Showing papers on "Scintillation published in 1981"

Simultaneous Rocket-Borne Beacon and In Situ Measurements of Equatorial Spread F - Intermediate Wavelength Results

Abstract: A high-performance rocket carrying a four-frequency, phase-coherent beacon and full complement of in situ diagnostic instrumentation was launched into active equatorial spread F on July 17, 1979. In this paper we report the results of spectrally analyzing the beacon phase-scintillation and Langmuir probe data. By using simultaneous backscatter data from the Altair radar we were able to establish that the scintillation develops in high-density regions adjacent to the prominent plume structures and associated depletions. In these high-density regions the in situ spectra show a pronounced change in the power law slope near a spatial wavelength of 500 m. Larger scale structures admit a systematically varying power law index that is generally less than 2, in good agreement with a large body of Wideband satellite data and recently analyzed Atmospheric Explorer E data. Smaller-scale structures admit a spectral index much larger than 2. A single, overall power law near k−2 was found only in low-density regions that did not contribute significantly to the scintillation. The results presented here and in a companion paper suggest that refinements in the current theories of equatorial spread F near and above the F region peak are needed.

Wind measurements by the temporal cross-correlation of the optical scintillations.

Abstract: Various methods of correlation analysis that have been used to deduce crosswind from a drifting scintillation pattern are briefly described and then compared with regard to their immunity to noise and their accuracy when faced with nonuniformities along the propagation path or changes in the characteristics of the turbulence. Of the techniques considered, none is ideal; but a new technique, using complete knowledge of the cross-covariance function, proves to be advantageous in a wide variety of situations.

Spectral characteristics of medium‐scale equatorial F region irregularities

Abstract: The spectral characteristics of equatorial F region irregularities with scale sizes from a few kilometers to several hundred kilometers have been measured using data from the Atmospheric Explorer-E satellite. The spectra admit the expected power law characterization with a mean spectral index near 2. There is, however, a systematic decrease in the spectral index with increasing perturbation strength which has not previously been reported. Nearly simultaneous phase scintillation data from the Wideband satellite were also spectrally analyzed for comparison with the AE-E data. The same systematic decrease of the spectral index with increasing perturbation strength was found in the measured phase spectral indices. A completely unambiguous comparison between the in situ and scintillation deduced turbulence levels cannot be made. However, for realistic propagation model parameters, the spectral characterization of turbulence based on the phase scintillation data is generally consistent with that observed in situ.

Microwave equatorial scintillation intensity during solar maximum

Abstract: A comparison of scintillation levels at 1.5 GHz made from the Appleton anomaly region of the magnetic equator and from the region close to the magnetic equator (termed the electrojet latitudes) showed increased F region irregularity intensity over the anomaly region during years of high sunspot number. Peak to peak fading greater than 27 dB was noted from Ascension Island (through a dip latitude of 17°) in the anomaly region while only 7–9 dB from Natal, Brazil, and Huancayo, Peru, were noted, the last two paths being close to the magnetic equator. The hypothesis advanced is that the dominant factor responsible for the intense gigahertz scintillation is the traversal of the propagation path through the anomaly region. During years of high sunspot numbers the high levels of ΔN constituting the F region irregularity structure are due to (1) very high electron density in the anomaly region (compared to the electrojet region) and (2) the late appearance of these high electron densities (to 2200 local time) in the anomaly region. The patches or plumes of irregularities seen in the postsunset time period then produce high ΔN; scintillation excursions are proportional to this parameter. The postulation of vertical irregularity sheets in the patches was examined to determine the possibility of this being an important factor in the difference between electrojet and anomaly scintillation levels. Older gigahertz data from the sunspot maximum years 1969–1970 were reanalyzed, and more recent observations from other studies were also reviewed. It was found that through the anomaly region, high scintillation indices were noted at a variety of azimuths of the propagation path rather than just along a path closely aligned with the magnetic meridian. A more complete evaluation of the geometrical factor, which must be of considerable importance in determining the absolute value of the scintillation intensity, awaits further observations.

UHF scintillation activity over polar latitudes

Abstract: Polar cap ionospheric irregularities have been monitored for several years from Thule Air Base, Greenland using 250 MHz satellite beacon scintillation measurements. The seasonal pattern of the polar cap irregularities shows very high intensity levels during the winter and lower levels during the summer (sunlit) months. This behavior is similar to in-situ polar cap electric field measurements which show larger fluctuations in the winter than in the summer, an effect which may be related to E layer conductivity changes. A striking contrast was noted between high scintillation levels observed during 1979/80, a year of high solar flux, and much lower levels observed during 1975, a year of low solar flux. This variation may be related to a corresponding solar cycle variation in polar cap F layer electron density. The data reveal little difference between periods of high and low Kp, and only a weak diurnal variation in any season. Direct optical and ionosonde measurements indicate that these scintillations are produced by ionospheric irregularities in the polar cap F-region. Results of spaced receiver drift measurements indicate that the small scale of irregularity drift was antisunward. Intense irregularities are associated with discrete sun aligned F layer auroras. A weak background level of scintillation persisted in the high solar flux years.

Solar wind motion within 30 R solar masses - Spacecraft radio scintillation observations

Abstract: New remote-sensing observations are reported of the solar wind motion within about 30 earth radii. Use is made of the interplanetary scintillation (IPS) spaced receiver technique with the radio source being a spacecraft signal (rather than a natural radio source as in previous spaced receiver studies). The spacecraft used are Helios A and B and the Viking orbiters. The purposes of the study are (1) to augment the scarce estimates of solar wind bulk flow speed near the sun and in the ecliptic with measurements made using spacecraft signals, and (2) to estimate random velocity components and identify the region where the random velocity is a significant fraction of the mean velocity. In addition, the radial evolution of speed and random velocity is compared with that of the plasma density fluctuation spectrum. Also reported are the first accurately normalized IPS scintillation index measurements using a monochromatic point source.

Observations of cloud‐produced amplitude scintillation on 19‐ and 28‐GHz earth‐space paths

Abstract: The amplitudes of satellite signals sometimes scintillate ± several decibels when heavy cumulus clouds pass through the radio path on hot summer days. These scintillations have been measured on a 19-GHz and 28-GHz earth-space path using 7-m and 0.6-m antennas at Crawford Hill, New Jersey. Scintillation intensity at 28 GHz is 1.2 times that at 19 GHz, consistent with the ƒ7/12 frequency dependence produced by a thin turbulent layer. The scintillation process is polarization-independent and has a low-pass power spectrum with a cutoff frequency of about 0.3 Hz. Rain attenuation often accompanies the more intense scintillation. The mean duration of scintillation-produced fades is short (about 1.3 s for fades greater than 1 dB), but over 1000 fades of over 1 dB at 28 GHz were observed in two summer months. Because of their weak frequency dependence these short but frequent events could produce repeated outages on 4- and 6-GHz earth-space links having low attenuation margins.

2-D Position-Sensitive Scintillation Detector for Neutrons

Abstract: A 2-di mensional neutron-position scintillation detector, based on the well-proven principle of the Anger ?-ray camera, has been developed. A 6Li-loaded, Ce-activated glass scintilator is coupled via a unique 44 mm thick light guide to a closely spaced hexagonal array of nineteen 5 cm diameter photomultipliers. The 1 mm thick x 22 cm diameter glass scintillator is made up of four optically cemented quadrants. The location of a scintillation is determined by calculating the X and Y centroids using a resistor weighting scheme. A spatial FWHM resolution of 2-3 mm was obtained. At 0.025 eV (1.8 A) the efficiency of the glass is 80% and the FWHM pulse height resolution is 14%. These characteristics in combination with low ?-ray sensitivity in the thin scintillator provide for excellent background rejection. Drift in the photomultipliers' gain results in output change of <0.05 mm/°C. The detector was evaluated in experiments at the pulsed spallation neutron source ZING-P'. In an experiment with a single crystal diffractometer a Laue pattern from a NaCl crystal was obtained. In a resonance radiography experiment Au and in, contained in a test object, were graphically resolved. For these applications the scintillation detector has several inherent advantages over a gas proportional counter.

The gas scintillation proportional counter on Exosat

Abstract: The inclusion of a gas scintillation proportional counter (GSPC) within the EXOSAT payload complement significantly improves the spectroscopic capability of the mission. This broad-band medium energy spectrometer used in conjunction with the large area proportioni counter array (ME) should provide additional spectroscopic details on strong X-ray sources at photon energies above ~ 2 keV. The novel type of detector has an energy resolution at least a factor of two better than the ME experiment over a similar bandwidth (2 – 40 keV).

One Dimensional Scintillation Cameras for Positron Ect Ring Detectors

Abstract: A one-dimensional scintillation camera configured as a continuous ring with coincidence-mode collimation avoids many of the problems associated with rings formed from an array of discrete detector elements. With the goal of achieving a stationary detector exhibiting high spatial resolution and sensitivity, several detector configurations have been designed. Both analytic and Monte-Carlo simulation techniques have been used to evaluate these designs. Results of the calculations and comparative measurements on the various detector designs are discussed.

Relation between irradiance and log-amplitude variance for optical scintillation described by the K distribution

Abstract: We derive the relationship between log-amplitude (or log-irradiance) variance and irradiance variance for optical scintillation described by a K probability distribution. The results indicate that all observations of maximum variance values, whether log-irradiance or irradiance data are used, are consistent. The K distribution appears to be a reasonable assumption for scintillation in the region of maximum variance. The K distribution is also consistent with data and theory in the very strong refractive turbulence region where exponential irradiance statistics are observed.

Dominant configurations of scintillation‐producing irregularities in the auroral zone

Abstract: Recent scintillation observations have disclosed the existence of sheetlike electron density irregularities aligned along L shells in the auroral zone ionosphere. In this paper we exploit the aspect sensitivity of phase scintillation to identify the dominant three-dimensional configuration of irregularities in four latitude/time zones: (1) equatorward of the high-latitude scintillation boundary on the nightside of the earth, (2) poleward of the nightside boundary, (3) equatorward of the boundary on the day side, and (4) poleward of the dayside boundary. We find the sheetlike irregularities to be confined to region 2, with few exceptions. The dominant configuration in the other zones, including zone 4, is that of axially symmetric rodlike irregularities aligned along the magnetic field.

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

Abstract: 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.

Ionospheric scintillation observations at Natal

Abstract: We report ionospheric scintillation observations made at Natal, Brazil, during a period of 2½ years. The nighttime scintillations of satellite signals at 250 MHz commonly exceeded 20 dB peak-to-peak. Seasonally, it showed a typical behavior for the South American sector with a single peak centered around October–November months and the local evening hours of 2100–2300 LMT. This seasonal behavior was different from that of the African sector and very different from that of the Pacific sector. On a spacing of 278 m the eastward irregularity drift speed was measured by the method of similar fades. This, together with the determination of Fresnel frequency from the power spectrum analysis, was used to calculate the height of the irregularity patch. A value of 294 km was obtained. Multisatellite scintillation observations indicated that even though the initiation of an irregularity cloud commonly occurred earlier on signals transmitted by the eastern satellite, there were substantial cases when the reverse was observed. One case which was analyzed in detail suggested that the irregularity clouds formed independently along two radio paths separated by 520 km.

Intense daytime radio wave scintillations and sporadic E layer near the dip equator

Abstract: The paper describes two cases of very intense scintillation activity (amplitude exceeding 10 dB) recorded at Huancayo during the daytime on radio beacons on 257 MHz from Marisat satellite and ATS 3 (137 MHz) and LES 6 (254 MHz) signals. Both these events are shown to be associated with an intense blanketing equatorial sporadic E layer at Huancayo. It is suggested that the bursts of strong daytime scintillations of VHF radio waves are due to densely ionized thin sheets of ionization over the equator and only normally weak (≈ 1 dB) daytime scintillations may be due to instabilities associated with the q type of equatorial Es.

Neutron position-sensitive scintillation detector

Abstract: A device is provided for mapping one- and two-dimensional distributions of neutron-positions in a scintillation detector. The device consists of a lithium glass scintillator coupled by an air gap and a light coupler to an array of photomultipliers. The air gap concentrates light flashes from the scintillator, whereas the light coupler disperses this concentrated light to a predetermined fraction of the photomultiplier tube array.

A Parallel Plate Imaging Proportional Counter with High Background Rejection Capability

Abstract: We present the results of studies of the scintillation produced by avalanches in a parallel plate proportional counter. It is shown, that for applications in X-ray astronomy, background rejection of greater than 90% may be achieved by risetime discrimination using the light pulse. We also present images of individual avalanches obtained by coupling an image intensifier to the counter with fibre optics. Electronic centroid analysis using such an intensifier is discussed. Measurements are also presented of the scintillation emission spectrum and position resolution with various gas mixtures.

Properties of an Imaging Gas Scintillation Proportional Counter

Abstract: We have mated a parallel-grid gas scintillation proportional counter (GSPC) to a multiwire proportional counter (MWPC) through a UV transparent window to produce a new instrument that combines the good energy resolution of the GSPC with the imaging capabilities of the MWPC. The imaging gas scintillation proportional counter (IGSPC) operates on the principle of detecting the centroid of the UV light excited by X-ray photons interacting in the noble gas of the GSPC with a UV sensitive gas [e.g. Tetrakis (dimethylamino) ethylene (TMAE)] in the MWPC. A prototype version of the counter has been built which yields a measured performance of 9% (FWHM) energy resolution and 0.9 mm (FWHM) spatial resolution at 6 keV. Further refinements of this counter and a new counter, designed for flight on a sounding rocket, should permit us to achieve 18% (FWHM) energy resolution and 0.6 mm (FWHM) spatial resolution at 1 keV.

Scintillation of VHF/UHF and L band satellite signals at Guam

Abstract: An investigation of the equatorial scintillation of satellite signals at 257.55 and 1541.5 MHz for about 10° and 50° elevation angles is reported. Diurnal and seasonal variations are shown for the two frequencies and the two elevation angles. A periodicity in the occurrence of the scintillation was observed and is attributed to the gravitational field of the moon. It is proposed that the moon's gravity may affect the occurrence and intensity of equatorial scintillation by modulating the velocity of the zonal winds at ionospheric heights. An additional possible contribution to the periodic variation in the scintillation is a periodic variation which occurred in the geomagnetic activity during the first half of the year.