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At least 397 records · Page 22
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NASA NTRS2006

Difficulties in Laboratory Studies and Astronomical Observations of Organic Molecules: Hydroxyacetone and Lactic Acid

For the past 35 years, radio astronomy has revealed a rich organic chemistry in the interstellar gas, which is exceptionally complex towards active star-forming regions. New solar systems condense out of this gas and may influence the evolution of life on newly formed planets. Much of the biologically important functionality is present among the some 130 gas-phase molecules found to date, including alcohols, aldehydes, ketones, acids, amines, amides and even the simplest sugar - glycolaldehyde. Still, many unidentified interstellar radio signals remain, and their identification relies on further laboratory study. The molecules hydroxyacetone and lactic acid are relatively small organic molecules, but possess rather complex rotational spectra owing to their high asymmetry. Hydroxyacetone is particularly problematic because it possess a very low barrier to internal rotation, and exhibits strong coupling of the free-rotor states with the overall rotation of the molecule. As in the case of acetamide, a full decomposition method was employed to order the resultant eigenstates onto normal asymmetric top eigenvectors.

Apponi, A. J.↗
NASA NTRS1987

Models of Uranium continuum radio emission

Uranium continuum radio emission detected by the Voyager 2 Planetary Radio Astronomy experiment during the January 1986 encounter is considered. The continuum emissions comprised four components (equatorial emissions, anomaly emissions, strong nightside emissions, and weak nightside emissions) associated with different sources. The equatorial emissions appeared most prominently during the days before closest approach and extended from 40 kHz or below to about 120 kHz. The anomaly emissions were seen about 12 hours before closest approach and extended to about 250 kHz. The agreement found between Miranda's phase and strong radio emission at 20.4 kHz, just after closest approach, suggests intense dynamic activity on the Miranda L shell.

Romig, Joseph H.↗
NASA NTRS2001

RXTE, VLBA, Optical, and Radio Monitoring of the Quasars 3C 279, PKS 1510--089, and 3C 273

We are continuing our combined RXTE X-ray, VLBA imaging (at 43 GHz), optical (several observatories), and radio (University of Michigan Radio Astronomy Observatory) monitoring of the quasars 3C 279 and PKS 1510-089, and have started similar monitoring of 3C 273. X-ray flares in 3C 279 and PKS 1510-089 are associated with ejections of superluminal components. In addition, there is a close connection between the optical and X-ray variability of 3C 279. There is a strong correlation between the 14.5 GHz and X-ray variability of PKS 1510-089 in 1997 and 1998 (with the radio leading the X-ray) that becomes weaker in subsequent years. X-ray fluctuations occur on a variety of timescales in 3C 273, with a major prolonged outburst in mid-2001. The lead author will discuss the correlations in terms of inverse Compton models for the X-ray emission coupled with synchrotron models for the lower-frequency radiation. Synchrotron self-Compton models can explain the "reverse" time lag in PKS 1510-089 is well as the variable correlation between the X-ray variations and those at lower frequencies in this object and in 3C 279.

Marscher, A. P.↗
NASA NTRS2013

Capabilities, Performance, and Status of the SOFIA Science Instrument Suite

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory, carrying a 2.5 m telescope onboard a heavily modified Boeing 747SP aircraft. SOFIA is optimized for operation at infrared wavelengths, much of which is obscured for ground-based observatories by atmospheric water vapor. The SOFIA science instrument complement consists of seven instruments: FORCAST (Faint Object InfraRed CAmera for the SOFIA Telescope), GREAT (German Receiver for Astronomy at Terahertz Frequencies), HIPO (High-speed Imaging Photometer for Occultations), FLITECAM (First Light Infrared Test Experiment CAMera), FIFI-LS (Far-Infrared Field-Imaging Line Spectrometer), EXES (Echelon-Cross-Echelle Spectrograph), and HAWC (High-resolution Airborne Wideband Camera). FORCAST is a 540 m imager with grism spectroscopy, developed at Cornell University. GREAT is a heterodyne spectrometer providing high-resolution spectroscopy in several bands from 60240 m, developed at the Max Planck Institute for Radio Astronomy. HIPO is a 0.31.1 m imager, developed at Lowell Observatory. FLITECAM is a 15 m wide-field imager with grism spectroscopy, developed at UCLA. FIFI-LS is a 42210 m integral field imaging grating spectrometer, developed at the University of Stuttgart. EXES is a 528 m high-resolution spectrograph, developed at UC Davis and NASA ARC. HAWC is a 50240 m imager, developed at the University of Chicago, and undergoing an upgrade at JPL to add polarimetry capability and substantially larger GSFC detectors. We describe the capabilities, performance, and status of each instrument, highlighting science results obtained using FORCAST, GREAT, and HIPO during SOFIA Early Science observations conducted in 2011.

SOFIA↗
NASA NTRS1992

Low Frequency Interstellar Scattering and Pulsar Observations

Radio astronomy at frequencies from 2 to 30 MHz challenges time tested methods for extracting usable information from observations. One fundamental reason for this is that propagation effects due to the magnetoionic ionosphere, interplanetary medium, and interstellar matter (ISM) increase strongly with wavelength. The problems associated with interstellar scattering off of small scale irregularities in the electron density are addressed. What is known about interstellar scattering is summarized on the basis of high frequency observations, including scintillation and temporal broadening of pulsars and angular broadening of various galactic and extragalactic radio sources. Then those high frequency phenomena are addressed that are important or detectable at low frequencies. The radio sky becomes much simpler at low frequencies, most pulsars will not be seen as time varying sources, intensity variations will be quenched or will occur on time scales much longer than a human lifetime, and many sources will be angularly broadened and/or absorbed into the noise. Angular broadening measurements will help delineate the galactic distribution and power spectrum of small scale electron density irregularities.

Cordes, James M.↗
NASA NTRS1991

Evidence of auroral plasma cavities at Uranus and Neptune from radio burst observations

Radio bursts originating from the stronger magnetic polar regions of both Uranus and Neptune were detected by the planetary radio astronomy experiment during the Voyager 2 encounters with the planets. It has previously been demonstrated that these bursts are beamed into a broad, hollow emission pattern from their auroral sources. It is now shown that the bursts at both planets also manifest similar detailed patterns, with the waves beamed into two separate and distinct radiation cones at intermediate wave frequencies. This double-cone emission pattern is predicted by relativistic cyclotron resonance theory, and application of this theory to the observed emission pattern yields the plasma density structure within the radio source region. Calculations indicate that at both Uranus and Neptune the plasma-to-cyclotron frequency ratio can drop well below 0.01 within the active region. Such low values indicate that the southern auroral zones at both planets contain an auroral plasma cavity that is similar to that found in earth's nightside auroral zone.

Farrell, W. M.↗
NASA NTRS1975

Radio detection of thunderstorm activity with an earth-orbiting satellite

A study was made to determine the feasibility of using artificial earth satellites to monitor thunderstorm activity. The nighttime noise-temperature measurements made with the earth-oriented vee antenna of the Radio Astronomy Explorer (RAE 1) satellite in the frequency range 0.2-9.2 MHz were correlated with reported surface thunderstorm activity. Analysis shows that the minimum nighttime HF noise level (in the absence of surface thunderstorms) at an altitude of 5850 km over the United States is fixed by man-made noise. When thunderstorms are active below the satellite, the noise level is increased by about 6-12 dB. The highest level is associated with the most intense storms. It is concluded that thunderstorm regions can be detected by an orbiting satellite using HF radio techniques, but ionospheric effects must be taken into account.

Herman, J. R.↗
NASA NTRS1990

Astrophysics from the moon; Proceedings of the Workshop, Annapolis, MD, Feb. 5-7, 1990

The present conference on astrophysics from the moon encompasses the study of the Galaxy, external planetary systems, solar physics, stars and stellar evolution, the frontiers of Galactic, extragalactic, and cosmological astronomy, an introduction to lunar-based astronomy, concepts for lunar observatories including high-energy observatories, solar observatories, and observatories for particle astrophysics and gravitational studies. Specific issues addressed include the dynamics of Jovian atmospheres, planetary magnetospheres, flare physics, exobiology and SETI from the lunar farside, and the study of interactive stars, star formation, H II regions in absorption at low frequencies, and normal galaxies. Also addressed are the potential lunar investigation of quasars, the formation epoch, and the large-scale structure of the universe, and observational issues related to X-ray large arrays, optical interferometers, VLF radio astronomy, a UV-solar reflecting coronagraph, and a heavy-nucleus detector.

Mumma, Michael J.↗
NASA NTRS1995

Neptune radio emission in dipole and multipole magnetic fields

We study Neptune's smooth radio emission in two ways: we simulate the observations and we then consider the radio effects of Neptune's magnetic multipoles. A procedure to deduce the characteristics of radio sources observed by the Planetary Radio Astronomy experiment minimizes limiting assumptions and maximizes use of the data, including quantitative measurement of circular polarization. Study of specific sources simulates time variation of intensity and apparent polarization of their integrated emission over an extended time period. The method is applied to Neptune smooth recurrent emission (SRE). Time series are modeled with both broad and beamed emission patterns, and at two frequencies which exhibit different time variation of polarization. These dipole-based results are overturned by consideration of more complex models of Neptune's magnetic field. Any smooth emission from the anticipated auroral radio source is weak and briefly observed. Dominant SRE originates complex fields at midlatitude. Possible SRE source locations overlap that of 'high-latitude' emission (HLE) between +(out) and -(in) quadrupoles. This is the first identification of multipolar magnetic structure with a major source of planetary radio emission.

Sawyer, C. B.↗
NASA NTRS1978

Low frequency spectra of type III solar radio bursts

Flux density spectra have been determined for 91 simple type III solar bursts observed by the Goddard Space Flight Center radio astronomy experiment on the IMP-6 spacecraft during 1971 and 1972. Spectral peaks were found to occur at frequencies ranging from 44 kHz up to 2500 kHz. Half of the bursts peaked between 250 kHz and 900 kHz, corresponding to emission at solar distances of about 0.3 to 0.1 AU. Maximum burst flux density sometimes exceeds 10 to the -14th W/sq m/Hz. The primary factor controlling the spectral peak frequency of these bursts appears to be a variation in intrinsic power radiated by the source as the exciter moves outward from the sun, rather than radio propagation effects between the source and IMP-6. Thus, a burst spectrum strongly reflects the evolution of the properties of the exciting electron beam, and according to current theory, beam deceleration could help account for the observations.

Weber, R. R.↗
NASA NTRS1987

Spacelab-2 plasma depletion experiments for ionospheric and radio astronomical studies

The Spacelab-2 Plasma Depletion Experiments were a series of studies to examine Shuttle-induced perturbations in the ionosphere and their application to ground-based radio astronomy. The Space Shuttle Challenger fired its orbital maneuvering subsystem engines, releasing large amounts of exhaust molecules that caused the electrons and ions in earth's upper atmosphere to chemically recombine, thereby creating so-called 'ionospheric holes'. Two burns conducted over New England produced ionospheric peak depletions ranging from 25 to 50 percent, affected the ionosphere over a 200-kilometer altitude range, and covered 1 to 2 deg of latitude. Optical emissions associated with the hole spanned an area of several hundred thousand square kilometers. A third burn was conducted over a low-frequency radio observatory in Hobart, Australia, to create an 'artificial window' for ground-based observations at frequencies normally below the natural ionospheric cutoff (penetration) frequency. The Hobart experiment succeeded in making high-resolution observations at 1.7 megahertz through the induced ionospheric hole.

Mendillo, M.↗
NASA NTRS2022

A Census of Young Stellar Objects in Two Line-of-Sight Star-Forming Regions Toward IRAS 22147+5948 in the Outer Galaxy

Context. Star formation in the outer Galaxy, namely, outside of the Solar circle, has not been extensively studied in part due to the low CO brightness of the molecular clouds linked with the negative metallicity gradient. Recent infrared surveys provide an overview of dust emission in large sections of the Galaxy, but they suffer from cloud confusion and poor spatial resolution at far-infrared wavelengths. Aims. We aim to develop a methodology to identify and classify young stellar objects (YSOs) in star-forming regions in the outer Galaxy and use it to resolve a long-standing disparity in terms of the distance and evolutionary status of IRAS 22147+5948. Methods. We used a support vector machine learning algorithm to complement standard color–color and color–magnitude diagrams in our search for YSOs in the IRAS 22147 region, based on publicly available data from the Spitzer Mapping of the Outer Galaxy survey. The agglomerative hierarchical clustering algorithm was used to identify clusters. Then the physical properties of individual YSOs were calculated. The distances were determined using CO 1–0 from the Five College Radio Astronomy Observatory survey. Results. We identified 13 Class I and 13 Class II YSO candidates using the color–color diagrams, along with an additional 2 and 21 sources, respectively, using the applied machine learning techniques. The spectral energy distributions of 23 sources were modeled with a star and a passive disk, corresponding to Class II objects. The models of three sources include envelopes that are typical for Class I objects. The objects were grouped into two clusters located at a distance of 2:2 kpc and 5 clusters at 5:6 kpc. The spatial extent of CO, radio continuum, and dust emission confirms the origin of YSOs in two distinct star-forming regions along a similar line of sight. Conclusions. The outer Galaxy may serve as a unique laboratory for exploring star formation across environments, on the condition that complementary methods and ancillary data are used to properly account for cloud confusion and distance uncertainties.

Agata Karska↗
NASA NTRS1985

Saturn radio emission and the solar wind - Voyager-2 studies

Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the relationship between parameters of the solar wind/interplanetary medium and the nonthermal Saturn radiation. Solar wind and interplanetary magnetic field properties were combined to form quantities known to be important in controlling terrestrial magnetospheric processes. The Voyager 2 data set used in this investigation consists of 237 days of Saturn preencounter measurements. However, due to the immersion of Saturn and the Voyager 2 spacecraft into the extended Jupiter magnetic tail, substantial periods of the time series were lacking solar wind data. To cope with this problem a superposed epoch method (CHREE analysis) was used. The results indicate the superiority of the quantities containing the solar wind density in stimulating the radio emission of Saturn - a result found earlier using Voyager 1 data - and the minor importance of quantities incorporating the interplanetary magnetic field.

Desch, M. D.↗
NASA NTRS1973

Applications of radio interferometry to navigation

Radio astronomy experiments have demonstrated the feasibility of making precise position measurements using interferometry techniques. The application of this method to navigation and marine geodesy is discussed, and comparisons are made with existing navigation systems. The very long baseline technique, with a master station, can use either an artificial satellite or natural sources as position references; a high-speed data link is required. A completely ship-borne system is shown to be feasible, at the cost of poorer sensitivity for natural sources. A comparison of Doppler, delay and phase-track modes of operating a very long baseline configuration is made, as that between instantaneous measurements and those where a source can be tracked from horizon to transit. Geometric limitations in latitude and longitude coverage are discussed. The characteristics of natural radio sources, their flux, distribution on the sky, and apparent size are shown to provide a limit on position measurements precision. The atmosphere and frequency standard used both contribute to position measurement uncertainty by affecting interferometric phase.

Knowles, S. H.↗
NASA NTRS1982

Facilities for US radioastronomy

An overview of the radio-astronomy field is given, and prospects ready for construction at NASA are presented. A very-long-baseline array consisting of ten 25 m antennas, with a limiting wavelength of 7 mm and an angular resolution at that wavelength of 2 x 10 to the 4th arcsec is discussed. Eighty percent of the phase information will be obtained by closure around the 36 independent triangles, and high quality aperture-synthesis maps will be produced at all wavelengths. The 25 m telescope will be capable of several applications including the discovery of new molecules in our galaxy (in particular, the envelope of the evolved carbon star IRC + 10216), the detection of CO to distances of perhaps 100 million light years, and the understanding of the events which occur as stars are formed from molecular clouds, and as energy is fed back into the molecular gas by new stars. The submillimeter-wave telescope contains the last atmospheric radio windows where astronomical observations can be made from the earth's surface. The need for funding is stressed.

Thaddeus, P.↗
NASA NTRS1980

The formation of arcs in the dynamic spectra of Jovian decameter bursts

A model is presented that can account for several features of the dynamic spectral arcs observed at decameter wavelengths by the planetary radio astronomy experiment on Voyagers 1 and 2. It is shown that refraction of an extraordinary mode wave initially excited nearly orthogonal to the local magnetic field is significantly influenced by the local plasma density, being greater the higher the density. It is assumed that the source of the decameter radiation lies along the L = 6 flux tube and that the highest frequencies are produced at the lowest altitudes, where both the plasma density and magnetic field gradients are largest. It is further assumed that the decameter radiation is emitted into a thin conical sheet, consistent with both observation and theory. In the model the emission cone angle of the sheet is chosen to vary with frequency so that it is relatively small at both high and low frequencies, but approximately 80 deg at intermediate frequencies. The resulting emission pattern as seen by a distant observer is shown to resemble the observed arc pattern. The model is compared and contrasted with examples of Voyager radio data.

Goldstein, M. L.↗
NASA NTRS1995

Changes in Jupiter's 13-cm Synchrotron Radio Emission Following the Impact of Comet Shoemaker-Levy-9

Results of an observing program to monitor the synchrotron radio emission from Jupiter's inner radiation belts before, during and after the impact of Comet SL-9 are reported. The observations were made at 2295 MHz as part of the NASA-JPL Jupiter Patrol, a long-term radio astronomy monitoring program begun in 1971. The data indicate that the intensity of the synchrotron emission at 13 cm wavelength increased by 27 percent within a few days after the comet impacts; the longitudinal beaming curve was distorted during the week of impacts; the magnetic latitude beaming curves flattened after the week of impacts suggesting an increase in the emission at higher magnetic latitudes; and the decay of the enhanced emission is consistent with an exponential with a time constant of ~125 days. The reported changes following the SL-9 impact are unprecedented in the 23-year history of the Jupiter Patrol.

Synchrotron↗
NASA NTRS2004

On-the-Fly Mapping for Calibrating Directional Antennas

An improved method of calibrating a large directional radio antenna of the type used in deep-space communication and radio astronomy has been developed. This method involves a raster-scanning-and-measurement technique denoted on-the-fly (OTF) mapping, applied in consideration of the results of a systematic analysis of the entire measurement procedure. Phenomena to which particular attention was paid in the analysis include (1) the noise characteristics of a total-power radiometer (TPR) that is used in the measurements and (2) tropospherically induced radiometer fluctuations. The method also involves the use of recently developed techniques for acquisition and reduction of data. In comparison with prior methods used to calibrate such antennas, this method yields an order-of-magnitude improvement in the precision of determinations of antenna aperture efficiency, and improvement by a factor of five or more in the precision of determination of pointing error and beam width. Prerequisite to a meaningful description of the present method is some background information concerning three aspects of the problem of calibrating an antenna of the type in question: In OTF mapping measurements in which a TPR is used, the desired data are the peak temperature corresponding to a radio source, the pointing offset when the antenna is commanded to point toward the source, and the shape of the main lobe of the antenna beam, all as functions of the antenna beam elevation and azimuth angles. These data enable one to calculate the (1) antenna aperture efficiency by comparing the measured peak temperature with that expected for a 100-percent-efficient antenna, (2) the mechanical pointing error resulting from small misalignments of various parts of the antenna structure, and (3) misalignments of the antenna subreflector and other mirrors. For practical reasons having to do with obtaining adequate angular resolution and all-sky coverage, it is necessary to perform azimuth and elevation scans fairly rapidly. Many natural radio sources used in calibrating antennas are only approximately pointlike: some sources subtend angles greater than the beam width of a given antenna. In such a case, the antenna partially resolves the source structure and does not collect all of the radiation emitted by the source. This makes it necessary to estimate how much of the total known radiation from the source would actually be collected by the antenna if it were 100-percent efficient. The resulting estimate, leading to a source-size correction factor, introduces another degree of uncertainty to the measurements. OTF mapping can remove this uncertainty

Rochblatt, David↗