Eric B. Burgh

Time-Resolved Observations of Jupiter's Far-Ultraviolet Aurora

Simultaneous imaging and spectroscopic observations of Jupiters far-ultraviolet aurora covering half a jovian rotation were made on 31 May 1994. The Hubble Space Telescope Wide Field Planetary Camera 2 images revealed dramatic and rapidly changing auroral features, including discrete longitudinal structures along the auroral ovals, with variable contrast; a poleward offset in a north oval sector, showing equatorward motion near dusk; emissions polewards of the ovals, apparently co-rotating; and a bright event developing near the dawn limb. Viewing geometry effects explain the rotational intensity modulation observed by the International Ultraviolet Explorer, without intrinsic longitudinal asymmetries.

Far‐Ultraviolet Spectroscopy of Venus and Mars at 4 A Resolution with the Hopkins Ultraviolet Telescope on Astro‐2

Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at ~4 A resolution were obtained on 1995 March 13 and 12, respectively, by the Hopkins Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the space shuttle Endeavour. Longward of 1250 A, the spectra of both planets are dominated by emission of the CO fourth positive (A1Π-X1Σ+) band system and strong O I and C I multiplets. In addition, CO Hopfield-Birge bands, B1Σ+-X1Σ+ (0, 0) at 1151 A and C1Σ+-X1Σ+ (0, 0) at 1088 A, are detected for the first time, and there is a weak indication of the E1Π-X1Σ+ (0, 0) band at 1076 A in the spectrum of Venus. The B-X band is blended with emission from O I λ1152. Modeling the relative intensities of these bands suggests that resonance fluorescence of CO is the dominant source of the emission, as it is for the fourth positive system. Shortward of Lyα, other emission features detected include O II λ834, O I λ989, H I Lyβ, and N I λλ1134 and 1200. For Venus, the derived disk brightnesses of the O I, O II, and H I features are about one-half of those reported by Hord et al. from Galileo EUV measurements made in 1990 February. This result is consistent with the expected variation from solar maximum to solar minimum. The Ar I λλ1048, 1066 doublet is detected only in the spectrum of Mars, and the derived mixing ratio of Ar is of the order of 2%, consistent with previous determinations.

Rocket-borne Long-Slit Ultraviolet Spectroscopy of Comet Hale-Bopp

A sounding rocket observation of comet Hale-Bopp was conducted on 1997 April 6 at 3:51 UT when the comet was at heliocentric and geocentric distances of 0.92 and 1.39 AU, respectively. The instrument was a telescope and long-slit (65 × 260), ultraviolet (1280-1880 A) spectrograph that sampled the coma parallel to the Sun-comet line from ~105 km sunward of the nucleus to ~2 × 105 km tailward with ~6 (6000 km) spatial resolution. Two spectral images were obtained with the slit offset 20 and 40 from the nucleus in the direction perpendicular to the long axis of the slit. Spatial profiles and production rates for C, O, CO, and S are presented. Modeling of the spatial profiles of CO and C emissions indicate that photodissociation of the detected CO can account for all of the C I emissions observed. The brightnesses of the strong bands of the CO Fourth Positive system and the S I λ1814 multiplet along near-nuclear lines of sight were diminished by self-absorption. A CO production rate of ~3 × 1030 molecules s-1 is derived.

Rocket Observations of Far-Ultraviolet Dust Scattering in NGC 2023

The reflection nebula NGC 2023 was observed by a rocket-borne long-slit imaging spectrograph in the 900-1400 ? bandpass on 2000 February 11. A spectrum of the star, as well as that of the nebular scattered light, was recorded. Through the use of a Monte Carlo modeling process, the scattering properties of the dust were derived. The albedo is low, 0.2-0.4, and decreasing toward shorter wavelengths, while the phase function asymmetry parameter is consistent with highly forward-scattering grains, g ~ 0.85. The decrease in albedo, while the optical depth increases to shorter wavelengths, implies that the far-UV rise in the extinction curve is due to an increase in absorption efficiency.

Far Ultraviolet Spectroscopic Explorer Observations of CO and H2 Emission in Comet C/2001 A2 (LINEAR)

Observations of comet C/2001 A2 (LINEAR) were made with the Far Ultraviolet Spectroscopic Explorer beginning 2001 July 12.58 and coinciding with a photometric increase of ~1.5 mag. Spectra were obtained in the 905-1180 A range at 0.25 A spectral resolution using the 30 × 30 aperture. Several new cometary emissions were identified, particularly the (0, 0) bands of the CO Hopfield-Birge systems C-X and B-X at 1088 and 1151 A, respectively, O I (1D-1D) at 1152 A, and three lines of the H2 Lyman system at 1071.6, 1118.6, and 1166.8 A, pumped by solar Lyβ fluorescence. Also detected were O I multiplets at 989, 1027, and 1040 A and several lines of the H I Lyman series. The rotational envelopes of the CO bands are resolved and appear to consist of both cold and hot components, the cold component accounting for 70% of the flux and with a rotational temperature of 55 ± 5 K. The hot component may be indicative of a CO2 source. The CO bands, H2 lines, and O I λ1152 all decreased by a factor of 2 over the 7.5 hr observation. The derived time-averaged production rates are Q(CO) = 1.3 × 1027 molecules s-1 and Q(H2O) = 2.1 × 1029 molecules s-1. These values may be uncertain by as much as a factor of 2 because of uncertainties in the solar flux and the electron impact contribution to the excitation.

Flyable windowless calibration lamps for Far-UV spectroscopy

We present the design of a pinhole lamp recently flown aboard two NASA/JHU sounding rocket missions as a wavelength standard for a far-UV spectrograph with a 900 - 1400 Angstrom bandpass. Lamp configuration, spectral output, gas supplies, payload accommodation and operation procedures are discussed. This lamp could easily be incorporated into future far-UV spectroscopic orbital missions and would benefit science return. We also discuss the use of Bayard-Alpert tubes (ionization gauges) as far-UV sources, which have the advantage of not requiring an external gas supply. At pressures between 10-5 and 10-7 Torr these tubes produce a strong emission line spectra, caused by electron impact with residual gas atoms in the vacuum. Below 10-7 Torr the residual gas line intensities have weaken enough to reveal the long wavelength tail of a 150 eV bremsstrahlung spectrum produced by electron impact onto tungsten grid. The use of ionization gauges in flat field and end-to-end calibration experiments is described. We show how an ionization gauge and spectrograph can be used as a real- time residual gas analyzer sensitive to atomic and molecular gas species that emit within the bandpass. Such a device could be useful in material processing and contamination control environments.

Windowless vacuum ultraviolet collimator

We describe a vacuum collimator that we have assembled to characterize the windowless ultraviolet properties of a sounding rocket spectrographic telescope. The collimator comprises a Cassegrain telescope, with SiC coated optics, used in the pre-flight calibration phase of the Far Ultraviolet Spectroscopic Explorer, and vacuum skins provided by NASA/Wallops. The collimator focal plane has a three-axis motorized stage, which we have used for the precision placement of a knife edge focal locator along with pinhole and flat-field sources. We describe the focusing procedure and present the results of a number of experiments whereby the collimator, telescope and spectrograph point spread functions were determined and a spectro-spatial flat-field of the detector was acquired.

Ultraviolet groove efficiency of a holographic grating: implications for a dual-order spectrograph.

The ultraviolet groove efficiency for a holographically ruled diffraction grating with a trapezoidal profile has been measured. The efficiencies for the +/-1 and the zero orders are in good agreement with those derived from scalar theory. The +/-1 orders have equal efficiency as a function of wavelength. The peak of the sum of fitted groove efficiency functions is 76%, a level that is competitive with the groove efficiency of a mechanically blazed grating. We suggest that a normal-incidence grating mount with detectors at both orders will offer a system with twice the efficiency and provide a built-in redundancy. We discuss design considerations for reducing astigmatism equally in both orders in such dual-order mountings.

ON THE CORRELATION BETWEEN CO ABSORPTION AND FAR-ULTRAVIOLET NONLINEAR EXTINCTION TOWARD GALACTIC OB STARS

A sample of 59 sight lines to reddened Galactic OB stars was examined for correlations of the strength of the CO Fourth Positive (A 1 � − X 1 � + ) absorption band system with the ultraviolet interstellar extinction curve parameters. We used archival high-dispersion NEWSIPS IUE spectra to measure the CO absorption for comparison to parametric fits of the extinction curves from the literature. A strong correlation with the non-linear far-UV curvature term was found with greater absorption, normalized to E(B − V ), being associated with more curvature. A weaker trend with the linear extinction term was also found. Mechanisms for enhancing CO in dust environments exhibiting high non-linear curvature are discussed. Subject headings: Dust, extinction — ISM: abundances — ISM: molecules — ultraviolet: ISM