Engineering topics
Publications and source records attributed to Moos, H. W..
Some recent results on planetary upper atmospheres obtained by means of orbiting ultraviolet observatories are reviewed with emphasis on Jupiter and Io torus. Consideration is given to long-term variation in Jovian Ly alpha emission, UV polar auroras on Jupiter, and UV emission from the Io torus. Requirements for UV planetary astronomy are briefly discussed.
Spatially resolved spectra of Jupiter taken with the International Ultraviolet Explorer satellite show enhanced emissions from the polar regions at H L-alpha (1216 A) and in the Lyman and Werner bands of H2 (1175-1650 A). Two types of variability in emission brightness have been observed in these aurorae: an increase in the observed emission as the auroral oval rotates with Jupiter's magnetic pole to face toward the earth and a general variation in brightness of more than an order of magnitude under nearly identical observing conditions. In addition, the spectral character of these aurorae (determined by the ratio of H L-alpha to H2 brightnesses) appears variable, indicating that the depth of penetration of the auroral particles is not constant.
A sounding rocket measurement of the H I L-alpha emission from Jupiter made on Dec. 1, 1978 shows limb darkening and an average disk brightness of 13 kR. This brightness is significantly higher than in previous measurements, and was confirmed by an IUE observation on Dec. 10, 1978. Comparison with a plane-parallel hydrogen layer model indicates that there is enhanced emission from the equatorial regions, reaching a peak near 80 deg longitude.
Observations of the optical extreme ultraviolet spectrum of the Jupiter planetary system during the Voyager 1 encounter have revealed previously undetected physical processes of significant proportions. Bright emission lines of S(+2), S(+3), O(+2) indicating an electron temperature of 100,000 K have been identified in preliminary analyses of the Io plasma torus spectrum. Strong auroral atomic and molecular hydrogen emissions have been observed in the polar regions of Jupiter near magnetic field lines that map the torus into the atmosphere of Jupiter. The observed resonance scattering of solar hydrogen Lyman alpha by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (greater than or equal to 1000 K) with a large atomic hydrogen abundance. A stellar occultation by Ganymede indicates that its atmosphere is at most an exosphere.
Observations of atmospheric ultraviolet emission between 1350 and 2200 A from the night side of Venus, made by the International Ultraviolet Explorer (IUE), are reported. Low dispersion spectra taken by the short and long wavelength IUE spectrographs clearly show the (0,0) band of the NO delta system at 1909 A; the delta (0,1) band at 1980 A and the delta (0,2) band are indicated, while the (2,0) Cameron band of CO at 1928 A does not seem to be present. The relative band intensities appear to be the same as in earth airglow, where the excitation mechanism has been identified as radiative association of O and N atoms. The column emission rate of 0.5 kR implies a larger N to O ratio in the atmosphere of Venus than in that of the earth.
Direct evidence for the presence of acetylene in the upper part of the Saturnian atmosphere is reported. This evidence consists of two spectra of Saturn obtained by using the low-dispersion mode of the short-wavelength spectrograph on the IUE satellite. A series of distinct absorption bands in the reflected solar radiation at 1750 A is attributed to acetylene. The reciprocal of the acetylene cross section at 1750 A is shown to imply 7 x 10 to the 17th molecules/sq cm in the reflecting layer. It is concluded that the radiation at 1750 A originates from less than 2.3 km-amagat within the atmosphere.
The profile of the chromospheric L-alpha emission line of the F5 IV-V star Procyon (Alpha CMi, d = 3.5 pc) has been measured using the high-resolution Princeton spectrometer aboard NASA's Copernicus satellite. L-alpha absorption lines of interstellar deuterium and hydrogen are distinctly present. The average number density of interstellar hydrogen along the line of sight is found to be 0.11 + or - 0.02 per cu cm, similar to the densities that have been found in the directions of the stars Epsilon Eri, Epsilon Ind, and Alpha Cen A. These stars are all within 3.5 pc of the earth. The ratio of deuterium to hydrogen in the direction of Procyon is found to be 1.3 (+1.2, -0.5) x 10 to the -5th.
High-resolution Copernicus spectra of Epsilon Eri and Epsilon Ind containing interstellar hydrogen and deuterium L-alpha absorption lines are presented, reduced, and analyzed. Parameters of the interstellar hydrogen and deuterium toward these two stars are derived independently, without any assumptions concerning the D/H ratio. Copernicus spectra of Alpha Aur and Alpha Cen A are reanalyzed, and limits on the D/H number-density ratio consistent with the data for all four stars are considered. A comparison of the present estimates for the parameters of the local interstellar medium with those obtained by other techniques shows that there is no compelling evidence for significant variations in the hydrogen density and D/H ratio in the local interstellar medium. On this basis the hypothesis of an approaching local interstellar cloud proposed by Vidal-Madjar et al. (1978) is rejected
Photoabsorption cross sections of methane and ethane have been determined in the wavelength range from 1380 to 1600 A at room (295 K) and dry-ice (200 K) temperatures. It is found that the room-temperature ethane data are in excellent agreement with the older measurements of Okabe and Becker (1963) rather than with more recent determinations and that a small systematic blueshift occurs at the foot of the molecular absorption edges of both gases as the gases are cooled from room temperature to 200 K, a value close to the actual temperature of the Jovian atmosphere. It is concluded that methane photoabsorption will dominate until its cross section is about 0.01 that of ethane, which occurs at about 1440 A, and that ethane should be the dominant photoabsorber in the Jovian atmosphere in the region from above 1440 A to not farther than 1575 A.
High-precision, high-resolution profiles of the L alpha and Mg II k chromospheric emission lines from Arcturus (alpha Boo) obtained with the Princeton Experimental Package aboard the Copernicus satellite are presented. Asymmetries seen in the profiles of these lines are probably intrinsic to the star, rather than the result of interstellar absorption. In contrast to previous observations of the Ca II K emission line, no evidence is found during a three-year period for variability in the profiles or in the total fluxes from these lines on time scales ranging from hours to months. Also presented is a flux profile of the O I 1302 line and flux upper limits for L beta, O VI 1032, Si III 1206, and O V 1218.
Far-ultraviolet (1160-1750 A) spectra of the Saturnian disk and the ring system have been obtained by using a very sensitive rocket-borne spectrograph with a microchannel plate detector. The use of two apertures of different diameter in the telescope focal plane permitted the separation of the contribution of the planetary disk from that of the rings. H I lambda 1216 was the only atomic spectral line emission detected in the planet and the rings. A weak signal from the disk between 1300 A and 1500 A was observed. Geometric disk albedos, averaged over 50 A, were determined from 1500 A to 1700 A. Measurements of the ring reflectivity longward of 1650 A are compatible with H2O frost but not NH3 frost.
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An objective grating spectrometer covering the wavelength range of 500 to 1700 A with a 10-A resolution is employed for the Voyager ultraviolet spectrometer experiment. In determining the composition and structure of the atmospheres of Saturn, Jupiter and several satellites, the ultraviolet spectrometer will rely on airglow mode observations to measure radiation from the atmospheres due to resonant scattering of solar flux, and the occultation mode for assessments of the atmospheric extinction of solar or stellar radiation as the spacecraft enters shadow zones. Since it is capable of prolonged stellar observations in the 500 to 1000 A wavelength range, the spectrometer is expected to make important contributions to exploratory studies of UV sources.
A rocket-borne spectrograph detected H I Lyman-alpha emission from the disk of Saturn and from the vicinity of the planet. The signal is consistent with an emission brightness of 700 rayleighs for the disk and 200 rayleighs for the vicinity of Saturn. The emission from the vicinity of the planet may be due to a hydrogen atmosphere associated with the Saturnian ring system.
Recent accomplishments in three areas of UV instrumentation for space astronomy are discussed. These areas include reliable UV photometry, sensitive photon-detection techniques, and precise telescope pointing. Calibration facilities for spectrometers designed to operate in the spectral regions above 1200 A and down to 400 A are described which employ a series of diodes calibrated against electron synchrotron radiation as well as other radiometric standards. Improvements in photon-detection sensitivity achieved with the aid of pulse-counting electronics and multispectral detectors are reported, and the technique of precise subarcsecond telescope pointing is briefly noted. Some observational results are presented which demonstrate the advantages and precision of the instruments and techniques considered.
Photoabsorption cross sections of methane in the 1400-1850-A spectral region have been measured. Cross sections at wavelengths greater than 1475 A are approximately 200 times smaller than those currently accepted. This has a significant effect on the interpretation of spectral measurements of the Jovian planets in this wavelength region.
Near-ultraviolet radiation from Nova Cygni 1975 was detected by the Copernicus satellite on five occasions from 1975 September 1 to 1975 September 9. The nova was not seen in the UV after this date. The principal result was the observation of a broad emission feature from the Mg II doublet at 2800 A. The absence of strong UV radiation at shorter wavelengths suggests that these lines are produced by collisional excitation in the outer layers of an expanding shell with electron temperature of approximately 4000 K. The absence of observed emission lines from highly ionized species indicates that the amount of material with log T between 4.4 and 5.7 is less than 0.001 times that which produces the Mg II emission. The continuum flux in the near-UV decreased as the nova evolved, showing that the total luminosity decreased as the nova faded in the visible.
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