framework of simple orientation-based quasar/radio galaxy unification schemes. Our observations suggest that the host galaxies of radio-loud steep-spectrum quasars are similar to those of radio galaxies and strengthen previous conclusions based on ground-based data that both types of objects are probably members of the same parent population.

Interplanetary magnetic field By control of dayside auroras

The effect of the interplanetary magnetic field (IMF) By component on the dayside auroral oval from Viking UV measurements for March–November 1986 is studied. Observations of dayside auroras from Viking UV images for large positive (15 cases) and negative (22 cases) IMF By (∣By∣>4 nT), suggest that: (1) the intensity of dayside auroras tends to increase for negative IMF By and to decrease for positive By, so that negative IMF By conditions seem preferable for observations of dayside auroras; (2) for negative IMF By, the auroral oval tends to be narrow and continuous throughout the noon meridian without any noon gap or any strong undulation in the auroral distribution. For positive IMF By, a sharp decrease and spreading of auroral activity is frequently observed in the post-noon sector, a strong undulation in the poleward boundary of the auroral oval around noon, and the formation of auroral forms poleward of the oval; and (3) the observed features of dayside auroras are in reasonable agreement with the expected distribution of upward field-aligned currents associated with the IMF By in the noon sector.

Objects in NGC 205 Resolved into Stellar Associationsby [ITAL]Hubble Space Telescope[/ITAL] Ultraviolet Imaging

We have obtained high-resolution UV images with the Hubble Space Telescope/Wide-Field Planetary Camera 2 of the central region of the dwarf elliptical galaxy NGC 205. Our images reveal that many of the hot UV stars previously detected and studied from the ground are actually multiple systems, open clusters, and star associations. We have performed photometry of two such clusters, and we find that our data are consistent with stellar ages of 50 and 100 Myr, respectively. From the number of massive stars in NGC 205, we estimate that the star formation episode in this galaxy has turned ~1000 M☉ of gas into stars over the last 100 Myr.

Size of the auroral oval: UV ovals and precipitation boundaries compared

The oval boundaries in 44 Viking UV images are compared with three critical boundaries as defined from simultaneous DMSP particle precipitation data. The particle boundaries are the equatorward boundary of the particle oval (often associated with the earthward edge of the main plasma sheet), the boundary between smooth and structured precipitation, and the poleward boundary of the particle oval (close to the open‐closed field line separatrix). The UV oval is characterized by the latitude of maximum UV intensity, equatorward boundary, and poleward boundary which are the latitudes corresponding to the half values of the maximum intensity. Differences between the UV and particle boundaries are quantified in various magnetic local time sectors and at different activity levels. The study shows that the poleward boundary of the particle oval is often at ≥2° higher latitudes than the most intense UV luminosity. Large differences are typical especially in the midnight and morning sectors. The present results suggest that caution is needed in interpreting the dramatic poleward expansion of the oval in the UV images, or more generally in using UV images to compute changes in the amount of open flux under different states of substorm activity.

Development of auroral streamers in association with localized impulsive injections to the inner magnetotail

During continuous magnetospheric activity it is not uncommon to observe narrow (in MLT) transient particle injections (duration about 1–2 minute at E=100 keV and local time extent ≤ 1 hour MLT) in the nightside part of geosynchronous orbit. Using global UV images from POLAR spacecraft we analyze the development of auroral activity on December 22, 1996 during a sequence of such injections observed by two LANL spacecraft. We found that narrow transient injections are associated with specific localized auroral form, the auroral streamer, which develops in this local time sector. The streamer first appear as a bright spot in the poleward part of the double oval ≈2–5 minutes before the geosynchronous plasma injection, and then develops equatorward, reaching in many cases the equatorward boundary of the UV aurora. We interprete the observations as evidence that some high speed flow bursts (BBFs) of small cross‐tail extent (less than 1 h MLT), formed in the distant tail or midtail, can intrude as close to the Earth as the geosynchronous distance before being stopped.

Cloning Hubble Deep Fields. I. A Model‐independent Measurement of Galaxy Evolution

We present a model-independent method of quantifying galaxy evolution in high- resolution images, which we apply to the Hubble Deep Field (HDF). Our procedure is to k-correct the pixels belonging to the images of a complete set of bright galaxies and then to replicate each galaxy image to higher redshift by the product of its space density, 1/V_{max}, and the cosmological volume. The set of bright galaxies is itself selected from the HDF because presently the HDF provides the highest quality UV images of a redshift-complete sample of galaxies (31 galaxies with I<21.9, \bar{z}=0.5, and for which V/V_{max} is spread fairly). These galaxies are bright enough to permit accurate pixel-by-pixel k-corrections into the restframe UV (\sim 2000 A). We match the shot noise, spatial sampling and PSF smoothing of the HDF, resulting in entirely empirical and parameter free ``no-evolution'' deep fields of galaxies for direct comparison with the HDF. We obtain the following results. Faint HDF galaxies (I>24) are much smaller, more numerous, and less regular than our ``no-evolution'' extrapolation, for any relevant geometry. A higher proportion of HDF galaxies ``dropout'' in both U and B, indicating that some galaxies were brighter at higher redshifts than our ``cloned'' z\sim0.5 population. By simple image transformations we demonstrate that bolometric luminosity evolution generates galaxies which are too large and the contribution of any evolving dwarf population is uninterestingly small. A plausible fit is provided by `mass-conserving' density-evolution, consistent with hierarchical growth of small-scale structure. Finally, we show the potential for improvement using the Advanced Camera, with its superior UV and optical performance.

Externally Illuminated Young Stellar Environments in the Orion Nebula: [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]Telescope[/ITAL] Planetary Camera and Ultraviolet Observations

We present new 005 resolution (22 AU) narrowband Hubble Space Telescope Planetary Camera images of externally illuminated young stellar objects embedded in the Orion Nebula. We also present 002 resolution (9 AU) UV images of seven externally illuminated protostellar environments and the first UV spectra that cover the spectral range between 1400 and 3000 Å. We discuss 43 objects for which the angular resolution has been improved over previous data by more than a factor of 2. These young stellar objects are either embedded inside the Orion Nebula and externally illuminated by the Trapezium stars or located in front of the nebula and seen in silhouette. The visibility of young stars surrounded by diffuse matter is dominated by intense line emission from ionization fronts (IFs) propagating into material photoablated from circumstellar disks by soft UV radiation. Near the Trapezium stars, the electron density at these IFs is around 105 to 106 cm-3 and the radial intensity profiles of emission lines decrease roughly as r-3, consistent with an approximately constant-velocity diverging flow with an r-2 density profile. However, some radial intensity profiles are better fitted with an exponential function. Low ionization fraction near the IF and the heating and acceleration of the photoablation flow can explain deviations from the r-3 intensity profiles. Many young stars located within 30'' of θ1 Ori C are surrounded by concentric arcs of [O III] and Hα emission located 05 to 3'' from the IF facing θ1 Ori C. These arcs may trace bow shocks formed by the interaction of the expanding photoablation flow with the fast stellar wind from θ1 Ori C. The [O III] emission may be enhanced by UV radiation from θ1 Ori C, thermal conduction, and/or turbulent mixing of weakly shocked, photoablated gas with the thermalized shocked stellar wind. About 30% of the bright externally illuminated young stellar objects contain dark regions seen in silhouette against background nebular emission in Hα and the forbidden transitions of common ions that may trace circumstellar protoplanetary disks. In some sources, the regions seen in silhouette in Hα and ionic transitions are bright in the 6300 Å [O I] line. Most externally illuminated young stellar objects have dusty tails pointing radially away from the source of ionizing photons. Tails have an average length of 500 AU, independent of the projected distance from θ1 Ori C, are limb-brightened in emission lines, and are sometimes seen in silhouette against background nebular light, indicating that they contain large column densities of gas and dust. We discuss a variety of tail formation mechanisms and conclude that initial conditions probably play a key role in their formation. The large fraction of young stellar objects with extended circumstellar structure, the mass limits on this structure, and the estimated mass-loss rates are combined to produce an estimate for the photoionization age of the Orion Nebula. The derived photoionization age of the Orion Nebula is less than 105 yr and possibly as short as 104 yr.

Detectability of High-Redshift Elliptical Galaxies in the Hubble Deep Field

Relatively few intensively star-forming galaxies at redshifts z>2.5 have been found in the Hubble Deep Field (HDF). This has been interpreted to imply a low space density of elliptical galaxies at high z, possibly due to a late (z<2.5) epoch of formation, or to dust obscuration of the ellipticals that are forming at z~3. I use HST UV (2300 Ang) images of 25 local early-type galaxies to investigate a third option, that ellipticals formed at z>4.5, and were fading passively by 2<z<4.5. Present-day early-types are faint and centrally concentrated in the UV. If ellipticals formed their stars in a short burst at z>4.5, and have faded passively to their present brightnesses at UV wavelengths, they would generally be below the HDF detection limits in any of its bands at z>2.5. Quiescent z ~ 3 ellipticals, if they exist, should turn up in sufficiently deep IR images.

On the Origin of the Ultraviolet Continuum in Seyfert 2 Galaxies

High-resolution Hubble Space Telescope ultraviolet images of the Seyfert 2 galaxies NGC 1667, NGC 3982, NGC 5427, and NGC 5953, previously known to have circumnuclear star-forming rings, are presented. The UV images of all four Seyfert 2 galaxies show the presence of several compact UV-bright knots at distances of ~0.7-2 kpc from the nucleus and associated to the circumnuclear star-forming ring. The optically bright Seyfert 2 nucleus is barely detected in the UV. The UV images provide the first direct empirical evidence that the UV flux emitted by galaxies having a Seyfert 2-type nucleus surrounded by a circumnuclear star-forming ring is dominated by radiation coming from clusters of young hot stars distributed along the star-forming ring. The contribution of the Seyfert 2 nucleus amounts to ~1%-10% of the observed UV flux in these four galaxies. If circumnuclear star-forming rings of 2-3 kpc in diameter, or smaller, are a common feature of Seyfert 2 galaxies, we conclude that (a) the observed UV flux in Seyfert 2 galaxies is not dominated by the radiation from the nuclear ionizing source and (b) the observed UV flux is dominated by emission from clusters of young hot stars located in circumnuclear star-forming regions. Whatever the nature of the nuclear ionizing source in these Seyfert 2 galaxies, this must be obscured. The UV and Hα luminosities associated to the Seyfert 2 nucleus in these galaxies are consistent with the scenario of the nuclear ionizing source being an obscured power-law ionizing source or a young (2-4 Myr) obscured high-metallicity (≥Z☉) nuclear starburst.

Hubble Space TelescopeObservations of the Gravitationally Lensed Cloverleaf Broad Absorption Line QSO H1413+1143: Imaging

An analysis of Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC) and Wide Field Planetary Camera 2 (WFPC2) images of the gravitationally lensed Cloverleaf broad absorption line quasi-stellar object (QSO) H1413+1143 is presented. Astrometric and photometric measurements are derived for the four components of the lensed QSO for five different epochs over a baseline of 2.76 yr. Because of the replacement of WFPC with WFPC2 and the change in the purpose of the observation at the various epochs, the data were not always taken with the same filter. With the exception of the declination of component D, the relative positions of the four components are measured to within ≈ 3 mas; these results are consistent with but considerably more accurate than earlier measurements. The relative photometric measurements at any one epoch are typically accurate to ≈ 0.02-0.03 mag (1 σ). The initial HST WFPC images cover a baseline of 1.26 yr (1992.21-1993.47 [1992 March 16-1993 June 22]), and over this time interval there is little evidence for brightness variations of any of the components relative to one another at levels >0.06 mag (>2 σ). Photometric measurements of the more accurate WFPC2 data obtained with different filters extends this baseline an additional 1.50 yr (to 1994.97 [1994 December 22]). The WFPC2 data also fail to reveal significant brightness variations among the components. In addition, the WFPC2 data include both UV (F336W) and near-infrared (F814W) images. These color data indicate the presence of sight-line-dependent extinction, causing the F336W-F814W color index of component B (the most reddened component) to be 0.56 ± 0.04 mag redder than that of component C (the least reddened component). The lack of evidence for significant component brightness variations at all HST observation epochs suggests that the data could be reliably extinction-corrected to derive the relative amplifications of the four image components. This is done for several reasonable dust-extinction models. Thus, the derived astrometry along with the photometric analysis set clear constraints on models for the Cloverleaf. Since component D shows some evidence for microlensing, the results on its relative amplification should be used with caution. While existing models can successfully reproduce the relative positions, the relative amplifications have not yet been successfully modeled. The WFPC2 imaging data has also permitted a sensitive search for component structure and the gravitational lens itself. There is marginal evidence for elongated structure between components A and C that may be part of an Einstein ring. However, no significant evidence for the lensing object is found. The various measurements are quantified in ways useful for setting model constraints. Limits on the mass-to-light ratio and detectability of the lensing galaxy are also discussed.

FAUST Observations of Ultraviolet Sources toward the Virgo Cluster

We analyze three UV images covering a 100 square degree field toward the Virgo cluster,obtained by the FAUST space experiment. We detect 191 sources to a signal-to-noise ratio of 4.4 and identify 94% of them. Most sources have optical counterparts in existing catalogs and about half are identified as galaxies. Some sources with no listed counterpart were observed at the Wise Observatory. We present the results of low resolution visible spectrophotometry and discuss the foreground 101 stellar sources and the 76 detected galaxies, both in the cluster and in the fore- or background. We derive conclusions on star-formation properties of galaxies and on the total UV flux from discrete and diffuse sources in the cluster. We test for the presence of intra-cluster dust, determine the clustering properties of UV emitting galaxies, and derive the UV luminosity function of Virgo galaxies.

An Atlas of Hubble Space Telescope Ultraviolet Images of Nearby Galaxies

We present an atlas of UV ($\sim 2300$ \AA) images, obtained with the Hubble Space Telescope (HST) Faint Object Camera, of the central $22''\times 22''$ of 110 galaxies. The observed galaxies are an unbiased selection constituting about one half of a complete sample of all large ($D>6'$) and nearby ($V< 2000$ km s$^{-1}$) galaxies. This is the first extensive UV imaging survey of normal galaxies. The data are useful for studying star formation, low-level nuclear activity, and UV emission by evolved stellar populations in galaxies. At the HST resolution ($\sim 0.05''$), the images display an assortment of morphologies and UV brightnesses. These include bright nuclear point sources, compact young star clusters scattered in the field or arranged in circumnuclear rings, centrally-peaked diffuse light distributions, and galaxies with weak or undetected UV emission. We measure the integrated $\sim 2300$ \AA flux in each image, classify the UV morphology, and examine trends between these parameters and the optical properties of the galaxies.

SO2Distributions on Io

We present an analysis of disk-resolved images of Io at 232.5, 260, and 285 nm taken with the FOC (Faint Object Camera) of the Hubble Space Telescope. The images at 232.5 and 260 nm were acquired in 1993 in an effort to separate the surface and SO2atmosphere contributions to the observed UV albedo. We combine these images to make UV color maps of the regions centered on the leading and trailing hemispheres. Between latitudes +60° and −60° we find that the UV colors are dominated by three distinctive components,B0,B1, andB2, one more than was found to be required to fit visible wavelength Voyager data at these latitudes. The Voyager component “B” (McEwen, A. S., T. V. Johnson, D. L. Matson, and L. A. Sonderblom 1988.Icarus75, 450–478) appears to be a combination of two distinct spectral components:B1andB2. We find thatB1, the darker component, represents either a new compositional unit or patches of SO2vapor overlying compositional unitB(=B2). To distinguish between these two possibilities, we propose two simple models of surface reflectivities and SO2vapor curve of growth designed to allow a crude separation between the effects of the absorptions by surface materials and SO2vapor.InModel 1, we recognize that all absorption bandmodels share a linear regime at the limit of small absorption pathlengths and assume that the SO2vapor absorption spectrum on Io is linearly dependent on absorption pathlength at all wavelengths without temperature dependencies. In this case Io's albedo must be dominated by the surface reflectance and the spectral differences between B1and B2are the signature of different surface units. In this model the two-way SO2vapor column density is either below our detectability limit of ∼4.1017mol·cm−2or is confined to denser patches below our spatial resolution limit of ∼250 km.InModel 2, we recognize that SO2absorptions on Io may be non-linear at 285 nm (a local maximum in SO2absorption cross sections) even in the presence of significant transmission through the gas. We retain the assumption of linearity at 232.5 nm where the SO2absorption cross sections are smallest and consider two variants of the model in which different assumptions are made about the underlying albedo of the surface materials. Invariant A, which is characterized by a relatively high assumed UV reflectivity for SO2frost, the amounts of gas are inconsistent with mm-wave and UV spectroscopic observations, and with the cold temperatures found for the pervasive thermal reservoir unit in Veederet al.'s (Veeder, G. J., D. L. Matson, T. V. Johnson, D. L. Blaney, and J. D. Goguen 1994.J. Geophys. Res.(Planets) 99, 17095–17162) thermophysical model of the surface. Invariant B, SO2frost is characterized by the lowest UV reflectivity consistent with the data. In this case there is no detectable SO2vapor over SO2frost rich regions and the FOC UV images are consistent with the presence of SO2vapor in patches of column densityN∼ 1018cm−2covering ∼11–15% of Io's projected surface. This variant of Model 2 is found to be in agreement with both the disk integrated UV spectroscopic and mm-wave observations and Veederet al.'s thermophysical model. In particular the longitude distribution of the SO2patches is similar to the longitude distribution of thermal anomalies in Veederet al.'s model.The identification of the composition of theBunit remains problematic. Polysulfur oxides (PSO) give a reasonable accounting of the UV reflectivities but may be too bright near 700 nm; Sulfur does not satisfy the UV albedos but cannot be ruled out because of uncertainties in its behavior under Io conditions. In any of the above models, regardless of the assumptions made concerning the curve of growth of SO2vapor absorption, we find that the percentage coverage of SO2frost in the regions of Io's surface that we observed is in the range of 50–60%. This is a similar result to those found in earlier spectroscopic studies of SO2frost features near 2 and 4 μm.

The Far-Infrared Radiation in M83

The fact that a galaxy is barred may have an effect on the source of the far-infrared (FIR) radiation observed. Yet, to date, no one has addressed the effects that that might have on interpreting the global FIR signal. We hope to add to the existing data on FIR radiation in galactic bulges by comparing a barred galaxy, M83, to an unbarred galaxy, M74.We use optical, Ha and UV images that are blurred to the same resolution as the IRAS HIRES images. Looking at the blurred UV images, bright "globs" were chosen and apertures were fit to them. The “globs” were then located on the optical, Hα and FIR images and fluxes were determined for each aperture used.

The UV cameras on the High Energy Transient Experiment (HETE)

The High Energy Transient Experiment (HETE), scheduled for launch this year, is a small satellite dedicated to multiwavelength observations of γ-ray and X-ray bursts. The HETE spacecraft will be equipped with gamma-ray detectors, X-ray detectors with a coded mask, and ultraviolet-sensitive CCD cameras. The UV cameras on HETE are wide-field imagers which will a) provide UV images of the regions in which γ-ray or X-ray bursts are detected, before, duringand after the burst, b) detect UV transients, whether associated with a high-energy transient or not, c) monitor the brightnesses of field stars for variability over a wide range of timescales, and d) serve as star trackers for HETE. In this poster, we describe the HETE UV instrumentation, control software, and data products.