Scintillation Regime Research Articles

Does transparent hidden matter generate optical scintillation?

Stars twinkle because their light goes through the atmosphere. The same phenomenon is expected when the light of extra-galactic stars goes through a Galactic - disk or halo - refractive medium. Because of the large distances involved here, the length and time scales of the optical intensity fluctuations resulting from the wave distortions are accessible to current technology. In this paper, we discuss the different possible scintillation regimes and we focus on the so-called strong diffractive regime that is likely to produce large intensity contrasts. The critical relationship between the source angular size and the inten- sity contrast in optical wavelengths is also discussed in detail. We propose to monitor small extra-galactic stars every ∼10 s to search for intensity scintillation produced by molecular hydrogen clouds. We discuss means to discriminate such hidden matter signals from the foreground effects on light propagation. Appropriate observation of the scintillation process described here should allow one to detect column density stochastic variations in Galactic molecular clouds of the order of ∼3 × 10 −5 g/cm 2 , that is 10 19 molecules/cm 2 per ∼10 000 km transverse distance.

Evaluation of phase-shifting approaches for a point-diffraction interferometer with the mutual coherence function.

The estimation accuracy of a point-diffraction interferometer is examined with two phase-shifting schemes: spatial and temporal. Under the assumption of plane- or spherical-wave propagation through isotropic turbulence that can be accurately represented as a series of thin phase screens, results that are valid for any scintillation regime are obtained by use of the invariance with a propagation of the mutual coherence function. It is established that the estimation accuracy of the spatial phase-shifting point-diffraction interferometer is invariant with scintillation. Upper and lower bounds on the perfor mance of the temporal phase-shifting point-diffraction interferometer are developed. Wave optical simulation results are presented that validate the analytic predictions for the two phase-shifting schemes. The results and techniques presented can be used to assess the appropriate phase-shifting scheme given finite resources, such as a limited number of pixels in a detector array or a restricted detector frame rate.

Interstellar Scintillation of the Polarized Flux Density in Quasar PKS 0405−385

The remarkable rapid variations in radio flux density and polarization of the quasar PKS 0405-385 observed in 1996 are subject to a correlation analysis, from which characteristic time scales and amplitudes are derived. The variations are interpreted as interstellar scintillations. The cm wavelength observations are in the weak scintillation regime for which models for the various auto- and cross-correlations of the Stokes parameters are derived and fitted to the observations. These are well modelled by interstellar scintillation (ISS) of a 30 by 22 micro-as source, with about 180 degree rotation of the polarization angle along its long dimension. This success in explaining the remarkable intra-day variations (IDV)in polarization confirms that ISS gives rise to the IDV in this quasar. However, the fit requires the scintillations to be occurring much closer to the Earth than expected according to the standard model for the ionized interstellar medium (IISM). Scattering at distances in the range 3-30 parsec are required to explain the observations. The associated source model has a peak brightness temperature near 2.0 10^{13}K, which is about twenty-five times smaller than previously derived for this source. This reduces the implied Doppler factor in the relativistic jet, presumed responsible to 10-20, high but just compatible with cm wavelength VLBI estimates for the Doppler factors in Active Galactic Nuclei (AGNs).

Lensing of ultra-high energy cosmic rays in turbulent magnetic fields

We consider the propagation of ultra high energy cosmic rays through turbulent magnetic fields and study the transition between the regimes of single and multiple images of point-like sources. The transition occurs at energies around Ec Z 41 EeV(Brms/5 μG)(L/2 kpc)3/2(50 pc/Lc)1/2, where L is the distance traversed by the CR's with electric charge Ze in the turbulent magnetic field of root mean square strength Brms and coherence length Lc. We find that above 2Ec only sources located in a fraction of a few % of the sky can reach large amplifications of its principal image or start developing multiple images. New images appear in pairs with huge magnifications, and they remain amplified over a significant range of energies. At decreasing energies the fraction of the sky in which sources can develop multiple images increases, reaching about 50% for E > Ec/2. The magnification peaks become however increasingly narrower and for E < Ec/3 their integrated effect becomes less noticeable. If a uniform magnetic field component is also present it would further narrow down the peaks, shrinking the energy range in which they can be relevant. Below E Ec/10 some kind of scintillation regime is reached, where many demagnified images of a source are present but with overall total magnification of order unity. We also search for lensing signatures in the AGASA data studying two-dimensional correlations in angle and energy and find some interesting hints.

Fractional moments and their usefulness in atmospheric laser scintillation

Use of moments of non-integer order (called fractional moments) in statistical optics is proposed for discriminating between candidates for probability density functions (PDFs) of the scintillation intensity to overcome the experimental problems encountered by using integer moments. Low-order (< 2) fractional moments allow comparison of experimental data with theoretical PDFs of the intensity in different scintillation regimes. A comparison procedure, based on two low-order measured fractional moments, is described and tested by utilizing Monte Carlo samples from three distributions commonly used in atmospheric propagation (Ln, LnME and K). The bin width required to discriminate between different distributions and the effect of noise are investigated. Examples of application to experimental data are presented.

Investigation of pulsar diffractive scintillation at 4.75 and 10.55 GHz

Many papers were in the past devoted to the investigation of the interstellar medium (ISM) with the help of pulsars, most of them studying the time and frequency structure of pulsar flux variations in the strong scintillation regime. Weak scintillation is typically found at cm- and dcm-wavelengths, but depends on distance so that the critical frequency (νc) increases with distance. Pulsar intensities weaken, however, at high frequencies, since as a rule pulsar spectra have spectral indices steeper than −1.5. This explains why there are practically no measurements of pulsars in the weak scintillation regime.We report in this paper on first measurements of the correlation function of pulsar intensity fluctuations, of the decorrelation timeτdand of the scintillation indexmdat high frequencies, i.e. at 4.75 and 10.55 GHz in the weak scintillation regime. The detail data analysis, theoretical relations and comparison with theory will presented by Malofeev et al. (1996).

Deducing turbulence parameters from transionospheric scintillation measurements

The theoretical framework and experimental methodology used to interpret observations of ionospheric scintillations in terms of geophysical processes are reviewed and recent experimental observations of ionospheric scintillations are discussed in this paper. During the past 15 years significant progress has been made in several areas. In particular, significant advances have been made in theoretical understanding of the strong scintillation regime and the effects of short-term temporal variations of the scintillation producing irregularities on observations made with spaced-receiver geometries in both weak and strong scintillations. This improved understanding of the scintillation process has significantly increased the utility of the technique particularly in the equatorial latitudes where geometrical effects are least important.

Radio Source Scintillations Through Comet Tails Revisited: Comet Wilson (1987)

Observations of the quasars 0606-795 and 0637-752 as the tail of Comet Wilson swept across them on May 1 and May 2, 1987, showed a three-fold increase in scintillation index over that of nearby compact radio sources outside the tail. Two scintillation regimes have been identified: (1) small-scale turbulence of 10-40 km develops near the tail-axis; (2) large-scale turbulence of 90-350 km is present in the off-axis transition region between the tail plasma and solar wind. At a distance 0�12 AU downstream from the nucleus the r.m.s. electron-density variation in these turbules is 4-8 cm-3 on axis and 0�8-1� 7 cm-3 in the transition region between the tail and the solar wind. The reported negative results from earlier comets are shown to be of doubtful significance.

Refractive turbulence effects on truncated Gaussian beam heterodyne lidar

A monostatic heterodyne lidar performance model is formulated to study the combined effects of beam truncation and refractive turbulence in the weak scintillation regime. The results show that there is a loss of signal power due to beam truncation and coherence loss, but there is also an enhancement of signal power due to log-amplitude covariance in suitable conditions of long paths with weak turbulence.

Strong turbulence and atmospheric waves in stellar occultations

Many of the problems of stellar occultation observations stem from the difficulty of determining the effects of realistic atmospheric structure on the lightcurves. General techniques for producing model lightcurves for a variety of realistic atmospheric irregularities, including turbulence and inertia-gravity waves, are presented and applied. Using numerical simulations which model the propagation of a wave through a phase-changing screen, the limit of strong scintillations for one-dimensional, Kolmogorov-like turbulence, both for a point source and for extended sources, is investigated in some detail, and significant departures from the behavior in the weak scintillation regime are found. The results are compared with published analytical results and recent occultation data. The effects of large-scale atmospheric waves with realistic horizontal structure are examined, and the reliability of the numerical inversion method of retrieving the true atmospheric vertical structure under circumstances of strong ray crossing and horizontal inhomogeneities is assessed. The simulations confirm that large-scale layered features of the atmosphere are accurately recovered; horizontally inhomogeneous structures (including turbulence) with coherence scale L ⪡ (2πRH) 1 2 (where R = planetary radius and H = scale height) have little effect on the derived temperature profiles. It is concluded that analysis of occultations may eventually allow us to determine both the quasiglobal atmospheric structure and the statistical characteristics of small-scale refractivity variations.

Scintillating confusion - Evaluation of a technique for measuring compact structure in weak radio sources

An attractive scheme for investigating compact structure in weak radio sources is to study the scintillation properties of confusion in a large single-dish radio telescope. We have investigated the utility of this technique by observing the scintillations of 860-MHz confusion of the NRAO 300' (91 m) telescope. Analysis of these data indicated a reduction in the mean scintillation index with decreasing flux density which implied that weaker sources possessed less compact structure. More direct observations indicated that the weak sources of interest were not significantly deficient in compact structure, so the first result is probably due to properties of the IPS process in the strong scintillation regime. Our results may be due to overresolution (by the IPS process in the strong scintillation regime) of the ''hot spots'' responsible for scintillation in most strong sources at frequencies below 1000 MHz, or may indicate abnormally strong turbulence in the solar wind during August, 1977. Future applications of this method would be best conducted at lower frequencies with larger reflectors or short-spacing interferometers.

Probability distribution of turbulent irradiance in a saturation regime

Application of the central limit theorem to the stochastic equation of propagation suggests that the probability distribution of the complex wave amplitude defined on the geometrical phase front is approximately normal. The resulting irradiance probability density function, valid in the strong scintillation regime, is an exponential multiplied by the modified Bessel function I(0) both of argument proportional to the irradiance; it is not the Rice-Nakagami density function. Quantitative tests show that this exponential-Bessel function constitutes as good a fit as the log-normal to the irradiance probability data reported in this paper. Since the normal distribution hypothesis is consistent with the stochastic wave equation, the model proposed here should be a simple substitute to the often used but theoretically incorrect log-normal irradiance probability distribution model.

Correlated measurements of scintillations and in‐situ F‐region irregularities from OGO‐6

Scintillation estimates obtained from in‐situ irregularity measurements by OGO‐6 are compared with simultaneous 137 MHz and 6 GHz scintillations recorded at equatorial stations in the American and African sectors. In this first such correlated study, it is found that the equatorial irregularity amplitudes in these sectors are so large that for a moderately thick irregularity layer the commonly observed monotonic power law spectrum with outer scale dimensions much larger than the Fresnel dimension at VHF can explain the observed GHz scintillations and can locate the VHF band in the saturated scintillation regime. This finding is in contrast to other numerical modelling attempts which usually call for artificial tailoring of irregularity spectra to account for the observed GHz scintillations.