Hopkins Ultraviolet Telescope Research Articles

Morphology of Extreme and Far Ultraviolet Martian Airglow Emissions Observed by the EMUS Instrument on Board the Emirates Mars Mission

AbstractWe present the first continuous observations of the extreme and far ultraviolet (EUV and FUV) dayglow emissions measured by Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission. We found excellent agreement between the previous observations from the Hopkins Ultraviolet Telescope and recent observations by EMUS both in shape and magnitude. We presented the average disk brightness of major EUV and FUV emissions for about 10 months of data from April 2021 to February 2022. The solar activity was mild/minimum during the first half of the period presented in this study, but we noticed significant day‐to‐day variations in the major dayglow emissions independent of solar activity, indicating possible coupling from the lower atmosphere via waves/tides. The solar activity increased significantly during the second half of the study period. Our analysis showed that all major EUV and FUV emissions are highly correlated with solar forcing as well as seasonal changes.

The Hopkins Ultraviolet Telescope: The Final Archive

ABSTRACTThe Hopkins Ultraviolet Telescope (HUT) was a 0.9 m telescope and moderate-resolution (Δλ = 3 Å) far-ultraviolet (820–1850 Å) spectrograph that flew twice on the space shuttle, in 1990 December (Astro-1, STS-35) and 1995 March (Astro-2, STS-67). The resulting spectra were originally archived in a nonstandard format that lacked important descriptive metadata. To increase their utility, we have modified the original data-reduction software to produce a new and more user-friendly data product, a time-tagged photon list similar in format to the Intermediate Data Files (IDFs) produced by the Far Ultraviolet Spectroscopic Explorer calibration pipeline. We have transferred all relevant pointing and instrument-status information from locally-archived science and engineering databases into new FITS header keywords for each data set. Using this new pipeline, we have reprocessed the entire HUT archive from both missions, producing a new set of calibrated spectral products in a modern FITS format that is fully compliant with Virtual Observatory requirements. For each exposure, we have generated quick-look plots of the fully-calibrated spectrum and associated pointing history information. Finally, we have retrieved from our archives HUT TV guider images, which provide information on aperture positioning relative to guide stars, and converted them into FITS-format image files. All of these new data products are available in the new HUT section of the Mikulski Archive for Space Telescopes (MAST), along with historical and reference documents from both missions. In this article, we document the improved data-processing steps applied to the data and show examples of the new data products.

10.1007/s11755-008-1001-y

We describe a library of reference spectra of galaxies developed using published data and public-domain data of IUE (International Ultraviolet Explorer) and HUT (Hopkins Ultraviolet Telescope) ultraviolet missions. We describe the procedures that we use to create composite spectra of objects and report some of the parameters of the resulting spectra.

The Venus ultraviolet oxygen dayglow and aurora: Model comparison with observations

We compare the intensity of the OI 130.4 and 135.6 nm emissions calculated using the soft electron precipitation measured on board the Pioneer Venus (PV) Orbiter with the auroral brightness observed with the ultraviolet spectrometer (OUVS) on board the PV. For this purpose, we use a new electron transport model based on a Monte Carlo implementation of the Boltzmann equation and a multi-stream radiative transfer model to calculate the effects of multiple scattering on the intensity field of the 130.4-nm triplet. We show that the consideration of the enhancement of the emergent 130.4-nm to the 135.6-nm intensity by multiple scattering in the optically thick Venus atmosphere increases the auroral 130.4/135.6 ratio by a factor of about 3. We find agreement with the mean 130.4/135.6 ratio observed with PV-OUVS using the typical suprathermal electron energy spectrum reported from PV in situ measurements showing a characteristic energy of about 14 eV. To account for the average OI auroral emissions, the required precipitated energy flux is 2×10 −3 mW m −2, that is about 30% of the measured suprathermal night-side soft electron spectrum used as a reference. The calculated brightness of the CO Cameron bands is about twice as large as the weak observed emission, but within the error bars of the observations and the uncertainties of the dissociative excitation cross-section of CO 2. The electron transport model, coupled with calculations of excitation processes is also applied to an analysis of the FUV oxygen day airglow observations made with PV-OUVS and the Hopkins Ultraviolet Telescope (HUT) spectrograph. Comparisons indicate that the model accounts for both the disc-averaged intensities observed with the HUT spectrograph, the limb scans and the 130.4-nm images obtained with PV-OUVS. The relative contribution of resonance scattering of the solar line and photoelectron impact to the excitation of the 130.4-nm triplet depends on the altitude, but is globally dominated by resonance scattering. The intensity of the 130.4-nm dayglow emission does not vary proportionally with the O density in the lower thermosphere, but provides nevertheless a useful tool to remotely probe the atomic oxygen density and its variations.

Far‐Ultraviolet Spectroscopy of the Dwarf Novae SS Cygni and WX Hydri in Quiescence

We present time-resolved FUV spectra of the dwarf novae SS Cyg and WX Hyi in quiescence from observations using the Hopkins Ultraviolet Telescope on the Astro-1 and Astro-2 space shuttle missions and the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Both dwarf novae are characterized by blue continua that extend to the Lyman limit punctuated by broad emission lines including transitions of O VI, N V, Si IV, and C IV. The continuum of WX Hyi can be fitted with a white dwarf model with physically reasonable model parameters, but neither system actually shows unambiguous signatures of white dwarf emission. The shape and flux of the spectrum of SS Cyg cannot be self-consistently reconciled with a white dwarf providing all of the FUV continuum flux. Combination white dwarf/disk or white dwarf/optically thin plasma models improve the fit but still do not give physically reasonable model parameters for a quiescent dwarf nova. Assuming that the UV emission lines arise from the disk, the line shapes indicate that surface fluxes fall roughly as R-2 in both systems. Fits to the double-peaked line profiles in SS Cyg indicate that the FUV line-forming region is concentrated closer to the white dwarf than that of the optical lines and provide no evidence of a hole in the inner disk. Although the flux from SS Cyg was relatively constant during all of our observations, WX Hyi showed significant variability during the GHRS observations. In WX Hyi, the line and continuum fluxes are (with the exception of He II) highly correlated, indicating a link between the formation mechanisms of the line and continuum regions.

Fluorescent Molecular Hydrogen Emission in IC 63:FUSE, Hopkins Ultraviolet Telescope, and Rocket Observations

We present far-ultraviolet observations of IC 63, an emission/reflection nebula illuminated by the B0.5 IV star γ Cas, located 1.3 pc from the nebula. Molecular hydrogen fluorescence was detected first in IC 63 by IUE and later at shorter wavelengths by ORFEUS. Here we present Far Ultraviolet Spectroscopic Explorer (FUSE) observations toward three locations in the nebula, complemented by Hopkins Ultraviolet Telescope (HUT) data on the central nebular position. In addition, we present a sounding rocket calibration of a FUSE spectrum of γ Cas. Molecular hydrogen fluorescence is detected in all three FUSE pointings. The intensity of this emission, as well as the contributions from other species, are seen to vary with position. The absolute flux calibration of the sounding rocket data allows us to reliably predict the radiation field incident on IC 63. We use these data to test models of the fluorescent process. Our modeling resolves the perceived discrepancy between the existing ultraviolet observations and achieves a satisfactory agreement with the H2 rotational structure observed with FUSE.

원자외선 분광기(FIMS)의 감도 측정

We describe the in-flight sensitivity calibration of the Far ultraviolet Imaging Spectrograph (FIMS, also known as SPEAR) onboard the first Korean science satellite, STSAT-1, which was launched in September 2003. The sensitivity calibration is based on a comparison of the FIMS observations of the hot white dwarf G191B2B, and two O-type stars Alpha-Cam, HD93521 with the HUT (Hopkins Ultraviolet Telescope) observations. The FIMS observations for the calibration targets have been conducted from November 2003 through May 2004. The effective areas calculated from the targets are compared with each other.

Jovian auroral spectroscopy with FUSE: analysis of self-absorption and implications for electron precipitation

High-resolution ( ∼ 0.22 Å ) spectra of the north jovian aurora were obtained in the 905–1180 Å window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H 2 spectra and the spectral range allows the study of self-absorption. Below 1100 Å, transitions connecting to the v ″ ⩽ 2 levels of the H 2 ground state are partially or totally absorbed by the overlying H 2 molecules. The FUSE spectra provide information on the overlying H 2 column and on the vibrational distribution of H 2. Transitions from high-energy H 2 Rydberg states and treatment of self-absorption are considered in our synthetic spectral generator. We show comparisons between synthetic and observed spectra in the 920–970, 1030–1080, and 1090–1180 Å spectral windows. In a first approach ( single-layer model ), the synthetic spectra are generated in a thin emitting layer and the emerging photons are absorbed by a layer located above the source. It is found that the parameters of the single-layer model best fitting the three spectral windows are 850, 800, and 800 K respectively for the H 2 gas temperature and 1.3 × 10 18 , 1.5 × 10 20 , and 1.3 × 10 20 cm − 2 for the H 2 self-absorbing vertical column respectively. Comparison between the H 2 column and a 1-D atmospheric model indicates that the short-wavelength FUV auroral emission originates from just above the homopause. This is confirmed by the high H 2 rovibrational temperatures, close to those deduced from spectral analyses of H + 3 auroral emission. In a second approach, the synthetic spectral generator is coupled with a vertically distributed energy degradation model, where the only input is the energy distribution of incoming electrons ( multi-layer model ). The model that best fits globally the three FUSE spectra is a sum of Maxwellian functions, with characteristic energies ranging from 1 to 100 keV, giving rise to an emission peak located at 5 μbar, that is ∼ 100 km below the methane homopause. This multi-layer model is also applied to a re-analysis of the Hopkins Ultraviolet Telescope (HUT) auroral spectrum and accounts for the H 2 self-absorption as well as the methane absorption. It is found that no additional discrete soft electron precipitation is necessary to fit either the FUSE or the HUT observations.

Analysis of the Astro‐1/Hopkins Ultraviolet Telescope EUV–FUV dayside nadir spectral radiance measurements

The Hopkins Ultraviolet Telescope (HUT) was one of three ultraviolet Astro‐1 observatory instruments on space shuttle Columbia in December 1990 (STS‐35), covering the 830–1850 Å wavelength interval (first order) at ∼3.3 Å spectral resolution. Satellite altitude was 360 km. Dayside nadir measurements were performed during a single orbit on 7 December 1990 under solar maximum (F10.7 = 222), geomagnetically quiet (Ap = 4) conditions, covering late morning local times (solar zenith angle < 48°). These data constitute a reference dayglow radiance spectrum comprising a number of thermospheric emission features, including several weak features neighboring bright optically thick emissions, that have not yet received adequate explanations, in part owing to questions regarding spectral intensity calibration, dynamic range, etc., associated with older data sets. In this paper, the HUT extreme ultraviolet (EUV)–far ultraviolet (FUV) nadir dayglow spectrum is presented along with the results of a modeling analysis using a development version of the Atmospheric Ultraviolet Radiance Integrated Code (AURIC). The analysis relies on currently available laboratory data and first‐principles excitation and transport modeling codes and utilizes constraints drawn from the FUV spectral region data to study several EUV emissions of interest for which past airglow data are sparse. The focus is on the airglow in the 980–1200 Å region, which is particularly rich in emission features affected by thermospheric conditions. Emissions investigated in detail include (1) the optically thick OI 989 Å multiplet and associated 1172 Å fluorescence, (2) the OI 1026 Å sextuplet blended with atomic hydrogen Lyman β, (3) the atomic nitrogen multiplets at 1134 and 1200 Å consisting of optically thick components excited by e− + N and optically thin components excited by e− + N2 and hν + N2, (4) a number of other NI and N+ features (e.g., N+ 1085 Å) excited by N2 dissociative ionization, (5) the Birge‐Hopfield I (N2 BH‐1) system, and (6) the resonance lines of argon (Ar 1048 and 1067 Å).

Optical and Far‐Ultraviolet Spectroscopy of Knot D in the Vela Supernova Remnant

We present spectra of optical filaments associated with the X-ray knot D in the Vela supernova remnant. It has been suggested that knot D is formed by a bullet of supernova ejecta, that it is a breakout of the shock front of the Vela supernova remnant, and also that it is an outflow from the recently discovered remnant RX J0852.0� 4622. We find that knot D is a bow shock propagating into an interstellar cloud with normal abundances and typical cloud densities (nH � 4 11 cm � 3 ). Optical long-slit spectra show that the (S ii) �� 6716, 6731 to Hline ratio is greater than unity, proving that the optical filaments are shock excited. The analysis of far-ultraviolet spectra obtained with the Hopkins Ultraviolet Telescope and with the Far Ultravio- let Spectroscopic Explorer (FUSE) LWRS aperture show that slower shocks (� 100 km s � 1 ) produce most of the low-ionization lines such as O iii) � 1662, while faster shocks (� 180 km s � 1 ) produce the O vi �� 1032, 1038 and other high-ionization lines. C iii and O vi lines are also detected in the FUSE MDRS aperture, which was located on an X-ray-bright region away from the optical filaments. The lines have two velocity components consistent with � 150 km s � 1 shocks on the near and far sides of the knot. The driving pressure in the X-ray knot, P=kB � 1:8 � 10 7 cm � 3 K, is derived from the shock properties. This is over an order of mag- nitude larger than the characteristic X-ray pressure in the Vela supernova remnant. The velocity distribution of the emission and the overpressure support the idea that knot D is a bow shock around a bullet or cloud that originated near the center of the Vela remnant. Subject headings: ISM: individual (Vela Supernova Remnant) — shock waves — supernova remnants — ultraviolet: ISM

Far‐Ultraviolet Spectra of a Nonradiative Shock Wave in the Cygnus Loop

Spatial and spectral profiles of O VI emission behind a shock wave on the northern edge of the Cygnus Loop were obtained with the Far Ultraviolet Spectroscopic Explorer. The velocity width of the narrowest O VI profile places a tight constraint on the electron-ion and ion-ion thermal equilibration in this 350 km s-1 collisionless shock. Unlike faster shocks in SN 1006 and in the heliosphere, this shock brings oxygen ions and protons to within a factor of 2.5 of the same temperature. Comparison with other shocks suggests that shock speed, rather than Alfven Mach number, may control the degree of thermal equilibration. We combine the O VI observations with a low-resolution far-UV spectrum from the Hopkins Ultraviolet Telescope, an Hα image, and ROSAT PSPC X-ray data to constrain the preshock density and the structure along the line of sight. As part of this effort, we model the effects of resonance scattering of O VI photons within the shocked gas and compute time-dependent ionization models of the X-ray emissivity. Resonance scattering affects the O VI intensities at the factor of 2 level, and the soft spectrum of the X-ray rim can be mostly attributed to departures from ionization equilibrium. The preshock density is about twice the canonical value for the Cygnus Loop X-ray-emitting shocks.

Modeling the Spectral Signatures of Accretion Disk Winds: A New Monte Carlo Approach

Bipolar outflows are present in many disk-accreting astrophysical systems. In disk-accreting cataclysmic variables (CVs), these outflows are responsible for most of the strong features in the ultraviolet spectra of these systems. However, there have been few attempts to model these features quantitatively. Here we describe a new, hybrid Monte Carlo/Sobolev code, which allows us to synthesize the complete spectrum of a disk-dominated, mass-losing CV. The line profiles that we calculate for C IV resemble those calculated by previous workers when an identical geometry is assumed. However, our synthetic spectra exhibit not only the well-known resonance lines of O VI, N V, Si IV, and C IV but, with an appropriate choice of mass-loss rate and wind geometry, also many lines originating from excited lower states. Many of these lines have already been seen in the far-ultraviolet spectra of CVs obtained with the Hopkins Ultraviolet Telescope, the Far Ultraviolet Spectroscopic Explorer, and the Hubble Space Telescope. In order to illustrate the degree to which we are currently able to reproduce observed spectra, we finally present a preliminary fit to the Hopkins Ultraviolet Telescope spectrum of the dwarf nova Z Cam in outburst.

Ultraviolet Spectra of Star‐forming Galaxies with Time‐dependent Dust Obscuration

We study the effect of time-dependent dust obscuration on synthetic ultraviolet line profiles of a young stellar population. If the youngest and most massive stars are more obscured than the older, less massive stars, the C IV ?1550 and other stellar wind lines are significantly diluted with respect to a simple foreground screen model for the dust. We propose to use stellar wind lines as a probe of the dust-obscuration model instead of the previously employed nebular emission lines. Since purely stellar diagnostics are utilized, uncertain assumptions on the nebular properties are unnecessary. Photoionization models demonstrate that the C IV ?1550 emission is typically dominated by stellar winds and nebular contamination is negligible. A first comparison with a galaxy sample observed with the Hopkins Ultraviolet Telescope favors a dust geometry affecting ionizing and nonionizing stars equally. We point out the need for higher quality data for a more rigorous comparison. The Hubble Space Telescope is capable of obtaining such data in the future.

Far Ultraviolet Spectroscopic Explorer and Hopkins Ultraviolet Telescope Observations of Radiative Shocks in the Cygnus Loop

We report a combination of high and low spectral resolution far-ultraviolet spectral data for several groupings of bright radiative shock filaments in the Cygnus Loop supernova remnant. The high-resolution spectra were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite and cover the 905-1187 A spectral region at 0.1-0.3 A resolution, depending on observing aperture. The low-resolution data were obtained with the Hopkins Ultraviolet Telescope (HUT) and cover the 850-1850 A spectral region at ~3 A resolution. These data sets complement each other, with the HUT data providing the broader context of a wider range of emission lines and the FUSE data resolving line profiles from nearby airglow emissions and allowing detailed line profiles to be studied. Relative line intensities and line profiles change on a wide variety of spatial scales. O VI λλ1032, 1038 emission is much more generally distributed than the optical filaments. The FUSE data provide direct insight into the effects and variability of self-absorption in the strong resonance lines of O VI and C III λ977. Variability of the central reversals in O VI with position indicate a patchy O VI distribution, with self-absorption occurring locally within the remnant. Self-absorption in C III could be due to the interstellar medium, the local regions of the remnant, or a combination. We also find evidence for overlying molecular hydrogen absorption effects on some lines and suggest an interstellar (foreground) origin. It is likely that these effects have had significant impacts on many previous investigations that based their results on the interpretation of low-resolution spectral data where these effects could not be seen.

Global Far‐Ultraviolet (912–1800 A) Properties of Star‐forming Galaxies

We present the results of an archival study of the Hopkins Ultraviolet Telescope (HUT) Astro-2 database. Nineteen spectra of star-forming regions and starburst galaxies were retrieved, reprocessed, and analyzed. The spectra cover the wavelength region 912-1800 A, providing access to the domain of peak luminosity from a young stellar population. We created an atlas of galaxy spectra documenting the continuum and line properties with an emphasis on the relatively unexplored spectral region below 1200 A. The dust obscuration law was derived from a comparison of the HUT spectra with synthetic population models. The law is similar to the commonly adopted starburst reddening curve at longer wavelengths and approaches the Milky Way law near the Lyman break. A simple power-law parameterization is given, which allows users to express the reddening law in terms of the stellar or nebular color excess at ultraviolet or optical wavelengths. The star formation histories were derived from the reddening-corrected continua and the ultraviolet line profiles. We find typical ages of tens of Myr and star formation densities ranging from less than 0.1 to more than 10 M⊙ yr-1 kpc-2. The absorbed ultraviolet luminosity correlates very well with the far-infrared luminosity, as expected if the dominant dust absorption occurs in a foreground screen. This correlation holds even for the most luminous galaxies in our sample, whose IRAS luminosity is in excess of 1011 L⊙.

Shock Wave Emission at Miller’s Position 1 in the Cygnus Loop

We present the far-UV spectrum (900-1800 A) of the bright filament known as Miller's (1974) position 1 in the Cygnus Loop obtained with the Hopkins Ultraviolet Telescope during the Astro-2 space shuttle mission. Longer wavelength IUE and ground-based spectra of adjoining regions were scaled to produce a combined spectrum ranging from 900 to 7500 A. We compare this spectrum with models of radiative shock waves to infer shock velocities and elemental abundances. A power-law distribution of shock velocities within the spectrograph apertures can reproduce the observed spectrum. We consider the question of whether a simpler shock structure, thermally unstable cooling, or a mixing layer could produce a similar spectrum. We also discuss the elemental abundances with particular attention to C and Si, whose lines are only available in the UV.

A Detailed Analysis of a Cygnus Loop Shock-Cloud Interaction

The XA region of the Cygnus Loop is a complex zone of radiative and nonradiative shocks interacting with interstellar clouds. We combine five far ultraviolet spectral observations from the Hopkins Ultraviolet Telescope (HUT), a grid of 24 IUE spectra and a high-resolution longslit Halpha spectrum to study the spatial emission line variations across the region. These spectral data are placed in context using ground-based, optical emission line images of the region and a far-UV image obtained by the Ultraviolet Imaging Telescope (UIT). The presence of high-ionization ions (OVI, NV, CIV) indicates a shock velocity near 170 km/s while other diagnostics indicate v_shock=140 km/s. It is likely that a large range of shock velocities may exist at a spatial scale smaller than we are able to resolve. By comparing CIV 1550, CIII 977 and CIII] 1909, we explore resonance scattering across the region. We find that a significant column depth is present at all positions, including those not near bright optical/UV filaments. Analysis of the OVI doublet ratio suggests an average optical depth of about unity in that ion while flux measurements of [SiVIII] 1443 suggest a hot component in the region at just below 10^6K. Given the brightness of the OVI emission and the age of the interaction, we rule out the mixing layer interpretation of the UV emission. Furthermore, we formulate a picture of the XA region as the encounter of the blast wave with a finger of dense gas protruding inward from the pre-SN cavity.

Hubble Space TelescopeSpectroscopy of the Dwarf Nova RX Andromedae. I. The Underlying White Dwarf

We obtained Hubble Space Telescope Goddard High Resolution Spectrograph phase-resolved spectroscopic observations of the dwarf nova RX Andromedae at three times in its outburst cycle: (1) near the end of an extraordinarily deep and long dwarf nova quiescence, 3 months after the last outburst; (2) during the rise to outburst; and (3) near the end of a decline from outburst. The spectral wavelength range covered was 1149-1435 A. All of the spectra are dominated by absorption lines with weak to moderately strong emission wings due to the continued presence of disk material. Uncertainties in line velocities preclude a K1 determination or mass information. Our best-fitting model yielded Twd/1000 = 34.0 ± 0.1 K, log g = 8.0 ± 0.1, and Vrot= 600 km s-1. The Teff value is very similar to the Teff of the white dwarf in U Geminorum, but the rotational velocity appears to be higher than U Gem's value. We report approximate subsolar chemical abundances of carbon and silicon for RX And with C being 0.05 times solar and Si being 0.1 times solar while other elements are at essentially their solar values. However, accurate abundances are complicated by line emission, and we cannot exclude the possibility that the abundances of all species are essentially at the solar values. We see no evidence of thermonuclear-processed abundance ratios. If the white dwarf mass is 0.8 M☉, then the corresponding white dwarf cooling age, 4 × 106 years, is a lower limit to the age of this cataclysmic variable (CV). If the peculiar line features seen in the spectrum on the late decline from outburst are inverse P Cygni in nature, then infall velocities of ~ 2000 km s-1 are indicated during the decline from outburst. We compare the surface properties of the RX And white dwarf with the properties of other CV degenerates studied to date with the Hubble Space Telescope, the Hopkins Ultraviolet Telescope, and the International Ultraviolet Explorer.

Spectroscopic Evidence for High-Altitude Aurora at Jupiter from Galileo Extreme Ultraviolet Spectrometer and Hopkins Ultraviolet Telescope Observations

The Galileo Extreme Ultraviolet Spectrometer (EUVS) and the Hopkins Ultraviolet Telescope (HUT) acquired UV spectra of Jupiter Aurora in the period from 1995 through 1997. The EUVS spectra spanned the wavelength range 540–1280 Å and the HUT spectra measured the extreme ultraviolet and far ultraviolet (EUV+FUV) wavelength range 830–1850 Å. Both sets of spectra present evidence of high-altitude, optically thin H 2 band emissions from the exobase region. The analysis of the UV spectra with a two-stream electron transport model and a jovian model auroral atmosphere indicates that the primary electron flux is composed of both soft and hard electrons with characteristic energies in the soft electron energy range of 20–200 eV and the hard electron range of 5–100 keV. The soft electron flux causes enhanced EUV emission intensities below 1100 Å. The soft electron flux may explain the high temperature of the upper atmosphere above the homopause as measured from H 3 + rovibrational temperatures in the IR. For the deep aurora, a high primary characteristic energy above 5 keV is known to be present. The Galileo Energetic Particle Detector (EPD) has measured the electron distribution functions for energies above 15 keV in the middle magnetosphere. The high-energy distribution functions can be modeled by a combination of Maxwellian and kappa distributions. However, the EUV (800–1200 Å) portion of the HUT spectrum cannot be modeled with a single distribution of hard electrons as was possible in the past for the FUV (1200–1650 Å) spectrum measured by itself. The combination of EUV and FUV spectral observations by HUT serves to identify the amount of soft electron flux relative to the hard primary flux required to produce the high-altitude aurora in the neighborhood of the exobase.

Detection of Chlorine Ions in the [ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Spectrum of the Io Plasma Torus

The spectrum of the Io plasma torus in the range of 995-1187 A was recorded at 0.26 A resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE) on 2000 January 20. Five orbits of data were obtained in point-and-shoot mode (no tracking of the moving target), with the east ansa of the torus initially centered in the 30'' × 30'' aperture of the FUSE LiF spectrographs yielding a total observation time of 3405 s. The spectral resolution exceeds by a factor of 10 that of the data obtained by the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 mission. This region of the spectrum is dominated by resonance multiplets of S III and S IV, whose multiplet structures are nearly completely resolved, as well as numerous S II multiplets originating on the 2Do state of the ground configuration. Weak emission from a few lines of the resonance multiplets of Cl III and Cl II is detected at or above the 3 σ level, Cl III being stronger with two components roughly one-tenth the brightness of the main components of S III λ1018. We derive an abundance of Cl+2 of 3% relative to S+2, leading to an overall chlorine ion abundance in the torus of ≈1%. The ratio of S IV to S III brightness is about twice that observed by HUT, which, when the different slit geometries are accounted for, supports the earlier analysis that S IV emissions originate from a region more extended out of the centrifugal plane than the S III emissions.