Ultraviolet Spectrometer Experiment Research Articles

Rosetta-Alice observations of exospheric hydrogen and oxygen on Mars

The European Space Agency’s Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on 25 February 2007, closest approach being at 01:54 UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750–2000 Å spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the H i Lyman-α and Lyman-β emissions from exospheric hydrogen out beyond 30,000 km from the planet’s center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304 Å is detected at altitudes of 400–1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal oxygen atoms by the dissociative recombination of O 2 + .

William George Fastie

William George Fastie, a preeminent optical physicist and spectroscopist who helped start the Johns Hopkins University space program in the late 1950s, died of pneumonia on 14 July 2000 in Baltimore.Born in Baltimore on 6 December 1916, Fastie grew up during the Depression. After graduating from high school with what he called an “undistinguished scholastic record,” he worked at a grocery store to help support his family. From the time he was a teenager, he displayed a flair for physics. He attended the Johns Hopkins University from 1934 through 1941, first as a student in the evening college and later as a graduate student in physics, working under the supervision of A. Herman Pfund, Robert W. Wood, and Gerhardt H. Dieke. Remarkably, he did not complete the requirements for either an undergraduate or graduate degree.During World War II, he worked as a research assistant in the department of physics and developed infrared detectors. In 1945, he left Hopkins to become a research physicist at Leeds and Northrup Co; however, in 1951, professor of physics John Strong lured Fastie back to academe at Hopkins.Fastie’s scientific work at Hopkins was extremely inventive. In his first publications, he described a new type of spectrometer that today bears his name. Because of the subsequent widespread adoption of this type of instrumentation, he became well known among spectroscopists. With the advent of Sputnik, Fastie recognized the promise of spectroscopy from spaceborne platforms, so he began a program of rocket research that has continued at Hopkins. At first, the rocket program emphasized the spectroscopic study of the constituents of Earth’s upper atmosphere. But from the mid-1960s, it quickly evolved into an astronomy program, with accurately pointed telescopes carried on space vehicles. Fastie was a coinvestigator in the Mariner 5 flyby of Venus in 1967 and the Mariner 6 and 7 flybys of Mars in 1969, and was also the principal investigator for the ultraviolet spectrometer experiment on Apollo 17 in 1972. All the missions except for the first flyby carried ultraviolet spectrometers based on the design discovered by Fastie in 1952. Known today as the Ebert-Fastie spectrometer, it is similar to a design proposed by Hermann Ebert at the beginning of the 20th century that was rejected and subsequently forgotten because of concerns with fabrication feasibility. Without prior knowledge, Fastie rediscovered and significantly improved the design. With characteristic humility, when he learned of Ebert’s prior work, he insisted that the design be named solely for Ebert. Fastie’s recollections about the reinvention of the Ebert spectrometer were published in Physics Today (January 1991, page 37).During the mid-1960s, Fastie also shifted his interest to astronomy. He designed several precision-pointing telescopes whose designs are still used by the sounding rocket community. He designed the prototype for the Hopkins Ultraviolet Telescope that flew on the space shuttle in December 1990 and March 1995 and tested it on a sounding rocket. Fastie’s skills as a scientist were recognized with his 1977 appointment by NASA as a member of the Hubble Space Telescope (HST) science working group. In 1979, the Association of Universities for Research in Astronomy Inc requested that Hopkins serve as the proposed site for the Space Telescope Science Institute. Fastie played a significant role in preparing the proposal, particularly in the detailed formation of the concept of the institute.Fastie also worked on diverse topics such as the bioluminescence of the Chesapeake Bay and the development of new kinds of spectroscopic instrumentation. He had a gift of encouraging enthusiastic coworkers and young scientists.Fastie retired from Hopkins in 1982. However, he continued to work on campus through 1997 as a telescope scientist and a member of the HST science working group. Until 1996, he also continued to lead a program to search for extrasolar planets using HST. In recognition of his scientific achievements, Fastie received the David Richardson Medal for 1972 from the Optical Society of America and the Exceptional Scientific Achievement Medal for Outstanding Contributions to the Space Program from NASA in 1973.Fastie, who had a sharp wit and an infectious sense of humor, will be missed. William George Fastie PPT|High resolution© 2001 American Institute of Physics.

Venus Lyα: A Morphological and Radiative Transfer Analysis

The Venus Lya corona is caused by resonance scattering of the solar 1215.67 A Lya line by hydrogen atoms in the Venus upper atmosphere. The atmospheric atomic hydrogen content is probed remotely via Lya observations. On 1990 February 10 the Galileo spacecraft flew by Venus, obtaining a series of Venus scans with the Ultraviolet Spectrometer Experiment. The Pioneer Venus (PV) Orbiter Ultraviolet Spectrometer obtained Venus Lya images approximately weekly throughout its 14 year mission (1978–1992), spanning the 11 year solar cycle. I develop a morphology of the behavior of the Venus Lya signal with respect to location on Venus and solar activity level. The predawn hydrogen density enhancement extends to high latitudes (160 ). I find an equatorial minimum of hydrogen and evidence for a polar hood of enhanced hydrogen abundance. This hood may be produced by a hydrogen influx from the predawn region that is not removed as efficiently as hydrogen in the subsolar region. All features examined persist throughout the solar cycle and increase in hydrogen abundance with solar activity. A search for small-scale (1000 km) features produced a null result. However, the timescale for ballistic transport of atomic hydrogen in the exosphere is ∼8 minutes, too short for the detection of nonequilibrium density features. I analyze the data using a two-dimensional nonisothermal complete-frequency-redistribution multiple-scattering code modified from the LYAB code provided by James Bishop for the geocorona. I employ the VTS3 neutral thermosphere model (A. Hedin, H. Niemann, W. Kasprzak, & A. Seiff, J. Geophys. Res., 88, 73 [1983]) and calculate diffusive profiles for the vertical distribution of atomic hydrogen, characterized by the hydrogen number density and vertical flux at the exobase n f 0 0 (L. Paxton, D. Anderson, & A. Stewart, J. Geophys. Res., 93, 1766 [1988]). The flux parameter regulates the amount of hydrogen in the lower thermosphere, and the exobase density controls the amount in the upper thermosphere and exosphere. The almost linear relationship between the PV Langmuir probe photoelectron current and the observed solar Lya output provides a daily proxy for solar illumination of Venus hydrogen. I determine the parameter values that best fit the data for selected segments of the sunlit disk and examine their behavior with respect to location and solar activity. Numerical results are limited to the region of spatially uniform temperature in the exosphere and thermosphere. For high solar activity ( ), subsolar values are F ∼ 200 n (5.6 10.7 0 cm 3 and cm 2 s , cor4 7 0.8) # 10 f (5.8 0.9) # 10 0 responding to hydrogen column densities of (2.0 0.3) # cm 2 above 200 km and between 135 12 13 10 (1.0 0.2) # 10 km and the CO2 Lya absorbing layer at 112 km. These values agree with the subsolar work of Paxton et al. and with densities derived from in situ measurements by H. Brinton, H. Taylor, H. Niemann, H. Mayr, A. Nagy, T. Cravens, & D. Strobel (Geophys. Res. Lett., 7, 865 [1980]). Exobase density, , den0 creases from a local solar time (LST) of 10 hr to 15 hr by to cm . At 12 hr LST, increases from the 4 4 7 # 10 4 # 10 n0 equator to 60 latitude by to cm . This exo4 4 4 # 10 7 # 10 base density minimum at low latitudes on the dayside is consistent with solar EUV heating driving the transport removal of hydrogen atoms from the dayside upper atmosphere. The hydrogen abundance in the lower thermosphere, , does not f0 vary significantly. However, because of the opacity of the overlying upper thermosphere and exosphere, the variation of hydrogen column abundance for the values observed would n0 not be detectable in . Both and increase with solar f n f 0 0 0 activity, and there is evidence suggesting solar cycle phase dependence. A detailed interpretation of these quantitative results requires a photochemical model of the Venus upper atmosphere. The details of suprathermal hydrogen atom production are essential to understanding the transport behavior of hydrogen in the upper thermosphere and exosphere.

Exploring the fine structure at the limb in coronal holes

The fine structure of the solar limb in coronal holes is explored at temperatures ranging from 10(exp 4) to 10(exp 6) K. An image enhancement algorithm orignally developed for solar eclipse observations is applied to a number of simultaneous multiwavelength observations made with the Harvard Extreme Ultraviolet Spectrometer experiment on Skylab. The enhanced images reveal the presence of filamentary structures above the limb with a characteristic separation of approximately 10 to 15 sec . Some of the structures extend from the solar limb into the corona to at least 4 min above the solar limb. The brightness of these structures changes as a function of height above the limb. The brightest emission is associated with spiculelike structures in the proximity of the limb. The emission characteristic of high-temperature plasma is not cospatial with the emission at lower temperatures, indicating the presence of different temperature plasmas in the field of view.

Solar wind latitude variations and multiple scattering from Galileo interplanetary Lyman-α observations

The Galileo Ultraviolet Spectrometer Experiment (UVS) obtained a map of the celestial sphere from interplanetary Lyman-α (IP Lα) on 13,14 December 1990 during the Earth1 encounter. The Galileo spacecraft was near the downwind interstellar axis during the encounter and the map view directions filled the downwind hemisphere. The ratio between the observation and a single scattering model is attributed to a direct measurement of the multiple scattering correction required to model IP Lα in the inner solar system. Analysis of this data set, referred to as an antisun map, shows that the solar wind charge exchange rate with interplanetary gas is 25% less over the solar poles than in the ecliptic at solar maximum. A model of the interstellar wind based on the antisun map observation exhibits a number density of atomic hydrogen far from the solar system, inside the heliosphere, of 0.16 ± 0.05 cm−3.

Limits to the lunar atmosphere

The presence of sodium and potassium on the Moon implies that other more abundant species should be present. Volatile molecules like H2O are significantly more abundant than sodium in any of the proposed external atmospheric sources. Source mechanisms which derive atoms from the surface should favor abundant elements in the regolith. It is therefore puzzling that the Apollo ultraviolet spectrometer experiment set limits on the density of oxygen of NO < 5×10² cm−3, and that the Apollo Lunar Atmospheric Composition Experiment data imply NO < 50 cm−3 above the subsolar point. These limits are surprisingly small relative to the measured value for sodium. A simple consideration of sources and sinks predicts significantly greater densities of oxygen. It is possible but doubtful that the Apollo measurements occurred during an epoch in which source rates were small. A preferential loss process for oxygen on the darkside of the Moon is considered in which ionization by electron capture in surface collisions leads to escape through acceleration in the local electric field. Cold trapping in permanently shadowed regions as a net sink is considered and discounted, but the episodic nature of cometary insertion may allow formation of ice layers which act as a stabilized source of OH. On the basis of an assumed meteoroid impact source we predict a possible emission brightness of ∼50 R in the OH(A ‐ X)(0,0) band above the lunar bright limb. A very uncertain small comet source of H2O could raise this value by more than two orders of magnitude.

Atomic and molecular hydrogen from Titan in the Kronian magnetosphere

We consider the question of the neutral gas and plasma population in the region of the Kronian magnetosphere, outside the plasma mantle and inside the magnetopause, dominated by the efflux of Titan's atmosphere. A model that has been used successfully to describe the inner magnetosphere is applied to this region under constraints derived from the Plasma Science (PLS) and Ultraviolet Spectrometer (UVS) experiments on Voyager. We find that it is not possible to reconcile the results of these two experiments simultaneously with the values of the atomic and molecular source strengths of the atmosphere of Titan extant in the literature. The possible sources of the discrepancy are delineated.

Satellite observations of the oi 1304, 1356 and 1641 Å dayglow and the abundance of atomic oxygen in the thermosphere

The far ultraviolet spectrometer experiment on the S3-4 satellite made nadir-viewing observations of the atomic oxygen dayglow during the spring of 1978. Comparison of 27 orbits of OI 1304 Å ( 3P- 3S), 1356 Å ( 3p- 5S), and 1641 Å ( 1D- 3S) intensities shows important orbit-to-orbit variations. For four selected orbits of data, airglow intensities have been computed using the O abundance predicted by the MSIS-83 model atmosphere for the appropriate geophysical conditions. In general, the MSIS-83 empirical model reproduces the airglow rather well. To match the absolute magnitude of the airglow, the computed photoelectron flux was increased by 40%, also the observed 1641 1304 intensity ratio required a branching ratio of 2.4 × 10 −6. Reductions in the airglow during geomagnetically disturbed conditions suggest decreases in the O concentration which can be observed remotely.

Solar Mesosphere Explorer measurements of polar mesospheric clouds (noctilucent clouds)

The Ultraviolet Spectrometer experiment on board the Solar Mesosphere Explorer satellite has measured scattering of sunlight from the polar mesospheric cloud layer in the 0.2–0.3 μm spectral range. The layer is manifested in the limb-scanning measurements as large increases in radiance at heights near 80 km, at latitudes 60–90°N and 60–90°S and during the summer season. They are similar to noctilucent clouds with respect to their height, geometrical thickness (less than 3.5 km) and morphology. However, as shown by Donahue et al. (1972), they are much brighter and occupy the entire polar ‘cap’ area, a region largely inaccesible to ground-based observation. The measurements have revealed a forward-scattering asymmetry, which increases with layer brightness. This behaviour shows that the brighter clouds are composed of particles with radii according to Mie theory up to 0.07 μm, at least for the limited set of data studied so far. The variation of layer brightness with asymmetry factor is consistent with the layer being limited by the available water content of the atmosphere. The calculated water content of the particles, assuming them to consist of pure water ice, is about 100 μg m −2, provided the particle distribution is monodisperse. This corresponds to the total amount of atmospheric water vapor residing in a vertical column above 80 km for a water vapor mixing ratio of 1.2 ppmv. This is consistent with the amount of water vapor believed to exist at mesopause heights (a few ppmv). The large amounts of ice reported by Donahue et al. (1972) are too large by a factor ten. Their corrected values are consistent with the present analysis. A brief description is given concerning additional research topics which are being pursued using the extensive SME data base, which now consists of five complete summer seasons from 1981 to the present.

Mariner 10 observations of hydrogen Lyman alpha emission from the venus exosphere: Evidence of complex structure

The Ultraviolet Spectrometer Experiment on the MARINER 10 spacecraft measured the hydrogen Lyman α emmission resonantly scattered in the Venus exosphere at several viewing aspects during the encounter period. Venus encounter occurred at 17:01 GMT on 5 February 1974. Exospheric emissions above the planet's limb were measured and were analyzed with a spherically symmetric, single scattering, two-temperature model. On the sunlit hemisphere the emission profile was represented by an exospheric hydrogen atmosphere with T c = 275±50 K and n c = 1.5 × 10 5 cm −3 and a non-thermal contribution represented by T H = 1250±100 K with n H = 500±100 cm −3. The observations of the dark limb showed that the spherically symmetric model used for the sunlit hemisphere was inappropriate for the analysis of the antisolar hemisphere. The density of the non-thermal component had increased at low altitudes, < 12,000 km, and decreased at high altitudes, > 20,000 km, by comparison. We conclude that the non-thermal source is on the sunward side of the planet. Analysis of the dark limb crossing suggests that the exospheric temperature on the dark side is <125 K if the exospheric density remains constant over the planet; upper limits are discussed. An additional source of Lyman α emission, 70 ± 15 R, was detected on the dark side of the planet and is believed to be a planetary albedo in contrast to multiple scattering from the sunlit side. Our analysis of the MARINER 10 data is consistent when applied to the MARINER 5 data.

Mariner 9 ultraviolet spectrometer experiment: Vertical distribution of ozone on Mars

The vertical distribution of ozone in the atmosphere of Mars is computed from ultraviolet spectra obtained by the Mariner 9 spacecraft. In the Northern Hemisphere the ozone scale height is much smaller than the atmospheric scale height in midlatitudes and increases rapidly to a maximum farther north. At high latitudes (above 60°) there is no significant difference between the scale heights of ozone in the Northern (winter) Hemisphere and the Southern (summer) Hemisphere. Comparison of the ozone distribution with atmospheric temperature structure indicates that at some locations in the North, the density of water vapor increases with altitude, and the time for vertical mixing is about 3 days or more.

Aeronomical reference spectrum for solar UV below 2000 Å

The solar reference spectrum named R74113 announced at the 1975 International Association of Geomagnetism and Aeronomy symposium in Grenoble refers to the solar conditions of April 23, 1974, and covers the wavelength range 14–2000 Å. The fluxes for R74113 between 250 and 1940 Å were based on a preliminary evaluation of data obtained from a rocket experiment flown on April 23, 1974. The fluxes outside of the wavelength range covered by the rocket experiment were estimated values for the solar condition of April 23, 1974. This reference spectrum has been revised because of refined evaluation of the rocket data and detailed experience gained from many scans of the solar spectrum for the entire wavelength range 140–1850 Å obtained from the extreme ultraviolet spectrometer experiment on the AE‐E satellite. Although numerous changes have been made in the revised spectrum named F74113, summarized here, these changes have only a small effect on aeronomical calculations that have been based on the original reference spectrum.

Mariner 9 ultraviolet spectrometer experiment - Strong stellar lines in OB stars

view Abstract Citations (14) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Mariner 9 ultraviolet spectrometer experiment: strong stellar lines in OB stars. Molnar, M. R. Abstract Data from the ultraviolet spectrometer on the Mariner 9 Mars orbiter have been used to measure the strong stellar lines of Si III (1300 A), Si IV (1400 A), and C IV (1550 A). The C IV equivalent widths appear to be in good agreement with preliminary OAO-2 results; however, the Mariner 9 Si IV equivalent widths for B1 stars appear to be smaller by a factor of about 0.7. We have also made LTE calculations for the equivalent widths of these Si III and Si IV lines. Significant differences are found between various theoretical investigations which can be attributed to different models and, in particular, f-values. The observations are compared with the theoretical results. We suspect that line-blending effects make any interpretation uncertain. Publication: The Astrophysical Journal Pub Date: August 1975 DOI: 10.1086/153766 Bibcode: 1975ApJ...200..106M Keywords: B Stars; Mariner 9 Space Probe; O Stars; Spectral Line Width; Stellar Spectra; Ultraviolet Spectroscopy; Absorption Spectra; Early Stars; Spaceborne Astronomy; Stellar Evolution; Astrophysics full text sources ADS | data products SIMBAD (28)

Mariner 9 Ultraviolet Spectrometer Experiment: Upper Limits on the Lyman-Alpha Flux from Clusters of Galaxies

view Abstract Citations (10) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Mariner 9 Ultraviolet Spectrometer Experiment: Upper Limits on the Lyman-Alpha Flux from Clusters of Galaxies Bohlin, Ralph C. ; Henry, Richard C. ; Swandic, J. R. Abstract Data from the ultraviolet spectrometer on the Mariner 9 Mars orbiter have been used to set upper limits on the redshifted La flux from the Perseus and Pegasus I (NGC 7619) clusters of galaxies. The limit for the Perseus cluster implies that the temperature of the intergalactic gas required for gravitational binding must be greater than 10 K, if it exists. A less severe limit is set for the Pegasus I cluster. Subject headings: galaxies, clusters of - intergalactic medium - spectra, ultraviolet Publication: The Astrophysical Journal Pub Date: May 1973 DOI: 10.1086/152112 Bibcode: 1973ApJ...182....1B full text sources ADS | data products NED (2)

Extreme ultraviolet line intensities from the Sun

Anextreme ultraviolet spectrometer experiment by University College London for observation of whole-Sun spectrum -line fluxes, and flown on Orbiting Solar Observatory-6, is described. Results for the four lines with absolute calibration, are show n for the first 6 months of flight, from August 1969 to February 1970. The mean fluxes for the 30.4, 53.7, 58.4 and 121.6 nm lines are 8.1 x 10 9 ± 35 % ph cm-2 s-1 , 3.2 x 10 8 + 40% ph cm -2 s-1 , 2 .5 x 10 9 + 40% ph cm -2 s-1 , and 3 x 10 11 ± 50% ph cm -2 s -1 respectively. Relations between these fluxes and the 10.7 cm radio flux and Zurich sunspot number are presented.

Correction [to “Mariner 6 and 7 Ultraviolet Spectrometer Experiment: Analysis of hydrogen Lyman-alpha data”

Correction [to “Mariner 6 and 7 Ultraviolet Spectrometer Experiment: Analysis of hydrogen Lyman-alpha data”

Mariner 9 ultraviolet spectrometer experiment: Photometry and topography of Mars

Reflectance properties of Mars are measures in a 100-Å band centered at 3050 Å by the ultraviolet spectrometer. The instrument has an angular resolution which is equivalent to an area of 10 × 30 km on the surface of Mars. The transition from dusty conditions, which prevailed at the time of arrival of Mariner 9 on 14 November 1971, began on 1 January 1972, and relatively clear conditions existed after 23 January 1972. As the atmosphere became clearer, the scattering properties began to show a morning enhancement in both terminator and illuminated disk reflectance. A topographic map of Mars based on the scattering of ultraviolet light from the Mars atmosphere is shown. This map is based upon 3050-Å data obtained after the Mars atmosphere had cleared in the ultraviolet. Ultraviolet light which is Rayleigh-scattered by the Mars molecular atmosphere, with allowance for uniform turbidity, is proportional to surface pressure independent of atmospheric temperature structure. Comparison with Mariner 9 radio occultation measurements determines the fraction of total reflectance that is due to atmospheric scattering.

Mariner 6 and 7 Ultraviolet Spectrometer Experiment: Analysis of the O I 1304- and 1356-A emissions

Analysis of the O I 1304- and 1356-A data from Mariner 6 and 7, and estimation of the atomic oxygen concentration of the Martian atmosphere. Derived values of the parameters describing the sources of excitation differ considerably from previous estimates. A detailed statistical treatment is used in the analysis. At an exospheric temperature of 350 K the atomic oxygen concentration at 135 km is estimated to be between 0.5 and 1%. The likely source of the 1304-A intensity is resonance scattering of solar 1304-A photons. For the 1356-A emission, the likely source is photodissociation of CO2. There is an indication of a diurnal variation of the Martian upper atmosphere.

Mariner 6 and 7 ultraviolet spectrometer experiment: Photometry and topography of mars

From data obtained by the ultraviolet spectrometers aboard the Mariner 6 and 7 Mars missions the reflectivity of the illuminated disk of Mars was found to have two distinct spectral types, associated with polar and nonpolar regions. In this work the characteristic spectrum of nonpolar regions on Mars is discussed. An interpretation of the variation of the intensity at 3050Å due to surface pressure variations is goven. A comparison of the ultraviolet observations with pressures inferred by the infrared spectrometer is used to test this interpretation and to determine physical parameters of Mars which lead to the observed reflectance. Earth based pressures obtained by broadening of the 1 μ band of CO 2 and radar topography are compared with the ultraviolet results.

Planetary Ultraviolet Spectroscopy

The emission spectra of a planetary dayglow are produced by the resonance scattering of atoms, the fluorescence scattering of molecules, and the photoelectron excitation of the atoms and molecules that are the major constituents of the atmosphere. The absorption spectra of a planetary atmosphere are the result of the combination of ground reflection, atmospheric scattering, and atmospheric absorption. The observation of these spectra from a planetary spacecraft is accomplished by observing the bright limb, disk, terminator, and dark limb of the planet with a telescope and scanning uv spectrometer. Ultraviolet spectroscopy can determine if the atoms and molecules basic to life are present in a planetary atmosphere: in particular, molecular nitrogen and the photodissociation products of water vapor. Ultraviolet spectrometer experiments can also tell if the atmosphere has been changed by the presence of life such as the production of oxygen from photosynthesis. Local regions of photosynthetic oxygen production will produce ozone which is detectable by uv spectroscopy.