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NASA NTRS1984

Radar echoes at 2.66 and 40.92 MHz from the mesosphere, part 2.6A

During recent decades, the ionospheric D region has been scanned extensively by radar in the frequency range from 1 to 60 MHz. Progress has been made in understanding the reflection/scattering of radio waves in that area. Rocket measurement of ion density irregularities were compared to radar echo observations at 2.75 MHz with the conclusion that the received radar signal was due to scattering from isotropic and homogeneous turbulence in the altitude region between 70 and 95 km. However, scattering cross sections at 2 and 6 mHz suggest that the radar echo from the region below 80 km is in part due to partial reflection from stratified layers. The VHF scattering cross section is aspect sensitive in the D region below about 75 km and tends to be isotropic at higher altitudes. Positive correlations between scattered signal power and signal correlation time (P/C) have been observed by VHF radars in the lower mesosphere, with the conclusion that it might be an additional indication of partial reflection from stratified layers. In the upper mesosphere where the P/C correlation is negative, it is generally believed that the scattering is caused by isotropic turbulence. Radar echoes at the 2.16 and 40.92 MHz ranges are compared, assuming that both result from turbulent scatter. Adjusting the radar Bragg wavelength, it was found that both sets are due to scattering from the same layer of turbulence-generated irregularities.

Royrvik, O.↗

NASA NTRS2010

ARCADE 2 Observations of Galactic Radio Emission

We use absolutely calibrated data from the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE 2) flight in July 2006 to model Galactic emission at frequencies 3, 8, and 10 GHz. The spatial structure in the data is consistent with a superposition of free-free and synchrotron emission. Emission with spatial morphology traced by the Haslam 408 MHz survey has spectral index beta_synch = -2.5 +/- 0.1, with free-free emission contributing 0.10 +/- 0.01 of the total Galactic plane emission in the lowest ARCADE 2 band at 3.15 GHz. We estimate the total Galactic emission toward the polar caps using either a simple plane-parallel model with csc|b| dependence or a model of high-latitude radio emission traced by the COBE/FIRAS map of CII emission. Both methods are consistent with a single power-law over the frequency range 22 MHz to 10 GHz, with total Galactic emission towards the north polar cap T_Gal = 0.498 +/- 0.028 K and spectral index beta = -2.55 +/- 0.03 at reference frequency 0.31 GHz. The well calibrated ARCADE 2 maps provide a new test for spinning dust emission, based on the integrated intensity of emission from the Galactic plane instead of cross-correlations with the thermal dust spatial morphology. The Galactic plane intensity measured by ARCADE 2 is fainter than predicted by models without spinning dust, and is consistent with spinning dust contributing 0.4 +/- 0.1 of the Galactic plane emission at 23 GHz.

Kogut, A.↗

NASA NTRS1990

Numerical simulation of solar wind density fluctuations and their effects on VLF radio interferometry

A mission consisting of an array of small satellites has been proposed to investigate radio sources at frequencies from approximately 2 to 20 MHz, a range unobservable from the ground because of ionospheric absorption and aberrations. Such a mission seems feasible and relatively inexpensive, but further investigation is still necessary. The solar wind is a major concern because it contains turbulence-induced random density variations that cause a fluctuating phase shift in any electromagnetic signal passing through, sometimes making radio source imaging by interferometry impossible. A numerical simulation has been developed which attempts to accurately characterize solar wind density fluctuations. Results have allowed more concrete conclusions to be drawn about observing constraints and have shown that interferometric imaging at radio frequencies below 10 MHz with baselines up to 100 km is possible at solar elongation angles of about 90 deg or more.

Williamson, Robert S., III↗

NASA NTRS1973

A new ion and electron detector for ion cyclotron resonance spectroscopy.

A new detector using an extremelly driven tuned circuit has been developed for use in ion cyclotron resonance spectroscopy experiments. Based on the Q-meter circuit, this detector will operate at frequencies greater than 1 MHz at rf levels less than 1 mV. Operation in the frequency range 1-15 MHz allows the use of higher magnetic fields for more efficient storage of low mass ions in the trapping mode of operation. In the frequency range 2-6 MHz electrons can be detected in the ion cyclotron resonance cell by their resonant motion in the trapping plane.

Huntress, W. T., Jr.↗

NASA NTRS1974

Scientific instrumentation of the Radio-Astronomy-Explorer-2 satellite

The RAE-2 spacecraft has been collecting radio astronomical measurements in the 25 kHz to 13 MHz frequency range from lunar orbit since June, 1973. A summary is given of the technical aspects of the program including the calibration, instrumentation and operation of the RAE-2 experiments. Performance of the experiments over the first 18 months of the flight is summarized and illustrated. Among the unique features of the RAE-2 is the capability to observe repeated lunar occultations of strong radio sources at very low frequencies.

Alexander, J. K.↗

NASA NTRS1975

Directivity of low frequency solar type 3 radio bursts

The occurrence rate of type 3 solar bursts in the frequency range 4.9 MHz to 30 kHz was analyzed as a function of burst intensity and burst arrival direction. Results show that: (1) the occurrence rate of bursts falls off with increasing flux and (2) the distribution of burst arrival directions at each frequency shows a significantly larger number of bursts observed west of the earth-sun line than east of it. This western excess in occurrence rate appears to be correlated with the direction of the average interplanetary magnetic field, and is interpreted as beaming of the observed burst radiation along the magnetic field direction.

Fitzenreiter, R. J.↗

NASA NTRS1976

Directivity of low frequency solar type III radio bursts

The occurrence rate of type III solar bursts in the frequency range 4.9 MHz to 30 kHz is analyzed as a function of burst intensity and burst arrival direction. We find that (1) the occurrence rate of bursts varies inversely with the 1.5 power of the flux, and (2) the distribution of burst arrival directions at each frequency shows a significantly larger number of bursts observed west of the earth-sun line than east of it. This western excess in occurrence rate appears to be correlated with the direction of the average interplanetary magnetic field, and is interpreted as beaming of the observed burst radiation along the magnetic field direction.

Fitzenreiter, R. J.↗

NASA NTRS1977

A refracting radio telescope

Observations of extraterrestrial radio sources at the lower end of the radio frequency spectrum are limited by reflection of waves from the topside ionosphere and by the large size of antenna apertures necessary for the realization of narrow beamwidths. The use of the ionosphere as a lens is considered. The lens is formed by the release of chemicals such as H2 and H2O at the F2-layer peak. These chemicals promote dissociative recombination of O(+) in the ionosphere resulting in a local reduction in plasma density. Gradients in electron density in the vicinity of the gas release tend to focus rays propagating through the depleted region. Preliminary calculations indicate that a lens capable of focusing cosmic radio waves in the 1 to 10 MHz frequency range may be produced by the release of 100 kg of H2 at the peak of the nighttime F layer. The beamwidth of a refracting radio telescope using this lens may be less than 1/5 degree.

Bernhardt, P.↗

NASA NTRS1981

UVSP/SMM observations of transition region oscillations in sunspots

Using Ultraviolet Spectrometer and Polarimeter data obtained in emission lines formed at temperatures of 70,000 K to 130,000 K, transition region oscillations in sunspots have been observed. The frequency of these oscillations lies in the range 5.8 mHz to 7.8 mHz. Their regular appearance in line-of-sight velocity and their frequent occurrence in intensity in phase with maximum blue shift leads to the interpretation of the oscillations as upward-propagating acoustic waves. The presence in two of the C IV wavelength 1548.19 time series of a phase-shifted oscillation in the line width may be caused by the presence of unidentified blends in the line wings. The energy flux carried by the umbral acoustic waves is less than 2000 erg/sq cm/s, some seven orders of magnitude smaller than the missing radiative flux of sunspots.

Gurman, J. B.↗

NASA NTRS1982

Analysis of electromagnetic fields on an F-106B aircraft during lightning strikes

Information on the exterior electromagnetic environment of an aircraft when it is struck by lightning has been obtained during thunderstorm penetrations with an F-106B aircraft. Electric and magnetic fields were observed, using mainly time-derivative type sensors, with bandwidths to 50 MHz. Lightning pulse lengths ranging from 25 ns to 7 microsec have been recorded. Sufficient high-frequency content was present to excite electromagnetic resonances of the aircraft, and peaks in the frequency spectra of the waveforms in the range 7 to 23 MHz are in agreement with the resonant frequencies determined in laboratory scale-model tests. Both positively and negatively charged strikes were experienced, and most of the data suggest low values of peak current.

Trost, T. F.↗

NASA NTRS1985

Broad-band UHF dipole array

A 6X6 array of fan-dipoles was designed to operate in the 510 to 660 MHz frequency range for aircraft flight test and evaluation of a UHF radiometer system. A broad-band dipole design operating near the first resonance is detailed. Measured VSWR and radiation patterns for the dipole array demonstrate achievable bandwidths in the 35 percent to 40 percent range.

Bailey, M. C.↗

NASA NTRS1986

Ultrasonic verification of microstructural changes due to heat treatment

Ultrasonic attenuation was measured for polycrystalline samples of nickel and copper with various grain-size distributions produced by heat treatment. Attenuation as a function of frequency was determined for a sample having a known mean grain diameter. Once this function was determined, it could be scaled to determine the mean grain size of other samples of the same material with different mean grain diameters. These results were obtained by using broadband pulse-echo ultrasound in the 25 to 100 MHz frequency range. The results suggest an ultrasonic, nondestructive approach for verifying heat treatment of metals.

Generazio, E. R.↗

NASA NTRS1986

Scaling attenuation data characterizes changes in material microstructure

Ultrasonic attenuation was measured for polycrystalline samples of nickel and copper with various grain-size distributions produced by heat treatment. Attenuation as a function of frequency was determined for a sample having a known mean grain diameter D. It was found that, once this function is determined, it can be scaled to determine the mean grain size of other samples of the same material with different mean grain diameters. These results were obtained using broadband pulse-echo ultrasound in the 25 to 100 MHz frequency range. The results suggest an ultrasonic, nondestructive approach for verifying heat treatment of metals.

Generazio, E. R.↗

NASA NTRS1986

Ultrasonic verification of microstructural changes due to heat treatment

Generazio, E. R.↗

NASA NTRS1986

Harmonically structured ULF pulsations observed by the AMPTE CCE magnetic field experiment

Spectrograms of ULF waves in the 0 to 80 mHz frequency range have been prepared from magnetic field data obtained by the elliptically orbiting AMPTE/CCE satellite (with an apogee of approximately 8.8 earth radii). The most prominent feature of these spectrograms (which cover a full 15.6-h orbit) is the presence of harmonically structured, azimuthally polarized pulsations in the outer magnetosphere during daytime hours. The frequencies of these pulsations decrease with increasing radial distance from the earth, indicating that they represent independent resonances of local magnetic flux tubes. The latitudinal structure of these harmonic pulsations, observed as AMPTE/CCE traveled to + or - 16 deg magnetic latitude, is consistent with accepted field line resonance models.

Engebretson, M. J.↗

NASA NTRS1988

Ionospheric wave and irregularity measurements using passive radio astronomy techniques

The observation of midlatitude structures using passive radio astronomy techniques is discussed, with particular attention being given to the low-frequency radio telescope at the Clark Lake Radio Observatory. The present telescope operates in the 10-125-MHz frequency range. Observations of the ionosphere at separations of a few kilometers to a few hundreds of kilometers by the lines of sight to sources are possible, allowing the determination of the amplitude, wavelength, direction of propagation, and propagation speed of ionospheric waves. Data are considered on large-scale ionospheric gradients and the two-dimensional shapes and sizes of ionospheric irregularities.

Erickson, W. C.↗

NASA NTRS1987

Simultaneous radio and white light observations of the 1984 June 27 coronal mass ejection event

Two-dimensional images of radio bursts in the 25-50-MHz frequency range obtained with the SMM Coronagraph/Polarimeter and Mauna Loa K-coronameter during a coronal mass ejection event on June 27, 1984 are discussed. The present event is shown to be spatially and temporally associated with precursors in the form of meter-decameter type III bursts, soft X-ray emission, and a H-alpha flare spray. The results suggest gyrosychrotron emission from a plasmoid containing a magnetic field of about 2.5 G and nonthermal electrons with a number density of about 100,000/cu cm and energy of greater than about 350 keV.

Gopalswamy, N.↗

NASA NTRS1990

Satellite sound broadcast propagation studies and measurements

Satellite Sound Broadcasting is an attractive satellite application. Before regulatory decisions can be made in 1992, the propagation effects encountered have to be characterized. The Electrical Engineering Research Laboratory has nearly completed a system which will allow amplitude measurements to be made over 10 MHz bandwidths in the 800 to 1800 MHz frequency range. The system uses transmission from a transportable tower, and reception inside buildings or in the shadow of trees or utility poles. The goal is to derive propagation models for use by systems engineers who are about to design satellite broadcast systems. The advance of fiber-optics technology has helped to focus future development of satellite services into areas where satellites are uniquely competitive. One of these preferred satellite applications is the broadcasting of high-quality sound for stationary or mobile reception by listeners using low-cost, consumer-grade receivers. Before such services can be provided, however, the political hurdles of spectrum allocation have to be surmounted and the technical questions of standardization for world-wide compatibility have to be resolved. In order to arrive at an optimal system design, efficient in the use of our scarce spectral resources, affordable both to the broadcaster and the listener, and providing predictable performance, the propagation effects to which the service is subjected have to be characterized. Consequently, the objective of the research project is to make basic propagation measurements for direct Satellite Sound Broadcasting Service (SSBS). The data obtained should allow the development of propagation models to be used by communications engineers designing the operational systems. Such models shall describe the effects of shadowing and multipath propagation on SSBS receivers operating in a specified environment, such as inside commercial or residential buildings of various construction and also in the shadow of trees or utility poles as might be encountered by transporting or mobile listeners.

Vogel, Wolfhard J.↗