Small-scale Anisotropy Research Articles

Anomalous scaling of a passive scalar advected by a turbulent velocity field with finite correlation time and uniaxial small-scale anisotropy

The influence of uniaxial small-scale anisotropy on the stability of the scaling regimes and on the anomalous scaling of the structure functions of a passive scalar advected by a Gaussian solenoidal velocity field with finite correlation time is investigated by the field theoretic renormalization group and operator product expansion within one-loop approximation. Possible scaling regimes are found and classified in the plane of exponents epsilon-eta , where epsilon characterizes the energy spectrum of the velocity field in the inertial range E proportional, variantk;{1-2epsilon} , and eta is related to the correlation time of the velocity field at the wave number k which is scaled as k;{-2+eta} . It is shown that the presence of anisotropy does not disturb the stability of the infrared fixed points of the renormalization group equations, which are directly related to the corresponding scaling regimes. The influence of anisotropy on the anomalous scaling of the structure functions of the passive scalar field is studied as a function of the fixed point value of the parameter u , which represents the ratio of turnover time of scalar field and velocity correlation time. It is shown that the corresponding one-loop anomalous dimensions, which are the same (universal) for all particular models with a concrete value of u in the isotropic case, are different (nonuniversal) in the case with the presence of small-scale anisotropy and they are continuous functions of the anisotropy parameters, as well as the parameter u . The dependence of the anomalous dimensions on the anisotropy parameters of two special limits of the general model, namely, the rapid-change model and the frozen velocity field model, are found when u-->infinity and u-->0 , respectively.

Can We Detect the Anisotropic Shapes of Quasar HiiRegions during Reionization through the Small‐Scale Redshifted 21 cm Power Spectrum?

Light-travel time delays distort the apparent shapes of H II regions surrounding bright quasars during early stages of cosmic reionization. Individual H II regions may remain undetectable in forthcoming redshifted 21 cm experiments. However, the systematic deformation along the line of sight may be detectable statistically, either by stacking tomographic 21 cm images of quasars identified, for example, by the James Webb Space Telescope, or as small-scale anisotropy in the three-dimensional 21 cm power spectrum. Here we consider the detectability of this effect. The anisotropy is largest when H II regions are large and expand rapidly, and we find that if bright quasars contributed to the early stages of reionization, then they can produce significant anisotropy, on scales comparable to the typical sizes of H II regions of the bright quasars (30 Mpc). The effect therefore cannot be ignored when analyzing future 21 cm power spectra on small scales. If 10% of the volume of the intergalactic medium at -->z 10 is ionized by quasars with typical ionizing luminosity of -->S 5 × 1056 s−1, the distortions cause an 10 percent enhancement of the 21 cm power spectrum in the radial (redshift) direction, relative to the transverse directions. The level of this anisotropy exceeds that due to redshift-space distortion and has the opposite sign. We show that ongoing experiments such as Murchison Widefield Array (MWA, formerly known as the Mileura Widefield Array) should be able to detect this effect. A detection would reveal the presence of bright quasars and shed light on the ionizing yield and age of the ionizing sources and the distribution and small-scale clumping of neutral intergalactic gas in their vicinity.

Current models of the observable consequences of cosmic reionization and their detectability

A number of large current experiments aim to detect the signatures of the Cosmic Reionization at redshifts z>6. Their success depends crucially on understanding the character of the reionization process and its observable consequences and designing the best strategies to use. We use large-scale simulations of cosmic reionization to evaluate the reionization signatures at redshifted 21-cm and small-scale CMB anisotropies in the best current model for the background universe, with fundamental cosmological parameters given by WMAP 3-year results (WMAP3). We find that the optimal frequency range for observing the ``global step'' of the 21-cm emission is 120-150 MHz, while statistical studies should aim at 140-160 MHz, observable by GMRT. Some strongly-nongaussian brightness features should be detectable at frequencies up to ~190 MHz. In terms of sensitivity-signal trade-off relatively low resolutions, corresponding to beams of at least a few arcminutes, are preferable. The CMB anisotropy signal from the kinetic Sunyaev-Zel'dovich effect from reionized patches peaks at tens of muK at arcminute scales and has an rms of ~1 muK, and should be observable by the Atacama Cosmology Telescope and the South Pole Telescope. We discuss the various observational issues and the uncertainties involved, mostly related to the poorly-known reionization parameters and, to a lesser extend, to the uncertainties in the background cosmology.

Estimation prospects of the source number density of ultra-high-energy cosmic rays

We discuss the possibility of accurately estimating the source number density of ultra-high-energy cosmic rays (UHECRs) using small-scale anisotropy in their arrival distribution. The arrival distribution has information on their source and source distribution. We calculate the propagation of UHE protons in a structured extragalactic magnetic field (EGMF) and simulate their arrival distribution at the Earth using our previously developed method. The source number density that can best reproduce observational results by Akeno Giant Air Shower Array is estimated at about 10 −5 Mpc −3 in a simple source model. Despite having large uncertainties of about one order of magnitude, due to small number of observed events in current status, we find that more detection of UHECRs in the Auger era can sufficiently decrease this so that the source number density can be more robustly estimated. Two hundred event observation above 4 × 10 19 eV in a hemisphere can discriminate between 10 −5 and 10 −6 Mpc −3. Number of events to discriminate between 10 −4 and 10 −5 Mpc −3 is dependent on EGMF strength. We also discuss the same in another source model in this paper.

An alternative method to finding patterns in HiRes stereo data

In this paper ultra-high energy cosmic rays UHECRs data observed by the HiRes fluorescence detector in stereo mode is analyzed to search for events in the sky with an arrival direction lying on a great circle. Such structure is known as the arc structure. The arc structure is expected when the charged cosmic rays pass through the galactic magnetic field. The arcs searched for could represent a broad or a small scale anisotropy depending on the proposed source model for the UHECRs. The arcs in this paper are looked for using Hough transform were Hough transform is a technique used to looking for patterns in images. No statistically significant arcs were found in this study.

Small-scale anisotropy in magnetohydrodynamic turbulence under a strong uniform magnetic field

Small-scale statistics in magnetohydrodynamic turbulence under an externally applied strong stationary uniform magnetic field is studied theoretically and numerically. A simple expression for the velocity correlation spectrum is derived by applying the idea of the perturbation method by Ishihara et al. [Phys. Rev. Lett. 88 154501 (2002)]. It gives not only the scale dependence but also the angular dependence of the anisotropic velocity correlation spectrum at small scales. It is in good agreement with direct numerical simulation with 5123 grid points.

Migration of surface seismic data with VSP Green's functions

Seismic migration is the process by which wavefields recorded on or near the surface are mapped back into the subsurface to form an image of the subsurface structure. Common to all current migration methods is that they rely on a model of the subsurface for computing Green's functions. These computed Green's functions are approximations of the true Green's function in the subsurface. The more accurate the method used for computing the Green's functions, and to some extent also the choice of imaging condition, the better image we will get. Most state-of-the-art migration methods will use Green's functions that include multiple arrivals and some finite-frequency effects, allowing them to image fairly complex geologic structures. However, the computational model in most cases uses only scalar velocity that is smooth on a wavelength scale, thus ignoring scattering effects from small-scale structure, anisotropy, and other intrinsic rock effects. Further, whatever the model is, it has to be derived from the dat...

Search for cross-correlations of ultra-high-energy cosmic rays with BL Lacertae objects

The High Resolution Fly's Eye (HiRes) experiment is a stereo air fluorescence detector for the study of cosmic rays with energies above 5×1017eV. Arrival directions of cosmic rays observed in stereo can be resolved with a typical uncertainty on the order of 0.5°, making the experiment ideal for small-scale anisotropy studies. Here we present the results of searches for correlation with astronomical sources, in particular the recently observed correlations with objects of the BL Lac subclass of active galaxies.

Cosmic microwave background anisotropies from outflows in Lyman break galaxies

Abstract Thomson scattering of the cosmic microwave background (CMB) on moving electrons in the outflows of Lyman break galaxies (LBGs) at redshifts 2–8 contributes to the small-scale CMB anisotropies. The net effect produced by each outflow depends on its level of deviation from spherical symmetry, caused either by an anisotropic energy injection from the nuclear starburst or quasar activity, or by an inhomogeneous intergalactic environment. We find that for plausible outflow parameters consistent with spectroscopic observations of LBGs, the induced CMB anisotropies on arcminute scales reach up to ∼1 μK, comparable to the level produced during the epoch of reionization.

Constraints on the Evolution of the Primordial Magnetic Field from the Small‐Scale Cosmic Microwave Background Angular Anisotropy

Recent observations of the cosmic microwave background (CMB) have extended the measured power spectrum to higher multipoles $l\gtrsim$1000, and there appears to be possible evidence for excess power on small angular scales. The primordial magnetic field (PMF) can strongly affect the CMB power spectrum and the formation of large scale structure. In this paper, we calculate the CMB temperature anisotropies generated by including a power-law magnetic field at the photon last-scattering surface (PLSS). We then deduce an upper limit on the PMF based on our theoretical analysis of the power excess on small angular scales. We have taken into account several important effects such as the modified matter sound speed in the presence of a magnetic field. An upper limit to the field strength of $|B_\lambda|\lesssim$ 4.7 nG at the present scale of 1 Mpc is deduced. This is obtained by comparing the calculated theoretical result including the Sunyaev-Zeldovich (SZ) effect with recent observed data on the small-scale CMB anisotropies from the $Wilkinson Microwave Anisotropy Probe$ (WMAP), the Cosmic Background Imager (CBI), and the Arcminute Cosmology Bolometer Array Receiver (ACBAR). We discuss several possible mechanisms for the generation and evolution of the PMF.

Small scale anisotropy predictions for the Auger Observatory

We study the small scale anisotropy signal expected at the Pierre Auger Observatoryafter 1, 5, 10, and 15 years of operation, from sources of ultra-high energy (UHE)protons. We numerically propagate UHE protons over cosmological distancesusing an injection spectrum and normalization that fits current data up to∼1020 eV. We characterize possible sources of ultra-high energy cosmic rays (UHECRs) bytheir mean density in the local universe, , with r between 3 and 6. These densities span a wide range of extragalactic sites for UHECR sources,from common to rare galaxies or even clusters of galaxies. We simulate 100 realizations foreach model and calculate the two point correlation function for events with energies above4 × 1019 eV andabove 1020 eV with the exposure of the Auger South Observatory. We find that for , Auger should be able to detect small scale anisotropies in the near future. Distinguishingbetween different source densities based on cosmic ray data alone will be more challengingthan detecting a departure from isotropy and is likely to require larger statistics ofevents. Combining the angular distribution studies with the spectral shape aroundthe GZK feature will also help distinguish between different source scenarios.

Simulating cosmic reionization at large scales - I. The geometry of reionization

We present the first large-scale radiative transfer simulations of cosmic reionization, in a simulation volume of (100 h−1 Mpc)3. This is more than a two orders of magnitude improvement over previous simulations. We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and efficient code for 3D radiative transfer, c2-ray, which we have recently developed. These simulations allow us to do the first numerical studies of the large-scale structure of reionization which at the same time, and crucially, properly take account of the dwarf galaxy ionizing sources which are primarily responsible for reionization. In our realization, reionization starts around z∼ 21, and final overlap occurs by z∼ 11. The resulting electron-scattering optical depth is in good agreement with the first-year Wilkinson Microwave Anisotropy Probe (WMAP) polarization data. We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earlier, on average, than the voids. Ionization histories of smaller-size (5–10 comoving Mpc) subregions exabit a large scatter about the mean and do not describe the global reionization history well. This is true even when these subregions are at the mean density of the universe, which shows that small-box simulations of reionization have little predictive power for the evolution of the mean ionized fraction. The minimum reliable volume size for such predictions is ∼30 Mpc. We derive the power spectra of the neutral, ionized and total gas density fields and show that there is a significant boost of the density fluctuations in both the neutral and the ionized components relative to the total at arcmin and larger scales. We find two populations of H ii regions according to their size, numerous, mid-sized (∼10-Mpc) regions and a few, rare, very large regions tens of Mpc in size. Thus, local overlap on fairly large scales of tens of Mpc is reached by z∼ 13, when our volume is only about 50 per cent ionized, and well before the global overlap. We derive the statistical distributions of the ionized fraction and ionized gas density at various scales and for the first time show that both distributions are clearly non-Gaussian. All these quantities are critical for predicting and interpreting the observational signals from reionization from a variety of observations like 21-cm emission, Lyα emitter statistics, Gunn–Peterson optical depth and small-scale cosmic microwave background secondary anisotropies due to patchy reionization.

A closer look at the spectrum and small scale anisotropies of ultrahigh energy cosmicrays

We present results of numerical simulations of the propagation of ultrahigh energy cosmic rays(UHECRs) over cosmological distances, aiming at quantifying the statistical significance ofthe highest energy data on the spectrum and small scale anisotropies as detected by theAGASA experiment. We assess the significance of the lack of a GZK feature and itscompatibility with the reported small scale anisotropies. Assuming that UHECRs areprotons from extragalactic sources, we find that spectra with shape similar to that observedby AGASA can arise due to low statistics of detected events with a probability of6 × 10−4 for a continuous source distribution. However, when the spectrum iscombined with the information on the small scale anisotropies reported byAGASA, the probability of getting both at the same time becomes as low as2 × 10−5. Our analysis of the AGASA results shows the power that will derive combining spectrumand small scale anisotropy data in future high statistics experiments, such as Auger ones.

Searching for the origin of ultrahigh energy cosmic rays with the High Resolution Fly's Eye Stereo Detector

The High Resolution Fly's Eye (HiRes) experiment in Utah is an air fluorescence telescope mapping the sky in cosmic rays at energies above 10 18 eV. Since November 1999, HiRes has been operated in stereo mode to provide cosmic ray data of unprecedented quality of the northern sky. This paper focuses on recent results from the stereoscopic data. We present a measurement of the primary chemical composition above 10 18 eV, results on the search for small-scale anisotropies in the cosmic ray arrival distribution, and results on a search for point sources in the combined AGASA and HiRes stereo data set above 4.0 × 10 19 eV .

Characteristic scales during reionization

One of the key observables of the reionization era is the distribution of neutral and ionized gas. Recently, Furlanetto, Zaldarriaga and Hernquist developed a simple analytic model to describe the growth of H II regions during this era. Here, we examine some of the fundamental simplifying assumptions behind this model and generalize it in several important ways. The model predicts that the ionized regions attain a well-defined characteristic size Rc that ranges from ∼1 Mpc in the early phases to10 Mpc in the late phases. We show that Rc is determined primarily by the bias of the galaxies driving reionization; hence measurements of this scale constrain a fundamental property of the first galaxies. The variance around Rc ,o nthe other hand, is determined primarily by the underlying matter power spectrum. We then show that increasing the ionizing efficiency of massive galaxies shifts Rc to significantly larger scales and decreases the importance of recombinations. These differences can be observed with forthcoming redshifted 21-cm surveys (increasing the brightness temperature fluctuations by up to a factor of 2 on large scales) and with measurements of small-scale anisotropies in the cosmic microwave background. Finally, we show that stochastic fluctuations in the galaxy population only broaden the bubble size distribution significantly if massive galaxies are responsible for most of the ionizing photons. We argue that the key results of this model are robust to many of our uncertainties about the reionization process. Ke yw ords: galaxies: evolution ‐ intergalactic medium ‐ cosmology: theory.

COSMIC RAY ANISOTROPY WITH THE KASCADE EXPERIMENT

Anisotropies in the arrival directions of cosmic rays reflect their source distribution and propagation properties. In the presented analysis extensive air shower measurements with KASCADE are analyzed with respect to large and small scale anisotropies. The resulting upper limits on large scale anisotropy amplitudes of 10-3 to 10-2 restrict the parameter space of several propagation models. No cosmic ray point sources could be found in a sky survey of the northern hemisphere. Typical upper flux limits for point sources are around 10-10 m -2 s -1.

Anomalous scaling of passively advected magnetic field in the presence of strong anisotropy

Inertial-range scaling behavior of high-order (up to order N=51 ) two-point correlation functions of a passively advected vector field has been analyzed in the framework of the rapid-change model with strong small-scale anisotropy with the aid of the renormalization group and the operator-product expansion. Exponents of the power-like asymptotic behavior of the correlation functions have been calculated in the one-loop approximation. These exponents are shown to depend on anisotropy parameters in such a way that a specific hierarchy related to the degree of anisotropy is observed. Deviations from power-law behavior like oscillations or logarithmic behavior in the corrections to correlation functions have not been found.

Orientation of the cross-field anisotropy of small-scale ionospheric irregularities and direction of plasma convection

Abstract. The relationship between the orientation of the small-scale ionospheric irregularity anisotropy in a plane perpendicular to the geomagnetic field and the direction of plasma convection in the F region is investigated. The cross-field anisotropy of irregularities is obtained by fitting theoretical expectations for the amplitude scintillations of satellite radio signals to the actual measurements. Information on plasma convection was provided by the SuperDARN HF radars. Joint satellite/radar observations in both the auroral zone and the polar cap are considered. It is shown that the irregularity cross-field anisotropy agrees quite well with the direction of plasma convection with the best agreement for events with quasi-stationary convection patterns.

High energy gamma ray counterparts of astrophysical sources of ultra-high energy cosmic rays

If ultra-high energy cosmic rays (UHECRs) are accelerated at astrophysical point sources, the identification of such sources can be achieved if there is some kind of radiation at observable wavelengths that may be associated with the acceleration and/or propagation processes. No radiation of this type has so far been detected or at least no such connection has been claimed. The process of photopion production during the propagation of UHECRs from the sources to the Earth results in the generation of charged and neutral pions. The neutral (charged) pions in turn decay to gamma quanta and electrons that initiate an electromagnetic cascade in the universal photon background. We calculate the flux of this gamma radiation in the GeV–TeV energy range and find that for source luminosities compatible with those expected from small scale anisotropies in the directions of arrival of UHECRs, the fluxes can be detectable by future Cerenkov gamma ray telescopes, such as VERITAS and HESS, provided the intergalactic magnetic field is not larger than ∼10 −10 Gauss and for source distances comparable with the loss length for photopion production.

Anisotropy in large eddy simulations determined from SO(3) symmetry group

The issue of small-scale anisotropy in the context of eddy-viscosity-type subgrid-scale models is discussed in the present work. Recent developments in turbulence research suggest that anisotropy from large spatial scales is felt far into the inertial subrange of turbulence. This is of particular importance for subgrid-scale models of large eddy simulations. To address this issue, we present solutions of the random-phase Kolmogorov flow at moderate Reynolds numbers using direct numerical simulations and large eddy simulations with four subgrid-scale models: the Smagorinsky, the dynamic, the dynamic Clark, and the tensor-diffusivity models. The degree of anisotropy at different scales is analysed by decomposing the structure function into their irreducible representation of the SO(3) symmetry group. The results suggest that the dynamic model and the dynamic Clark model reproduce the statistical behaviour reasonably well, even in the anisotropic sectors at small length scales.