Dipole Amplitude Research Articles

The bulk motion of flat galaxies on scales of 100 Mpc in the quadrupole and octupole approximations

The peculiarities of non-Hubble bulk motions of galaxies are studied by analyzing a sample of 1271 thin edge-on spirals with distances determined using a multiparametric Tully-Fisher relation that includes the amplitude of the galaxy rotation, the blue and red diameters, surface brightness, and morphological type. In the purely dipole approximation, the bulk motion of galaxies relative to the cosmic microwave background frame can be described by the velocity of 336±96 km s−1 in the direction l=321°, b=−1° within radius Rmax=10000 km s−1. An analysis of more complex velocity field models shows that the anisotropy of the Hubble expansion described by the quadrupole term is equal to ∼5% on scale lengths Rmax=6000–10000 km s−1. The amplitude within the Local Supercluster (Rmax=3000 km s−1) is as high as ∼20%. The inclusion of the octupole component reduces the dipole amplitude to 134±111 km s−1 on scale lengths of ∼8000 km s−1. The most remarkable feature of the galaxy velocity field within Rmax=8000 km s−1 is the zone of minimum centered on l=80°, b=0° (the constellation of Cygnus) whose amplitude reaches 18% of the mean Hubble velocity.

Location changes enhance hemispheric asymmetry of magnetic fields evoked by lateralized sounds in humans

Auditory mismatch negativity, the brain's change-detection response, has been shown to be more sensitive than other early auditory cortex responses to the hemispheric specialization of speech processing. The present study used magnetoencephalography to assess hemispheric differences in cortical evoked responses during auditory spatial processing. We compared N1m to lateralized vowels presented with equal probabilities with mismatch fields (MMNm) to rare lateralized noises interspersed in a sequence of frequent midline sounds. Both N1m and MMNm dipole amplitudes were higher in the hemisphere contralaterally to the side of sound lateralization, but this effect was about four times bigger in the mismatch paradigm. Moreover, only MMNm dipoles showed shorter latencies in the hemisphere contralaterally to stimulation. Apparently stimulus changes activate specialized auditory networks more strongly than non-deviant events.

A magnetoencephalographic study on development of the somatosensory cortex in infants

In order to examine the sensori-motor correlation in infants, we recorded the somatosensory evoked magnetic fields to tactile stimulation by using a 37-channel magnetoencephalograph. Twelve healthy infants were examined at palmar grasp stage and pincers grasp stage. Air-tapping stimulation of the right thumb was performed. Three distinct components (W1-3) emerged, W3, with a latency of approximately 100 ms, being the most prominent. As infants grew up, the correlation coefficient and the amplitude of the equivalent current dipole of W3 for the thumb increased. These developmental changes may be attributable to increases in the stability and reproducibility of the cortex in response to somesthetic inputs. Moreover, this change along with motor development supports the presence of a sensori-motor correlation in infants.

Nonlinear evolution and saturation for heavy nuclei in DIS

The nonlinear evolution equation for the scattering amplitude of colour dipole off the heavy nucleus is solved in the double logarithmic approximation. It is found that if the initial parton density in a nucleus is smaller then some critical value, then the scattering amplitude is a function of one scaling variable inside the saturation region, whereas if it is greater then the critical value, then the scaling behaviour breaks down. Dependence of the saturation scale on the number of nucleons is discussed as well.

The geodynamo as a bistable oscillator

Our intent is to provide a simple and quantitative understanding of the variability of the axial dipole component of the geomagnetic field on both short and long time scales. To this end we study the statistical properties of a prototype nonlinear mean field model. An azimuthal average is employed, so that (1) we address only the axisymmetric component of the field, and (2) the dynamo parameters have a random component that fluctuates on the (fast) eddy turnover time scale. Numerical solutions with a rapidly fluctuating α reproduce several features of the geomagnetic field: (1) a variable, dominantly dipolar field with additional fine structure due to excited overtones, and sudden reversals during which the field becomes almost quadrupolar, (2) aborted reversals and excursions, (3) intervals between reversals having a Poisson distribution. These properties are robust, and appear regardless of the type of nonlinearity and the model parameters. A technique is presented for analysing the statistical properties of dynamo models of this type. The Fokker-Planck equation for the amplitude a of the fundamental dipole mode shows that a behaves as the position of a heavily damped particle in a bistable potential ∝(1 − a 2)2, subject to random forcing. The dipole amplitude oscillates near the bottom of one well and makes occasional jumps to the other. These reversals are induced solely by the overtones. Theoretical expressions are derived for the statistical distribution of the dipole amplitude, the variance of the dipole amplitude between reversals, and the mean reversal rate. The model explains why the reversal rate increases with increasing secular variation, as observed. Moreover, the present reversal rate of the geodynamo, once per (2−3) × 105 year, is shown to imply a secular variation of the axial dipole moment of ∼ 15% (about the current value). The theoretical dipole amplitude distribution agrees well with the Sint-800 data.

QCD dipole model and factorization

Abstract It is shown that the colour dipole approach to hard scattering at high energy is fully compatible with k T factorization at the leading logarithm approximation (in − log x Bj ). The relations between the dipole amplitudes and unintegrated diagonal and non-diagonal gluon distributions are given. It is also shown that including the exact gluon kinematics in the k T factorization formula destroys the conservation of transverse position vectors and thus is incompatible with the dipole model for both elastic and diffractive amplitudes.

Barotropic interaction between planetary- and synoptic-scale waves during the life cycles of blockings

In this paper, in an equivalent barotropic framework a new forced nonlinear Schroedinger equation is proposed to examine the interaction between the planetary-scale waves and the localized synoptic-scale eddies upstream. With the help of the perturbed inverse scattering transform method, nonlinear parameter equations can be derived to describe the evolution of the dipole soliton amplitude, frequency, group velocity and phase under the forcing of localized synoptic-scale eddies. The numerical solutions of these equations predict that in the interaction between the weak dipole soliton (weak incipient dipole anomaly) and the synoptic-scale eddies, only when the high-frequency eddies themselves have a moderate parameter match they can near resonantly enhance a quasi-stationary large-amplitude split flow. The instantaneous total streamfunction field (the sum of background westerly wind, envelope Rossby soliton and synoptic-scale waves) is found to be very similar to the observed Berggren-type blocking on the weather map(Berggren et al. 1949). The role of synoptic-scale eddies is to increase the amplitude of large-scale dipole anomaly flow, and to decrease its group velocity, phase velocity and zonal wavenumber so that the dipole anomaly system can be amplified and transferred from dispersive system to very weak dispersive one. This may explain why and how the synoptic-scale eddies can reinforce and maintain vortex pair block. Furthermore, it is clearly found that during the prevalence of the vortex pair block the synoptic-scale eddies are split into two branches around the vortex pair block due to the feedback of amplified dipole block.

The 2–10 keV X‐Ray Background Dipole and Its Cosmological Implications

The hard X-ray (>2 keV) emission of the local and distant Universe as observed with the HEAO1-A2 experiment is reconsidered in the context of large scale cosmic structure. Using all-sky X-ray samples of AGN and galaxy clusters we remove the dominant local X-ray flux from within a redshift of ~ 0.02. We evaluate the dipolar and higher order harmonic structure in 4 X-ray colours. The estimated dipole anisotropy of the unresolved flux appears to be consistent with a combination of the Compton-Getting effect due to the Local Group motion (dipole amplitude Delta = 0.0042) and remaining large scale structure (0.0023 2 keV Galactic X-ray emission. Comparison of the soft and hard colour maps with a harmonic analysis of the 1.5 keV ROSAT all-sky data qualitatively suggests that at least a third of the faint, unresolved ~ 18 deg scale structure in the HEAO1-A2 data may be Galactic in origin. However, the effect on measured flux dipoles is small (<~3%). We derive an expression for dipole anisotropy and acceleration and demonstrate how the dipole anisotropy of the distant X-ray frame can constrain the amplitude of bulk motions of the universe. From observed bulk motions over a local ~ 50 Mpc/h radius volume we determine 0.14 <~ Omega^0.6/b_x(0) <~ 0.59.

Simultaneous bilateral mismatch response to right- but not leftward sound lateralization.

Magnetoencephalography (MEG) was used to compare mismatch responses between hemispheres to changes in sound-source direction. Sixteen adults listened passively to two types of complex non-language sounds presented in separate blocks with midline standards and right- and left-lateralized deviants. Mismatch dipole amplitudes were larger contra- than ipsilaterally to the deviants. Both hemispheres processed right deviants simultaneously, whereas to left deviants, the left dipole peaked 20 ms later than the right dipole. A second experiment using the same standards but midline spectral deviants showed no interhemispheric differences. Here mismatch latencies were about 60 ms longer than in the location mismatch experiment. This suggested both fast, contralaterally dominant location mismatch responses and facilitated detection of auditory spatial deviance in the right hemifield.

The electroproduction of the Δ(1232) in the chiral quark–soliton model

We calculate the ratios E2 / M1 and C2 / M1 for the electroproduction of the Δ(1232) in the region of photon virtuality 0<− q 2<1 GeV 2 . The magnetic dipole amplitude M1 is also presented. The theory used is the chiral quark–soliton model, which is based on the instanton vacuum of the QCD. The calculations are performed in flavours SU(2) and SU(3) taking rotational ( 1/ N c ) corrections into account. The results for the ratios agree qualitatively with the available data, although the magnitude of both ratios seems to underestimate the latest experimental results.

Pion charge radius from charged pion electroproduction

We analyze a low-energy theorem of threshold pion electroproduction which allows one to determine the charge radius of the pion. We show that at the same order where the radius appears, pion loops induce a correction to the momentum dependence of the longitudinal dipole amplitude $L_{0+}^{(-)}$. This model-independent correction amounts to an increase of the pion charge radius squared from the electroproduction data by about 0.26~fm$^2$. It sheds light on the apparent discrepancy between the recent determination of the pion radius from electroproduction data and the one based on pion-electron scattering.

The IRAS PSCz dipole

We use the PSCzIRAS galaxy redshift survey to analyse the cosmological galaxy dipole out to a distance of 300 h−1 Mpc. The masked area is filled in three different ways, first by sampling the whole sky at random, secondly by using neighbouring areas to fill a masked region, and thirdly using a spherical harmonic analysis. The method of treatment of the mask is found to have a significant effect on the final calculated dipole. The conversion from redshift space to real space is accomplished by using an analytical model of the cluster and void distribution, based on 88 nearby groups, 854 clusters and 163 voids, with some of the clusters and all of the voids found from the PSCz data base. The dipole for the whole PSCz sample appears to have converged within a distance of 200 h−1 Mpc and yields a value for , consistent with earlier determinations from IRAS samples by a variety of methods. For b=1, the 2σ range for Ω0 is 0.43–1.02. The direction of the dipole is within 13° of the cosmic microwave background (CMB) dipole, the main uncertainty in direction being associated with the masked area behind the Galactic plane. The improbability of further major contributions to the dipole amplitude coming from volumes larger than those surveyed here means that the question of the origin of the CMB dipole is essentially resolved.

Measurement of theγp→π+nReaction near Threshold

Differential cross sections for the $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}n$ reaction have been measured at photon energies 1 to 2 MeV above threshold using tagged-photon beams. The electric dipole amplitude ${E}_{0+}$ deduced at threshold is compared to recent predictions from chiral perturbation theory. Constraints on the value of the $\ensuremath{\pi}N$ coupling constant are inferred by combining theory and experiment.

The QDOT all-sky IRAS galaxy redshift survey

We describe the construction of the QDOT survey, which is publicly available from an anonymous FTP account. The catalogue consists of infrared properties and redshifts of an all-sky sample of 2387 IRAS galaxies brighter than the IRAS PSC 60-μm completeness limit (S60>0.6 Jy), sparsely sampled at a rate of one-in-six. At |b|>10°, after removing a small number of Galactic sources, the redshift completeness is better than 98 per cent (2086/2127). New redshifts for 1401 IRAS sources were obtained to complete the catalogue; the measurement and reduction of these are described, and the new redshifts tabulated here. We also tabulate all sources at |b|>10° with no redshift so far, and sources with conflicting alternative redshifts either from our own work, or from published velocities. A list of 95 ultraluminous galaxies (i.e. with L60 μm>1012 L⊙) is also provided. Of these, ∼20 per cent are AGN of some kind; the broad-line objects typically show strong Fe ii emission. Since the publication of the first QDOT papers, there have been several hundred velocity changes: some velocities are new, some QDOT velocities have been replaced by more accurate values, and some errors have been corrected. We also present a new analysis of the accuracy and linearity of IRAS 60-μm fluxes. We find that the flux uncertainties are well described by a combination of 0.05-Jy fixed size uncertainty and 8 per cent fractional uncertainty. This is not enough to cause the large Malmquist-type errors in the rate of evolution postulated by Fisher et al. We do, however, find marginal evidence for non-linearity in the PSC 60-μm flux scale, in the sense that faint sources may have fluxes overestimated by about 5 per cent compared with bright sources. We update some of the previous scientific analyses to assess the changes. The main new results are as follows. (1) The luminosity function is very well determined overall but is uncertain by a factor of several at the very highest luminosities (L60 μm>5×1012 L⊙), as this is where the remaining unidentified objects are almost certainly concentrated. (2) The best-fitting rate of evolution is somewhat lower than our previous estimate; expressed as pure density evolution with density varying as (1+z)p, we find p=5.6±2.3. Making a rough correction for the possible (but very uncertain) non-linearity of fluxes, we find p=4.5±2.3. (3) The dipole amplitude decreases a little, and the implied value of the density parameter, assuming that IRAS galaxies trace the mass, is Ω=0.9(+0.45, −0.25). (4) Finally, the estimate of density variance on large scales changes negligibly, still indicating a significant discrepancy from the predictions of simple cold dark matter cosmogonies.

Measurement of the6Li(γ,π0)reaction near threshold

Total and differential cross sections for the reaction ${}^{6}\mathrm{Li}(\ensuremath{\gamma},{\ensuremath{\pi}}^{0})$ have been measured within 23 MeV of threshold using the tagged photon facility at SAL in conjunction with the ${\ensuremath{\pi}}^{0}$ spectrometer IGLOO. The differential measurements are the first to be reported in the threshold region, while the quality of the total cross section far surpasses previous investigations. The electric dipole amplitude for ${\ensuremath{\pi}}^{0}$ photoproduction has been deduced at threshold, and the relation to the corresponding free-deuteron amplitude as recently measured is examined within the context of an $\ensuremath{\alpha}\ensuremath{-}d$ cluster model of ${}^{6}\mathrm{Li}.$ The reduced P-wave amplitude ${P}_{3}^{(+)}/kq$ is deduced at threshold and is consistent with previous estimates from both the nucleon and nuclear sectors. The apparent stability of this amplitude as a function of environment remains a puzzle. Finally, the present cross sections compare favorably with the recent distorted-wave impulse approximation calculations of Kamalov et al.

Elastic QCD Dipole Amplitude at One Loop

We derive the analytic expression of the two one-loop dipole contributions to the elastic four-gluon amplitude in QCD. The first one corresponds to the double QCD Pomeron exchange, the other to an order ${\ensuremath{\alpha}}^{2}$ correction to one-Pomeron exchange. Both are expressed in terms of the square of the recently derived triple QCD Pomeron vertex and involve a summation over all conformal eigenvectors of the Balitsky-Fadin-Kuraev-Lipatov kernel.

The ROSAT X-ray background dipole

We estimate the dipole of the diffuse 1.5 keV X-ray background from the ROSAT all-sky survey map of Snowden et al (1995). We first subtract the diffuse Galactic emission by fitting to the data an exponential scale height, finite radius, disk model. We further exclude regions of low galactic latitudes, of local X-ray emission (eg the North Polar Spur) and model them using two different methods. We find that the ROSAT X-ray background (XRB) dipole points towards $(l,b) ~ (288, 25) \pm 19 degrees within 30 degrees of the CMB dipole. Its direction is also in good agreement with the HEAO1 X-ray dipole at harder energies. The normalised amplitude of the ROSAT XRB dipole is about 1.7 per cent. Subtracting from the ROSAT map the expected X-ray background dipole due to the reflex motion of the observer with respect to the cosmic rest frame (Compton-Geting effect) we find the large-scale dipole of the X-ray emitting extragalactic sources having an amplitude 90% of the total XRB, in general agreement with the predictions of Lahav et al (1997). We finally estimate that the Virgo cluster is responsible for about 20 per cent of the total measured XRB dipole amplitude.

A study of the 4p-excited autoionization resonances in Sr by measurement of the angular correlation between the photoejected electrons and polarized fluorescent photons

For five autoionization resonances of Sr in the 4p excitation region, we have determined the ratios of five amplitudes and one phase difference for the photo-ejected electron waves which leave the ion in the excited states. The technique employed is a combination of conventional angle-resolved photoelectron spectroscopy, and angular correlation measurements between photoejected electrons and polarized fluorescent photons. We discuss how the ratios of the dipole amplitudes are related to the spectroscopic descriptions of the resonances studied.

ERP Mapping in Phonological and Lexical Semantic Monitoring Tasks: A Study Complementing Previous PET Results

Previous PET results identified distinct neural systems involved with phonology (vicinity of the left sylvian fissure) and lexical semantics (left inferior temporal, left superior frontal, bilateral inferior parietal regions). In the time domain, the phonological task was thought to involve serial parsing of pseudo-words, whereas the lexical semantic task would correspond to probabilistic automated access to meaning. Auditory event-related potentials (ERPs) elicited by the same tasks were explored on 32 channels in 12 male volunteers. Subjects had to categorize as target a second element of a pair of pseudo-words or words if a preceding target was detected in the first element. Depending on the absence/presence of target in the first element, a RELEASE condition and a HOLD condition were distinguished. RELEASE and HOLD ERPs split earlier in the semantic task than in the phonological task (300 versus 412 ms after SOT, respectively), although words lasted longer than pseudo-words. Corresponding pairedttest maps showed a predominance of differences over left perisylvian regions for the phonological task and posterior bilateral regions for the semantic task. Underlying generators were investigated using BESA (Scherg, 1990) with global task ERPs. Six dipoles—constrained according to PET clusters—brought residual variance down to 0.36%, from 364 to 565 ms after SOT,in both tasks.Relative dipole amplitudes suggested a left-sided functional asymmetry for phonology. These results support the hypothesis of left perisylvian serial processing for phonology contrasting with bihemispheric parallel access for semantics and substantiates BESA for temporally tackling cognitive processes.

A complete photoionization experiment with polarized atoms using magnetic dichroism and phase tilt measurements

The study of the $5p$ photoemission from laser polarized (aligned or oriented) Eu atoms was used to deduce the ratio of the magnitude of the electric dipole amplitudes and the phase difference between the two outgoing $\ensuremath{\epsilon}s$ and $\ensuremath{\epsilon}d$ electron waves. The combination of magnetic dichroism experiments, the measurement of the difference spectra for two atomic polarizations, and phase tilt experiments, the observation of the modulation of the photoelectron signal on the angle \ensuremath{\eta} of linear laser polarization, is a very promising method for performing a ``complete'' photoionization experiment.