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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
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NASA NTRS1980

The Occurence Rate, Polarization Character, and Intensity of Broadband Jovian Kilometric Radiation

The major observational features of one new component of Jupiter's radio emission spectrum, the broadband kilometer-wavelenth radiation or bKOM are described. The Voyager planetary radio astronomy experiments reveal that the overall occurrence morphology, total power, and polarization character of bKOM are strong functions of the latitude and/or local time geometry of the observations. The post-encounter data show a decline in the mean occurrence rates and power level of bKOM and, in particular, a depletion in the occurrence rate at those same longitudes where the detection rate is a maximum before encounter. Additionally, the polarization sense undergoes a permanent reversal in sign after encounter, whereas the time-averaged wave axial ratio and degrees of polarization remain relatively unchanged. No evidence of any control by Io is found. The strong dependence of the morphology on local time suggests a source whose beam is nearly fixed relative to the Jupiter-sun line.

Desch, M. D.↗
NASA NTRS1980

The occurrence rate, polarization character, and intensity of broadband Jovian kilometric radiation

The paper describes the major observational features of one new component of Jupiter's radio emission spectrum, the broadband kilometer wavelength radiation, or bKOM. This study, using the Voyager Planetary Radio Astronomy (PRA) experiments, reveals that the overall occurrence morphology, dynamic spectra, and polarization character of bKOM are strong functions of the latitude and/or local time geometry of the observations. The postencounter data show a decline in the mean occurrence rates and power level of bKOM and, in particular, a depletion in the occurrence rate at those same longitudes where the detection rate is a maximum before encounter. Additionally, the polarization sense undergoes a permanent reversal in sign after encounter, whereas the time-averaged wave axial ratio and degree of polarization remain relatively unchanged. Finally, no evidence of any control by Io is found. The strong dependence of the morphology on local time suggests a source whose beam is nearly fixed relative to the Jupiter-sun line

Desch, M. D.↗
NASA NTRS1990

Dust and ionized gas in active radio elliptical galaxies

The authors present broad and narrow bandwidth imaging of three southern elliptical galaxies which have flat-spectrum active radio cores (NGC 1052, IC 1459 and NGC 6958). All three contain dust and extended low excitation optical line emission, particularly extensive in the case of NGC 1052 which has a large H alpha + (NII) luminosity. Both NGC 1052 and IC 1459 have a spiral morphology in emission-line images. All three display independent strong evidence that a merger or infall event has recently occurred, i.e., extensive and infalling HI gas in NGC 1052, a counter-rotating core in IC 1459 and Malin-Carter shells in NGC 6958. This infall event is the most likely origin for the emission-line gas and dust, and the authors are currently investigating possible excitation mechanisms (Sparks et al. 1990).

Forbes, D. A.↗
NASA NTRS2023

Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J Burst Observations

Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined, as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Frequency-ARray (LOFAR) and Parker Solar Probe (PSP) solar radio dynamic spectrum to identify 27 type III bursts and 27 J-bursts during a solar radio noise storm observed on 10 April 2019. We found that their exciter velocities were similar, implying a common acceleration region that injects electrons along open and closed magnetic structures. We describe a novel technique to estimate the density model in coronal loops from J-burst dynamic spectra, finding typical loop apex altitudes around 1.3 solar radius. At this altitude, the average scale heights were 0.36 solar radius, the average temperature was around 1 MK, the average pressure was 0.7mdyn cm -2 , and the average minimum magnetic field strength was 0.13 G. We discuss how these parameters compare with much smaller coronal loops.

Jinge Zhang↗
NASA NTRS2013

Radio Detection of the Fermi-LAT Blind Search Millisecond Pulsar J1311-3430

We report the detection of radio emission from PSR J1311.3430, the first millisecond pulsar (MSP) discovered in a blind search of Fermi Large Area Telescope (LAT) gamma-ray data. We detected radio pulsations at 2 GHz, visible for less than 10% of approximately 4.5 hr of observations using the Green Bank Telescope (GBT). Observations at 5 GHz with the GBT and at several lower frequencies with Parkes, Nan cay, and the Giant Metrewave Radio Telescope resulted in non-detections. We also report the faint detection of a steep spectrum continuum radio source (0.1 mJy at 5 GHz) in interferometric imaging observations with the Jansky Very Large Array. These detections demonstrate that PSR J1311.3430 is not radio quiet and provide additional evidence that radio-quiet MSPs are rare. The radio dispersion measure of 37.8 pc cm(exp -3) provides a distance estimate of 1.4 kpc for the system, yielding a gamma-ray efficiency of 30%, typical of LAT-detected MSPs. We see apparent excess delay in the radio pulses as the pulsar appears from eclipse and we speculate on possible mechanisms for the non-detections of the pulse at other orbital phases and observing frequencies.

Ray, P. S.↗
NASA NTRS1972

An experimental investigation of the power spectrum of phase modulation induced on a satellite radio signal by the ionosphere

The power spectrum of phase modulation imposed upon satellite radio signals by the inhomogeneous F-region of the ionosphere (100 - 500 km) was studied. Tapes of the S-66 Beacon B Satellite recorded during the period 1964 - 1966 were processed to yield or record the frequency of modulation induced on the signals by ionospheric dispersion. This modulation is produced from the sweeping across the receiving station as the satellite transits of the two dimensional spatial phase pattern are produced on the ground. From this a power spectrum of structure sizes comprising the diffracting mechanism was determined using digital techniques. Fresnel oscillations were observed and analyzed along with some comments on the statistical stationarity of the shape of the power spectrum observed.

Moser, D. T.↗
NASA NTRS1981

Power spectrum of small-scale density irregularities in the interstellar medium

Observations of the interstellar scintillation of radiation from 17 pulsars are reported which are used to place limits on the power spectrum of small-scale electron density irregularities in the interstellar medium. Measurements made at 340, 408, and 450 MHz in the dispersion measure range 3-57/cu cm pc of the time-dependent radio frequency spectrum of interstellar scintillations were analyzed to determine the scintillation index and a scintillation frequency-correlation scale based on the autocorrelation function in radio frequency of the fluctuations in scintillation. The dispersion-measure dependence of the scintillation frequency correlation scale is found to be consistent with both Gaussian model interstellar medium spectra and power-law spectra with indices between 3.0 and 4, while the radio-frequency scaling of the frequency correlation scale is consistent with power law indices between 2.8 and 3.9. However comparison of the shape of the radio-frequency autocorrelation function with model calculations indicates power law models with indices greater than 3.6 are possible. Data are also consistent with a local three-dimensional density spectrum at a wave number of 10 to the -9th/m of 3 x 10 to the 28th to 3 x 10 to the 29th/cu m.

Armstrong, J. W.↗
NASA NTRS1988

Radio properties of the highly polarized, quiescent quasar OI 287

The highly polarized, quiescent quasar OI 287 has been observed with high resolution at 2 cm and 6 cm in an effort to determine the origin of some of the object's peculiar properties. The results seem to rule out the classification of OI 287 as a blazar. Extrapolation of the radio core spectrum to midinfrared wavelengths fails to predict the infrared flux by at least an order of magnitude. This supports the conclusion that the infrared emission and the radio emission do not originate in the same synchrotron-emitting component. The high optical polarization could be related to possible broad absorption lines in the unobserved UV regime, or it could be caused by reflection into the line of sight by scattering agents distributed in a thin disk.

Ulvestad, James S.↗
NASA NTRS1981

PKS 2155-304 relativistically beamed synchrotron radiation from BL LAC object

The newly discovered BL Lacertae object, PKS 2155-304, was observed with the medium and high intensity energy detectors of the HEAO-1 A2 experiment. The variability by a factor of two in less than a day reported by Snyder, et al (1979) is confirmed. Two spectra, obtained a year apart, while the satellite was in scanning mode, are well fit by simple power laws with energy spectral index alpha sub 1 equals approximately 1.4. A third spectrum, of higher statistical quality, obtained while the satellite was pointed at its source, has has two components. An acceptable fit was obtained using a two power law model, with indices alpha sub 1 equals 2.0 (+1.2, -0.6) and alpha sub 2 equals -1.5 (+1.5, -2.3). An interpretation of the overall spectrum from radio through X-rays in terms of a synchrotron self-Compton model gives a good description of the data if allowance is made for relativistic beaming. Thus, from a consideration of the spectrum, combined with an estimate of the size of the source, the presence of jets is inferred without their observation.

Urry, C. M.↗
NASA NTRS1982

PKS 2155-304 - Relativistically beamed synchrotron radiation from a BL Lacertae object

The previously reported variability by a factor of two in less than a day is confirmed for the newly discovered BL Lac object PKS 2155-304, by means of the HEAO 1 A-2 experiment's medium- and high-energy detectors. An interpretation of the overall spectrum from radio through X-rays in terms of a synchrotron self-Compton model yields a good description of the data if relativistic beaming is allowed for. The presence of jets is inferred without their direct observation through consideration of the spectrum, combined with an estimate of the size of the source.

Urry, C. M.↗
NASA NTRS1987

Superluminal radio sources - What does X-ray emission tell us?

In a study on superluminal radio sources, statistical relationships between X-ray, optical, and radio luminosities among different categories of active galactic nuclei are compared to search for common energy mechanisms. The X-ray versus optical and X-ray versus radio correlations of radio-loud QSOs and superluminal radio sources are found to be similar, arguing against a model in which the emission in only one or two of the three wave bands is relativistically boosted. A regression analysis shows that highly polarized QSOs and optically violently variable QSOs are more similar to other flat-spectrum, radio-loud QSOs than to BL Lac objects, and it is reasonable to assume that self-Compton emission dominates the X-ray emission from at least half of the sources in this class. The X-ray versus radio correlation for BL Lac objects is poor, and there is support for the hypothesis that their X-ray emission is dominated by an isotropic component which is not directly related to relativistically boosted radio emission.

Worrall, Diana M.↗
NASA NTRS1973

Attenuation and phase dispersion in the atmosphere due to the microwave spectrum of oxygen

Radio wave propagation in the 40- to 140-GHz band through the earth's atmosphere is strongly influenced by the behavior of the O2 microwave spectrum. This behavior causes the transfer function to depend critically upon altitude. The spectroscopic properties of O2 are discussed and reduced to engineering formulas expressing attenuation and phase dispersion rates in terms of frequency and meteorological parameters. The theory is supported on several accounts by reliable spectroscopic measurements. Pressure scanning spectroscopy is used to investigate the O2 microwave spectrum under simulated atmospheric conditions. The application of spectroscopic information to analytical treatments of transfer properties for inhomogeneous slant paths is demonstrated. Attenuation and phase dispersion between 49 and 72 GHz are evaluated for zenith and tangential paths.

Liebe, H. J.↗
NASA NTRS2012

Coronal Magnetic Field Measurement from EUV Images Made by the Solar Dynamics Observatory

By measuring the geometrical properties of the coronal mass ejection (CME) flux rope and the leading shock observed on 2010 June 13 by the Solar Dynamics Observatory (SDO) mission's Atmospheric Imaging Assembly we determine the Alfven speed and the magnetic field strength in the inner corona at a heliocentric distance of approx. 1.4 Rs The basic measurements are the shock standoff distance (Delta R) ahead of the CME flux rope, the radius of curvature of the flux rope (R(sub c)), and the shock speed. We first derive the Alfvenic Mach number (M) using the relationship, Delta R/R(sub c) = 0.81[(gamma−1) M(exp 2) + 2] / [(gamma +1)(M2 − 1)], where gamma is the only parameter that needed to be assumed. For gamma = 4/3, the Mach number declined from 3.7 to 1.5 indicating shock weakening within the field of view of the imager. The shock formation coincided with the appearance of a type II radio burst at a frequency of approx. 300 MHz (harmonic component), providing an independent confirmation of the shock. The shock compression ratio derived from the radio dynamic spectrum was found to be consistent with that derived from the theory of fast-mode MHD shocks. From the measured shock speed and the derived Mach number, we found the Alfven speed to increase from approx 140 km/s to 460 km/s over the distance range 1.2-1.5 Rs. By deriving the upstream plasma density from the emission frequency of the associated type II radio burst, we determined the coronal magnetic field to be in the range 1.3-1.5 G. The derived magnetic field values are consistent with other estimates in a similar distance range. This work demonstrates that the EUV imagers, in the presence of radio dynamic spectra, can be used as coronal magnetometers

magnetic fields↗
NASA NTRS1981

Detection of radio emission from the jet in Centaurus A

The VLA has detected radio emission from the X-ray jet in Centaurus A, at 20 and 6 cm, whose radio morphology is similar to that of the X-ray jet. It is suggested that the same population of relativistic electrons is responsible for both radio and X-ray synchrotron emission, in which case in situ acceleration of electrons in the knots would be mandatory. The relativistic beam may alternatively heat the surrounding gas, resulting in X-ray emission. The static confinement of the knots of the jet seems to be accomplished by the presence of the ambient hot gas in the galaxy. The galaxy's nucleus has an inverted spectrum at radio frequencies, and it is noted that the jet is as bright as the nucleus at low frequencies.

Schreier, E. J.↗