Engineering topics

Scherb, F.

Publications and source records attributed to Scherb, F..

At least 19 records

NASA NTRS2003

Groundbased Observations of Io [OI]6300 A Emission During the Galileo 124, 125, and Cassini Encounters

For the past 12 years, we have conducted a synoptic study of [OI] 6300 A emission from Io using the high-resolution (R 120,000) stellar spectrograph at the National Solar Observatory McMath-Pierce telescope. We showed in a recent paper that this emission allows us to use Io as a localized probe of the three-dimensional plasma torus structure. We report on selected recent spectroscopic observations of Io [OI] 6300 A emission obtained during the Galileo I24 (1999-Oct-11) and I25 (1999-Nov-26) encounters with Io and the Cassini Jupiter encounter (closest approach 2000-Dec-30). The exposure time for each spectrum was 15 minutes, with a 5.2 x 5.2 aperture centered on Io. We obtained over 100 spectra for the I24 encounter during 1999 October 9-13, over 100 spectra for the I25 encounter during 1999 November 24-30, and for the Cassini Jupiter flyby almost 600 spectra from 2000 December to 2001 January 21. We use our database of observations to track long- and short-term variations in torus structure. We compare our results to Galileo, Cassini, HST, and other groundbased contemporaneous observations to gain insight into torus variability and structure.

Oliversen, R. J.↗

NASA NTRS2002

Groundbased IO [O I] 6300A Observations during the Galileo I24 and I25 and Cassini Encounters

We report on selected recent spectroscopic observations of Io [OI] 6300Angstrom emission, using the high-resolution (R approximately equal to 120,000) stellar spectrograph at the National Solar Observatory McMath-Pierce telescope. These data were obtained during the Galileo I24 (1999-Oct-11) and I25 (1999-Nov-26) encounters with Io and the Cassini Jupiter encounter (closest approach 2000-Dec-30). The exposure time for each spectrum was 15 minutes, with a 5.2 second x 5.2 second aperture centered on Io. We obtained 102 spectra for the I24 encounter during 1999 October 9-13, 82 spectra for the I25 encounter during 1999 November 24-28, 313 spectra during 2000 December 11-23, and 280 spectra during 2000 December 29-2001 January 21 for the Cassini Jupiter encounter. We showed in a recent paper (Oliversen et al. 2001, JGR, 106, 26183) that this emission allows us to use Io as a localized probe of the three-dimensional plasma torus structure. We will also present preliminary results on selected contemporaneous narrowband [SII]6731A torus images obtained at the McMath-Pierce west auxiliary telescope. We took 136, 112, and 277 torus images during the Galileo I24, Galileo I25 and Cassini Jupiter encounters, respectively. Jupiter was imaged directly onto the CCD through a ND 4 filter and the reflected light was used for guiding. Both sides of the torus were imaged simultaneously when there were no Galilean satellites between 3-8 Jovian radii from Jupiter.

Oliverson, R. J.↗

NASA NTRS2002

Measurements of [C I] 9850 A Emission from Comet Hale-Bopp

We present quantitative measurements of cometary [C I] 9850 A, emission obtained during observations of comet Hale-Bopp (C/1995 O1) in 1997 March and April. The observations were carried out using a high-resolution (lambda/Delta lambda approx. 40,000) Fabry-Perot/CCD spectrometer at the McMath-Pierce solar telescope on Kitt Peak. This forbidden line, the carbon analog of [O I] 6300 A, is emitted in the radiative decay of C(1D) atoms. In the absence of other sources and sinks, [C I] 9850 A emission may be used as a direct tracer of CO photodissociation in comets. However, in Hale-Bopp's large, dense coma, other processes, such as collisional excitation of ground-state C(3P), dissociative recombination of CO+, and collisional dissociation of CO and CO2 may produce significant amounts of C(1D). The long C(1D) radiative lifetime (approx. 4000 s) makes collisional de-excitation (quenching) the primary loss mechanism in the inner coma. Thus, a detailed, self-consistent global model of collisional and photochemical interactions is necessary to fully account for [C I] 9850 A emission in comet Hale-Bopp.

Oliversen, R. J.↗

NASA NTRS2001

HST/STIS Visible Images of Io in Eclipse

On 1999 August 27, Io was observed with HST STIS/CCD using a 2"x 2" aperture soon after it entered into eclipse (CML = 342 to 346). Two G750M/I spectral images, covering 5815 Angstroms to 6380 Angstroms and starting 3 and 17 minutes after 2nd contact, recorded emissions of Na I 5890,5896 Angstroms and [O I] 6300,6364 Angstroms. The oxygen and sodium emissions show several similar features. (1) Downstream wake emission that correlates in location with jovian magnetic field orientation; (2) Enhanced emissions near the sub-jovian region, albeit a relatively weak enhancement for sodium; and (3) Limb glow that is brighter in the hemisphere facing the jovian magnetic equator. These features are qualitatively similar to [O I] 6300 Angstrom images obtained with HST WFPC2 and to the UV STIS results of Roesler et al. 1999 (Science, 283, 353), as well as broadband images taken by the Galileo SSI (Geissler et al. JGR submitted) and more recent Cassini images. The integrated oxygen (6300 Angstrom + 6364 Angstrom) emission intensity (10.5 k Rayleighs (kR) disk averaged) did not change between the two exposures while the integrated disk averaged sodium D emission intensity decreased by 19% from 5.7 kR to 4.6 kR. Also, in contrast to the oxygen emission, sodium showed relatively enhanced emission in the northern polar regions. Likewise, on 1999 August 23, Io eclipse STIS observations were made in the near-UV and blue regions to try to identify the Galileo SSI violet filter emissions. Preliminary analysis reveals a residual signal after subtraction of a dominant scattered light background that is suggestive of a SO2 band structure. In addition, the largest individual emission at about 4075 Angstrom could be the S(+) doublet.

Oliversen, Ron J.↗

NASA NTRS1993

Variability of (O I) 6300-A emission near Io

Observations of (O I) 6300-A emission near Io have been obtained in 1990, 1991, and 1992 by the National Solar Observatory staff using the solar-stellar spectrography on the McMath-Pierce telescope at Kitt Peak. High-resolution spectra with a resolving power of about 1.2 x 10(exp 5) were obtained with an integration time of 10-15 min each. The viewing aperture for the observations was 5.2 arc sec x 5.2 arc sec centered on Io, with spatial resolution limited within this area by seeing conditions. Observations thus far have been reduced to obtain average brightness values over the aperture which range from approximately 200 to 1000 R for a number of different Io phase angles and Io system III longitudes. The (O I) 6300-A emission brightness exhibits an east-west asymmetry, where the average intensity for Io phase angles in the west (receding ansa) is 1.5 times brighter than in the east (approaching ansa). Similar east-west intensity ratios have also been observed for neutrals near Io and ions in the plasma torus for a number of other optical and ultraviolet emission lines which are excited by electron impact. In addition to the east-west asymmetry, the (O I) 6300-A emission brightness exhibits a strong dependence on the Io system III longitude angle, with a maximum value occurring in the range 200 deg +/- 50 deg. Earlier IUE observations of ultraviolet emission lines of O and S near Io obtained over a number of years have measured the east-west asymmetry, but the long IUE integration times of approximately 7-14 hours masked any detection of system III variability. For the (O I) 6300-A emission, the O(1D) state may be excited by electron impact of atomic oxygen and by electron impact dissociation of SO. The molecule SO may be present at the exobase or may be produced above the exobase as the dominant product of SO2 dissociation by electron impact. Preliminary assessment indicates that production of O(1D) by molecular dissociation may be more important. The (O I) 6300-A emission may therefore provide a remote signature for monitoring (1) the upward transport rates of molecular species in Io's atmosphere, (2) the relative abundance and time-variable dissociation of SO2 and/or SO at the exobase and in the corona of Io, and (3) the spatial distribution of these escaping molecular and atomic species and their ion production rates in the planetary magnetosphere.

Scherb, F.↗

NASA NTRS1993

H2O(+) production rates of Comets Austin 1990 V and P/Halley 1986 III

High-spectral-resolution scans of H2O(+) emissions from Austin 1990 V were obtained. A simple model of the ion distribution in the FOV is used to determine a lower limit phi(H2O+) to the H2O(+) ion flux away from the nucleus for each night of observations. A similar analysis is applied to previous observations of H2O(+) emissions from Comet P/Halley 1986 III. It is found that the mean ratio is phi(H2O+)/Q(H2O) is 1.2 x exp -3 for Halley (solar minimum) and 2.6 x 10 exp -3 for Austin (solar maximum).

Schultz, D.↗

NASA NTRS1993

The O(1D) distribution of Comet Austin 1989c1 = 1990 V

Flat field images of Comet Austin 1990 V were obtained from the morning sky, and a log of the observations is presented. Absolute intensity calibrations of the comet images are obtained along with scans of the cometary forbidden O I 6300 emission. The O(1D) distribution of the comet is found, and the OH mean lifetime at 1 AU is determined to be (1.1 +/- 0.3) 10 exp 5 s.

Schultz, D.↗

NASA NTRS1992

Comet Austin (1989c1) O(1D) and H2O production rates

The dual-etalon Fabry-Perot spectrometer of Kitt Peak's McMath solar telescope has been used to conduct Comet Austin observations with spectral scan resolutions of 0.21 A; this sufficed for resolution of cometary forbidden O I 6300 A emissions from nearby NH2, as well as telluric emissions of the same type. For these data, an O(1D) production rate is obtained which is noted to be nearly model-independent. The H2O production rate is determined by taking into account the photodissociation of H2O and OH as sources of O(1D).

Schultz, D.↗

NASA NTRS1990

Fabry-Perot observations of Comet Halley H2O(+)

Fabry-Perot scanning spectrometer observations of Comet Halley's H2O(+) emissions have yielded 6158.64 and 6158.85 A spin doublet data at distances in the range of 0 to 2 million km from the comet heat in the antisunward direction. Cometary plasma outflow velocities were ascertained on the basis of the emissions' Doppler shifts, yielding results that were mostly but not exclusively consistent with the plasma's constant antisunward acceleration; the acceleration varied from night to night of observations over a 30-300 cm/sec range. The unusual plasma kinematics of December 14-15, 1985, and January 10, 1986, may be associated with the tail-disconnection activity observed by others.

Scherb, F.↗

NASA NTRS1990

Comet Halley O(1D) and H2O production rates

Ground-based dual-etalon Fabry-Perot spectrometer observations have been made of Comet Halley's forbidden O I 6300 A emission. The 0.2 A resolution of the spectral scans was sufficient to resolve the O I forbidden line emissions from both nearby cometary NH2 and telluric emissions. On the basis of these measurements, the production rate Q of O(1D) was determined; it is then found, by taking into account the photodissociation of H2O and OH as sources of O(1D), that the ratio of H2O/O(1D) production rates is of the order of 6.

Magee-Sauer, K.↗

NASA NTRS1990

Fabry-Perot observations of comet Austin

Preliminary results of a program to observe Comet Austin (1990c1) from 16 April to 4 May and from 11 May to 27 May 1990 using the West Auxiliary of the McMath Solar Telescope on Kitt Peak, Arizona were presetned. The observations were made with a 15 cm duel-etalon Fabry-Perot scanning and imaging spectrometer with two modes of operation: a high resolution mode with a velocity resolution of 1.2 km/s and a medium resolution mode with a velocity resolution 10 km/s. Scanning data was obtained with an RCA C31034A photomultiplier tube and imaging data was obtained with a Photometrics LN2 cooled CCD camera with a 516 by 516 Ford chip. The results include: (1) information on the coma outflow velocity from high resolution spectral profiles of (OI)6300 and NH2 emissions, (2) gaseous water production rates from medium resolution observation of (OI)6300, (3) spectra of H2O(+) emissions in order to study the ionized component of the coma, (4) spatial distribution of H2O(+) emission features from sequences of velocity resolved images (data cubes), and (5) spatial distribution of (OI)6300 and NH2 emissions from medium resolution images. The field of view on the sky was 10.5 arcminutes in diameter. In the imaging mode the CCD was binned 4 by 4 resulting in 7.6 sec power pixel and a subarray readout for a field of view of 10.5 min.

Schultz, David↗

NASA NTRS1989

Fabry-Perot observations of NH2 emission from Comet Halley

The (0,8,0) cometary NH2 band lines have been isolated from both cometary and terrestrial OI 6300 A OH emissions in Fabry-Perot spectra of the emissions of Comet Halley near 6300 A. On the assumption that the major source of NH2 is NH3, these NH2 measurements limit NH3's abundance relative to H2O. The results thus obtained are noted to depend on the NH2 (0,8,0) band fluorescence efficiency factor g(B); using the g(B) value of A'Hearn (1982), the NH3 abundance is found to be about 0.1 percent of that of H2O, while the g(B) value of Tegler and Wyckoff (1989) yields a figure of 0.4 percent. Both results indicate a low NH3 abundance relative to water on Comet Halley.

Magee-Sauer, K.↗

NASA NTRS1988

Spatial distribution of O(1D) from Comet Halley

Images of comet Halley in forbidden OI 6300-A emission obtained using a 150 mm Fabry-Perot spectrometer in the imaging mode were combined with spectra taken in the scanning mode to deduce the distribution of cometary O(1D) within a 10-arcmin field of view centered on the comet head. The 10-km/sec bandpass of the system allows the distribution to be measured with little contamination from airglow O(1D) and cometary NH2 lines in the nearby spectrum. The results are modeled to provide photodestruction scale lengths for cometary H2O and OH, the predominant parents of O(1D). Results are compared with other experimental and theoretical results.

Magee-Sauer, K.↗

NASA NTRS1987

Spatial distribution of O(supra 1D) from Comet Halley

Images of Comet Halley in (OI)6300A emission obtained using a 150 mm Fabry-Perot spectrometer in the imaging mode were combined with spectra taken in the scanning mode to deduce the distribution of cometary O(1D) within a 10 arc min field of view centered on the comet head. The 10 km/sec bandpass of the system allows the distribution to be measured with little contamination from airglow O(1D) and cometary NH2 lines in the nearby spectrum. The results are modeled to provide photodestruction scale lengths for cometary H2O and OH, the predominant parents of O(1D). Results are compared with other experimental and theoretical results.

Roesler, F. L.↗

NASA NTRS1986

Spatial distribution of 6300A emission determined from narrow-band images of Halley's Comet

In March 1986 the Wisconsin dual-etalon Fabry-Perot spectrometer was adapted for use with an intensified CCD imaging detector and good quality images of OI 6300A emission from a 10 arcmin field centered on Halley's comet head were obtained in April and May 1986. The 10 km/sec bandpass was adequate to eliminate spectral contamination from cometary NH2 and, with appropriate Doppler shifts, terrestrial airglow. The observed distribution of OI emission indicates O(sup 1)D production from 2 sources, presumably photodissociation of H2O and OH. Scale lengths are determined for the distributions of these sources in the coma.

Roesler, F. L.↗

NASA NTRS1986

H-alpha scans of the intergalactic H I cloud in Leo

Scans of H-alpha were obtained for two fields within the large, intergalactic H I cloud in Leo. The data provide a conservative upper limit of 1.7 x 10 to the -7th ergs/sq cm/s/sr on the H-alpha surface brightness of the cloud. This limit corresponds to an emission measure of less than 2 cm exp -6 pc for gas at a temperature of 10,000 K and implies an ionized hydrogen mass of less than 9 x 10 to the 9th f exp 1/2 solar mass within the cloud, where f is the filling fraction of the gas. This intensity may have important implications for possible sources of the ionization.

Reynolds, R. J.↗

NASA NTRS1986

Ground-based observations of the Jupiter plasma torus

The intensity variation of (SIII) 9531 A emission from the hot plasma torus of Jupiter was measured using a scanning Fabry-Perot spectrometer. During the observational period, which covered 43 rotations of Jupiter, the (SIII) intensities showed a periodic variation consistent with a zone of enhanced emission approximately 90 deg in longitude, rotating with a period of 10.2 hours, or 2.8% longer than Jupiter's rotational period. Dessler developed a theoretical model of account for Jovian magnetospheric phenomena that differ in repetition period by 3%. In his model, periodic modulation of magnetospheric phenomena is controlled in part by the spin period of magnetic features below the surface of the planet. The differing observed periods are then due to differential rotation of the subsurface magnetic features at different latitudes. A consequence that follows from this concept of differential rotation of the internal planetary magnetic field is that not all the apparent subcorotational plasma flow in the torus is caused by plasma mass loading, as described by Hill and his associates. As a result, the mass loading required to produce the residual subcorotational flow is less than previously estimated.

Scherb, F.↗

NASA NTRS1986

Ground-based observation of Comet Halley

Approximately five hundred Fabry-Perot scans of H2O+, H alpha, and (OI) emissions from comets Halley, Hartley-Good, and Thiele were obtained at Kitt Peak in November, December and January. The analysis of the data is in a very early stage, but preliminary results were obtained on the production rate Q(H) of hydrogen, Q(D) of metastable oxygen, and Q(H2O) of water. The rate Q(H) varied with heliocentric distance R as R to the power -3.5 + or - 0.5. The rate Q(D) varied as R to the power -4.8 + or - 0.1. The ratio of Q(H2O) to Q(D) was 15 + or - 5.

Scherb, F.↗