Engineering topics

Christensen, A. B.

Publications and source records attributed to Christensen, A. B..

At least 19 records

NASA NTRS1989

An analysis of the effects of N2 absorption on the O(+) 834-A emission from rocket observations

Altitude profiles of the O(+) 834-A emission, obtained from rocket observations at moderate and high solar conditions, are compared with model calculations. The model showed that when viewing the atmosphere horizontally at altitudes below the F2 layer, the 834-A intensity was virtually independent of the O(+) density. The 834-A intensity was found to be highly sensitive to the N2 absorption cross section. Comparisons with model calculations suggest that absorption by N2 is a factor of 10 lower than the currently accepted values.

Cleary, D. D.↗

NASA NTRS1989

The ultraviolet spectrum of a dayside aurora - 530-1500 A

Observations by rocket-borne spectrometers of the high-latitude dayside aurora above Cape Perry, N.W.T. are reported. UV spectra of optical emissions produced by ambient precipitating particles are obtained in the 530-1500 A region, over a range of spectrometer line-of-sight orientations, from 100 km to the rocket apogee of 452 km. The spectrum below 1500 A is dominated by transitions from neutral and singly ionized atomic oxygen. N I, N II, and N2 emissions, which are prominent in day airglow and nighttime auroral spectra measured by the same instrumentation, are very weak, indicating energy from the dayside auroral particles is transferred to the atmosphere above most of the N2. Relative line strengths of O I and O II transitions in the high-latitude dayside aurora differ in comparison with either airglow or nighttime auroral observations.

Gentieu, E. P.↗

NASA NTRS1988

Development of the RAIDS extreme ultraviolet wedge and strip detector

In the next few years the Remote Atmospheric and Ionospheric Detector System (RAIDS) package will be flown on a Tiros spacecraft. The EUV spectrometer experiment contains a position-sensitive detector based on wedge and strip anode technology. A detector design has been implemented in brazed alumina and kovar to provide a rugged bakeable housing and anode. A stack of three 80:1 microchannel plates is operated at 3500-4100 V. to achieve a gain of about 10 to the 7th. The top MCP is to be coated with MgF for increased quantum efficiency in the range of 50-115 nm. A summary of fabrication techniques and detector performance characteristics is presented.

Kayser, D. C.↗

NASA NTRS1988

EUV properties of two diffraction gratings

The efficiency and scattering characteristics of a mechanically ruled grating (MRG) and a holographically ruled grating (HRG) are presented. One of these gratings will be employed in the Extreme Ultraviolet Spectrometer, an instrument of the Remote Atmospheric and Ionospheric Detector System to be flown aboard a TIROS satellite in 1991. The HRG showed much less Lyman alpha scattering, while the MRG had the better efficiency over most of the spectral range covered.

Cotton, D.↗

NASA NTRS1987

Magnetic field-aligned electric field acceleration and the characteristics of the optical aurora

The long-recognized association of brighter aurora with more deeply penetrating, and hence more energetic, electrons is examined. Using the Knight (1973) relation between the magnetic-field-aligned current density and potential drop (derived from the theory of single-particle motion in the presence of a magnetic-field-aligned electric field), an approximate expression relating the energy flux of the precipitating electrons over discrete aurora and the mean particle energy is derived. This expression is used in conjunction with an auroral optical excitation and emission model to specify the dependence of the red/blue ratio of auroral optical emissions on the brightness of the aurora. It is shown that the quantitative predictions of the discrete auroral theory are in accord with observations of the aurora.

Christensen, A. B.↗

NASA NTRS1986

Rocket observations of ultraviolet and optical emissions in the dayside aurora

Observations of the emissions extending from the UV to the near-IR region of the spectrum obtained at Cape Parry, Canada on December 7, 1981 are examined. The flight path of the rocket used to obtain the observations, launch conditions, and optical and UV instruments for measurement of the emission from the cleft precipitation region are described. Emission rates and altitude and zenith angles for emission features O I (989 A), O I (1304 A), O I (1356 A), O I (6300 A), O I (7774 A), O I (8446 A), H I (1216 A), and O II (834 A) are studied. The particle energy flux for the emission feature are estimated. The data suggested that there is an emission layer at 250-300 km.

Christensen, A. B.↗

NASA NTRS1985

The ultraviolet dayglow at solar maximum. 1 - Far UV spectroscopy at 3.5 A resolution

The earth's far ultraviolet dayglow (1080-1515 A) was observed at about 3.5 A resolution during a period of high solar activity near solar maximum om June 27, 1980. The observations were made at local noon by rocket-borne spectrometers viewing toward the earth's northern limb at 90 deg zenith angle (ZA) at altitudes between 100 and 245 km, and at 98 deg ZA between 245 and 260 km. The zenith angle was 8.9 deg. These spectra are compared with earlier lower-resolution dayglow data obtained during a period of lower solar activity and with auroral spectra. The brightness ratio of O I 1356 to the N2 Lyman-Birge-Hopfield (LBH) system, an indicator of the O to N2 density ratio, is lower than that previously measured at mid-latitudes and closer to the value found in aurorae. In the LBH system a depletion of the bands originating on the v-prime = 3 vibrational level of the excited state is found. Some weak N2 Birge-Hopfield bands and N I lines only marginally detected previously in the dayglow are confirmed.

Eastes, R. W.↗

NASA NTRS1984

EUV airglow during active solar conditions. II - Emission between 530 and 930 A

Instrumentation on the Astrobee F sounding rocket on June 27, 1980 gathered EUV airglow data in the 530-930 A range. The 2-4 A resolution data were examined specifically for O II emission lines below 800 A, which were centered at 537-539 A. The emision was attributed to doublet and quartet components and exhibited a branching ratio of 3. A separate identification was made of O II emissions at 581 A. Inclusion of the 2s2p(4)2P state is therefore recommended in photoinization models of cometry and planetary atmospheres.

Gentieu, E. P.↗

NASA NTRS1983

O I (7990 A) emission and radiative entrapment of auroral EUV

The auroral O I 7990-A multiplet intensity distribution has been observed by means of a 2-m grille spectrometer and a Fabry-Perot interferometer during 1977 and 1982, respectively, in Alaska. Grille spectrometer results indicate a nonstatistical distribution in the excited state, consistent with the preferential scattering of the O I 989-A EUV emission and in accord with recent radiative transfer calculations. A comparison of O I 7990-A intensities measured on the nightside with those from dayside magnetospheric cleft aurora shows a cascade contribution to the O I 8446-A line, and ultimately to the O I 1304-A transition, that depends on the altitude of the aurora. This is attributed to the greater radiative enhancement of O I 989-A at low altitude, and the correspondingly enhanced probability for branching from the 3 s-prime to the 3 p level in atomic O.

Christensen, A. B.↗

NASA NTRS1983

Vacuum ultraviolet monochromator calibration using measured atomic branching ratios

It is noted that there now exists a compendium of experimentally measured transition probability ratios for ionized transitions of oxygen, nitrogen, and sulfur, which can be used for calibration in the VUV spectral region. The emissions proposed for use in this context span the 485-1124 A wavelength interval, and can be easily excited using low pressure discharge lamps of conventional design and dispersed using a monochromator of moderate resolution.

Morrison, M. D.↗

NASA NTRS1983

EUV branching ratios for ionized nitrogen and oxygen emissions

Branching ratios of single ionized atomic nitrogen and oxygen EUV emissions that terminate on the metastable states of the respective ions are presented. The four NII ratios and the 482/515 A ratio in OII reported represent the first measured values. Details of the wavelength calibration procedures adopted and a brief discussion of two modeling implications of the measured ratios are included.

Morrison, M. D.↗

NASA NTRS1982

Long term variability of transmission of thin In-Sn and Sn-C films for EUV instrumentation

The transmission variability is reported for two filters in the EUV, where the filters were supported on an 80% transmission nickel mesh and fabricated by means of a vacuum deposition technique. The first film consisted of 2500 A-thick indium, deposited on 500 A-thick tin, and had its primary bandpass in the 740-960 A range. The second consisted of 1800 A-thick tin and 2000 A-thick carbon, with a bandpass of 500-750 A. At the end of 10 months, the transmission of both filters was measured to check variations. It is demonstrated that the EUV transmission of such filters can change over the typical lifetime of satellite missions, and that such variation depends on the storage environment. The use of vacuum, rather than dry nitrogen storage is recommended.

Chakrabarti, S.↗

NASA NTRS1982

High-resolution dayglow O I /1304 A/ and O I /989 A/ rocket observations

High-resolution (about 1 A) dayglow observations of the O I(1304) A) and O I(989 A) multiplets were made from an Astrobee-F rocket payload (25.046 CE) launched from White Sands, New Mexico at local noon on June 27, 1980 to an apogee of 260 km. Three components of the O I(1304 A) multiplet were measured at zenith angles of approximately 50 and 140 deg at 1.1 A resolution. Over the entire altitude range of observation, i.e., 100-260 km, the three components were found to be equal to within + or - 15%. The shape of the O I(989 A) multiplet distribution observed on the flight at 1.3 A resolution was indistinguishable from an optically thin source with the energy levels of the excited state populated according to their statistical weights in spite of the large optical depths of the O I(989 A) multiplet at rocket altitudes.

Christensen, A. B.↗

NASA NTRS1981

Spectroscopy of the extreme ultraviolet dayglow during active solar conditions

The spectral characteristics of the mid-latitude daytime airglow observed between 530 and 1500 A under conditions of high solar activity are compared with those obtained at the same location during markedly lower solar activity. The spectral observations were made by two scanning spectrometers and an N2 3371 A photometer carried aboard Astrobee-F rockets launched from White Sands Missile Range, NM, on January 9, 1978 and June 27, 1980. The more recent data allow the partial resolution of the emission spectrum between 800 and 1200 A into a large number of weak N I, O I and N2 transitions. Data taken at 220 km altitude suggest an increase in atomic nitrogen density of more than a factor of 3 between the two observations, along with a doubling of the solar EUV flux at wavelengths less than 688 A. No evidence of a corresponding increase in the 10 to 50 eV photoelectron flux is found, however, an ionospheric sounding data indicate the peak electron density to have decreased during this period. The mechanism for this electron flux decrease in the face of increased EUV flux and only a three-fold increase in N concentration remains unknown.

Gentieu, E. P.↗

NASA NTRS1981

Measured branching ratios for O II2D and 2P transitions in the wavelength range 530 to 800 A

Branching ratios for four sets of extreme ultraviolet transitions terminating on the 2D0 and 2P0 metastable levels of ionized oxygen have been measured. The emissions were excited in both an open window hollow cathode and a capillary discharge lamp, and the branching ratios were derived from the observed intensity ratios of the multiplet pairs. The results are in good agreement with theoretical values and compare favorably, within experimental uncertainties, with line ratios obtained by EUV spectroscopy of the airglow.

Morrison, D.↗

NASA NTRS1980

Ultraviolet downconverting phosphor for use with silicon CCD imagers

The properties and application of a UV downconverting phosphor (coronene) to silicon charge coupled devices are discussed. Measurements of the absorption spectrum have been extended to below 1000 A, and preliminary results indicate the existence of useful response to at least 584 A. The average conversion efficiency of coronene was measured to be approximately 20% at 2537 A. Imagery at 3650 A using a backside illuminated 800 x 800 CCD coated with coronene is presented.

Blouke, M. M.↗

NASA NTRS1979

Some implications of revised OI /7990 A/ and OI /11287 A/ branching ratios

Recent laboratory, geophysical, and theoretical determinations of the O I (7990 A) and O I (11,287 A) branching ratios indicate that the generally used values based on the NBS transition parameters need revision. The O I (7990 A) branching ratio is about 300 times smaller than the NBS value, and the O I (11,287 A) is about one-quarter as large. The transitions are important in the generation of permitted atomic oxygen spectra in several types of astronomical source, including the Orion Nebula (M42), the Seyfert galaxy NGC 4151, Arcturus, and comets. The absence of the O I (7990 A) emission in M42 and NGC 4151 is consistent with its very small branching ratio, and previous conclusions based on its absence, such as the rejection of starlight excitation in the Seyfert galaxy, require reinvestigation. The reduction of the O I (11,287 A) branching ratio is pertinent to hydrogen L-beta fluorescence excitation of O I (1304 A) and O I (8446 A) emission and requires an upward revision of oxygen production rates in comets.

Christensen, A. B.↗

NASA NTRS1978

Laboratory study of the O I /7990 A/ branching ratio

Branching ratios for several atomic oxygen 3p 3D transitions have been studied. A value of 0.0004 + or - 20% for the O I (7990 A) branching ratio was obtained, which is much smaller than the previously accepted value and in accord with recent calculations and interpretations of the auroral extreme-ultraviolet spectra. Measurements of the O I (7002 A) and O I (11,287 A) multiplets give branching ratios of 0.05 and 0.10, which are in better accord with calculations. As part of this experiment, results of a study of the highly structured O2 photoabsorption cross section in the neighborhood of the multiplets are also reported.

Christensen, A. B.↗