Intermediate Scale Structure Research Articles

A three-dimensional stochastic structure model derived from high-resolution isolated equatorial plasma bubble simulations

Ionospheric structure is characterized by the space–time variation of electron density. However, our understanding of the physical processes that initiate and sustain intermediate-scale structure development does not relate directly to statistical measures that characterize the structure. Consequently, high-resolution physics-based equatorial plasma bubble simulations are essential for identifying systematic relations between statistical structure measures and the underlying physics that initiates and sustains the structure evolution. An earlier paper summarized the analysis of simulated equatorial plasma bubble (EPB) structure initiated with a quasi-periodic bottom-side perturbation that generated five plasma bubbles. The results are representative of real environments. However, the association of the structure development with individual EPBs was difficult to ascertain. This paper summarizes the analysis of new results from single isolated EPB realizations with varying parameters that affect the structure development. The evolution of the single isolated EPB realizations reveal what we have identified as a canonical structure evolution pattern manifest in the space–time development of four quantitative spectral parameters. The onset of structure occurs when the plasma bubble penetrates the F-region peak. The parameter evolution from the initiation point have a fish-like appearance. The three-dimensional structure model can be used to interpret in situ and remote diagnostic measurements as well as predicting the deleterious effects of propagation disturbances on satellite communication, navigation, and surveillance systems.Graphical

Examining the dust of the tailless Oort-cloud comet C/2020 T2

We report our latest analysis of the Oort-cloud comet C/2020 T2 (also named Palomar or T2) observed at 2.06 au from the Sun (phase angle of 28.°5) roughly two weeks before perihelion. It lacks a significant dust tail in scattered light, showing a strong central condensation of the coma throughout the apparition that is reminiscent of so-called Manx comets. Its spectral slope of polarized light increases and decreases in the J (1.25 μm) and H (1.65 μm) bands, respectively, resulting in an overall negative (blue) slope (−0.31 ±0.14% μm−1 ) in contrast to the red polarimetric color of active comets observed at similar geometries. The average polarization degree of T2 is 2.86±0.17% for the J and 2.75±0.16% for the H bands. Given that near-infrared wavelengths are sensitive to the intermediate-scale structure of cometary dust (i.e., dust aggregates), our light-scattering modeling of ballistic aggregates with different porosities and compositions shows that the polarimetric properties of T2 are compatible with low-porosity (~66%), absorbing dust aggregates with negligible ice contents on a scale of 10–100 μm (density of ~652 kg m−3). This is supported by the coma morphology of T2 which has a viable β (the relative importance of solar radiation pressure on dust particles) range of ≲10−4. The secular evolution of r-band activity of T2 from archival data reveals that the increase in its brightness accelerates around 2.4 au pre-perihelion, with its overall dust production rate that is about 100 times smaller than those of active Oort-cloud comets. We also found an apparent concentration of T2 and Manx comets toward ecliptic orbits. This paper underlines the heterogeneous nature of Oort-cloud comets, which may be investigated in the near future with dedicated studies of their dust characteristics.

A hybrid two-dimensional orthogonal wavelet multiresolution and proper orthogonal decomposition technique for the analysis of turbulent wake flow

A hybrid two-dimensional orthogonal wavelet multiresolution and proper orthogonal decomposition technique is developed to analyze the wake flow behind a semi cylinder. The two-dimensional orthogonal wavelet analysis is first applied to decompose the measured velocity fields into four wavelet components based on their spatial scales. Then the extracted multi-scale structures are further investigated based on proper orthogonal decomposition. The modal energy distributions of multi-scale flow structures suggest that the dominance of first two POD modes is reduced and the relative energy becomes more distributed as the flow structure changes from large-scale to small-scale. The first two POD modes of intermediate-scale structure also appear in a mode pair and they are related to secondary resonance accompanied with large-scale flow oscillation. The POD mode 3 of intermediate-scale structure suggest the existence of symmetric vortex shedding pattern. The first two POD modes of small-scale structure also display a symmetrically distributed vortex pair and the distance between adjacent vortices is about half of the case of mode 3 of intermediate-scale structure. The PSD distributions of POD modes of multi-scale structures suggest that the bandwidth of PSD becomes less concentrated as the flow structure varies from large-scale to small-scale. Distinctive flow patterns associated with first and second order harmonics of fundamental flow oscillation can be successfully extracted from the dominating POD modes of intermediate-scale and small-scale structures. The proposed hybrid technique provides a tool for relate the most energetic flow events over a period of time with multi-scale structures cascaded in the turbulence flow.

Three-dimensional multiscale flow structures behind a wall-mounted short cylinder based on tomographic particle image velocimetry and three-dimensional orthogonal wavelet transform.

Three-dimensional (3D) flow structures around a wall-mounted short cylinder of height-to-diameter ratio 1 were instantaneously measured by a high-resolution tomographic particle image velocimetry (Tomo-PIV) at Reynolds number of 10 720 in a water tunnel. 3D velocity fields, 3D vorticity, the Q criterion, the rear separation region, and the characteristic of arch type vortex and tip vortices were first discussed. We found a strong 3D W-type arch vortex behind the short cylinder, which was originated by the interaction between upwash and downwash flows. This W-type arch vortex was reshaped to the M-shaped arch vortex downstream. It indicated that the head shape of the arch vortex structure depended on the aspect ratio of the cylinder. The large-scale streamwise vortices were originated by the downwash and upwash flows near the center location of W-type arch vortex. Then the 3D orthogonal wavelet multiresolution technique was developed to analyze instantaneous 3D velocity fields of Tomo-PIV in order to clarify 3D multiscale wake flow structures. The W-type shape arch vortex was extracted in the time-averaged intermediate-scale structure, while an M-shaped arch vortex was identified in the time-averaged large-scale structure. The tip vortices distributed in the time-averaged large- and intermediate-scale structures. The instantaneous intermediate-scale upwash vortices played an essential role in producing W-type head of arch structure. It was also observed that strong small-scale vortices appeared in the shear layer or near the bottom plate and most of them were contained in the intermediate-scale structures.

An Analysis of the Shapes of Interstellar Extinction Curves. VIII. The Optical Extinction Structure*

Abstract New Hubble Space Telescope/STIS optical spectra were obtained for a sample of early-type stars with existing International Ultraviolet Explorer UV spectra. These data were used to construct optical extinction curves whose general properties are discussed elsewhere. In this paper, we identify extinction features in the curves that are wider than diffuse interstellar bands (DIBs) but narrower than the well known broadband variability. This intermediate scale structure, or ISS, contains distinct features whose peaks can contribute a few percent to 20% of the total extinction. Most of the ISS variation can be captured by three principal components. We model the ISS with three Drude profiles and show that their strengths and widths vary from one sight line to another, but their central positions are stable, near 4370, 4870, and 6300 Å. The very broad structure (VBS) in optical curves appears to be a minimum between the 4870 and 6300 Å absorption peaks. We find relations among the fit parameters and provide a physical interpretation of them in terms of a simplistic grain model. Finally, we note that the strengths of the 4370 and 4870 Å features are correlated to the strength of the 2175 Å UV bump, but that the 6300 Å feature is not, and that none of the ISS features are related to R(V). However, we verify that the broadband curvature of the continuous optical extinction is strongly related to R(V).

Multi-scale wake structures around the dune

The three-dimensional wake structures behind a barchan dune model are investigated by the high-speed particle image velocimetry (PIV) at the Reynolds number of 5530. The measured velocity fields are first evaluated by time-averaged flow patterns and power spectra analysis. Then the one-dimensional orthogonal wavelet analysis is applied to reveal multi-scale turbulent structures and their corresponding second order statistics in the dune wake. The instantaneous multi-scale structures suggest that the large-scale structure is responsible for the formation of separation region. The intermediate-scale structure tends to be more active at the boundary of separation region, which may be due to the secondary vortex movement caused by the interactions between the wake and main flow. The appearance of small-scale vortices at the downstream of separation region indicates the breakdown process of large-scale vortices. The Reynolds stress and kinetic energy distribution suggest that the large-scale structures dominate the generation of turbulence in the dune wake. The lower kinetic energy contained in intermediate- and small- scale structures is considered to be related to smaller-scale interactions that reflect energy cascading phenomenon. The Reynolds shear stresses distributions suggest that the intermediate-scale structures also make significance in the separated boundary layer. As for small-scale structure, it is worthy to note that a region of high Reynolds shear stresses with an opposite sign is observed around the dune crest. The cross-correlation function of multi-scale structures suggest the quasi-periodicity of large-scale structure. The intermediate-scale structures also show a certain periodicity, however, such structures tend to be more unstable and the strength of them reduces as they travel downstream. When decreasing to small-scale, the vortical structures decays quickly at the downstream with larger convective velocity. This may indicate the breakdown process of large-scale vortices and the quick dissipation of small-scale vortices.

Dynamic spectral characteristics of high-resolution simulated equatorial plasma bubbles

Manifestations of severe nighttime equatorial ionospheric disturbances have been observed for decades. It is generally accepted that the phenomena are caused by large depletions, referred to as equatorial plasma bubbles (EPBs), which are initiated on the rising unstable bottom side of the nighttime F layer. Physics-based simulations have enhanced our understanding of the EPB phenomenon. However, until very recently, stochastic structure smaller than ∼ 10 km was not well resolved. Recent high-resolution EPB simulations have extended the resolution to hundreds of meters, which provides a unique opportunity to characterize intermediate-scale EPB structure.This paper presents a summary analysis of simulated high-resolution intermediate-scale EPB structure. Estimation of altitude-dependent power law spectral density function parameters provides an altitude versus time history of the intermediate-scale structure development. Local structure onset is associated with successive bifurcation of rising EPBs. Developed structure characterized by a two-component power law spectral density function ultimately subtends several hundred kilometers in altitude.Two-component inverse power-law structure was first observed in early in situ rocket measurements. It has been observed in diagnostic measurements of beacon-satellite and GPS scintillation data as well as in situ measurements from Atmospheric Explorer and C/NOFS satellites. The EPB simulation data fully support the reported EPB diagnostics as well as a correlation between the turbulent strength and the large-scale spectral index parameter estimates. However, recent analyses have shown that the correlation is an intrinsic property of power-law parameter estimation.

Multi-scale analysis on wake structures of asymmetric cylinders with different aspect ratios

The wake flow originated by three-dimensional bluff bodies having the different aspect ratio is experimentally studied in this paper. The turbulent wakes generated by asymmetric- and semi-cylinders were first measured by the high-speed PIV. The time-averaged and instantaneous flow structures were analyzed in various planes. Then, the orthogonal wavelet multi-resolution technique was used to analyze the multi-scale flow structures. The instantaneous vorticity and Reynolds shear stresses of various frequencies were evaluated and compared among the different aspect ratio and body shape. It was found that the asymmetric-cylinder produces the aerodynamic downforce and its separation region is smaller than that of semi-cylinder. The large-scale structure makes the most contribution to vorticities, and the intermediate-scale structure dominates the flow structures near the tip of body. The large-scale structure makes the most contribution to Reynolds shear stress except the tip of body, accounting for about 55.9–79.2%. The maximum value of large-scale Reynolds shear stress appears at the end of separation region, and the small-scale Reynolds shear stress exhibits the maximum value in the shear layer of the body surface.

Wavelet multi-resolution analysis on turbulent wakes of asymmetric bluff body

This paper focuses on the multi-scale wake structures of a new asymmetric side mirror of a race car, which can generate the aerodynamic downforce. The turbulent wake generated by symmetric bluff bodies of triangle and semi-cylinder and an asymmetric bluff body (called CT-cylinder) were experimentally investigated and were compared. Firstly, the instantaneous velocity and vorticity were quantitatively measured by high-speed PIV at a Reynolds number of 14,440 in the circulating water channel. Then a one- and two-dimensional wavelet multi-resolution techniques were used to analyze the instantaneous velocity and vorticity in order to decompose turbulent structures into a number of subsets based on their central frequencies. It is found that the CT-cylinder wake generates the asymmetric flow structures and separation region that is smaller than that of the triangle wake. The contribution to vorticity concentration comes from the large-scale structure of CT-cylinder wake in the (x, y)-plane, and the intermediate-scale structure of the CT-cylinder wake dominates the flow structure in the (x, z)-plane. There is no evident difference among the three wakes in the relatively small-scale structures. The large-scale structures of three wakes exhibit the main contributions to the Reynolds shear stress, accounting for almost 82–92%. However, the small-scale structures make less contribution to Reynolds shear stresses. The phase-averaging and spectral analyses indicate that the CT-cylinder wake generates the relatively smaller vortices and smaller separation region and the CT-cylinder wake exhibits lower fluctuation in the shear layer.

Development of intermediate scale structure near the peak of the F region within an equatorial plasma bubble

AbstractScintillation observations are used to study the evolution of intermediate scale (~100 m–few kilometers) irregularities through growth of the Rayleigh‐Taylor (R‐T) instability on the bottom side of the post‐sunset equatorial F region during magnetically quiet periods. Amplitude scintillations on a VHF signal from a geostationary satellite, recorded by spaced receivers at an equatorial station, are used to compute as a function of local time: (1) the coherence scale length for spatial variations of intensity in the ground scintillation pattern, which is linked with the spectrum of the intermediate scale irregularities near the peak of the equatorial F region that contribute the most to the observed scintillations; and (2) the “random velocity”, which accounts for the de‐correlation of the spaced receiver signals. The relationship between the coherence scale length and the random velocity for saturated scintillations at different local times suggests that (1) the random velocity is linked with fluctuations in the drift velocity of the irregularities caused by the perturbation electric fields associated with the R‐T instability rather than structural changes in the intermediate scale irregularities, (2) the spectrum of intermediate scale irregularities in the equatorial F peak region tends to be shallowest after the decay of the perturbation electric fields associated with the R‐T instability, and (3) evolution of intermediate‐scale irregularity spectrum in the equatorial plasma bubble near the equatorial F region peak depends on season and solar flux. These have implications for observation of low‐latitude L‐band scintillations.

Discovery of Localized Regions of Excess 10-TeV Cosmic Rays

The 7 year data set of the Milagro TeV observatory contains 2.2 x 10(11) events of which most are due to hadronic cosmic rays. These data are searched for evidence of intermediate scale structure. Excess emission on angular scales of approximately 10 degrees has been found in two localized regions of unknown origin with greater than 12sigma significance. Both regions are inconsistent with pure gamma-ray emission with high confidence. One of the regions has a different energy spectrum than the isotropic cosmic-ray flux at a level of 4.6sigma, and it is consistent with hard spectrum protons with an exponential cutoff, with the most significant excess at approximately 10 TeV. Potential causes of these excesses are explored, but no compelling explanations are found.

Conductivity−Viscosity−Structure: Unpicking the Relationship in an Ionic Liquid

The relationships between the ionic mobility, the viscosity, and the atomic-scale structure are investigated in computer simulations of mixtures of LiF and the network glass-forming material BeF(2). The simulations span a wide range of compositions, across which the fluidity of the system changes greatly due to the break-up of the Be-F network by the addition of the LiF. The relationship between the conductivity and viscosity passes from that expected for independently diffusing ions in the dilute mixtures to strongly decoupled Li+ migration through a viscous network at higher concentrations. The transition between these régimes is linked to the changing local and intermediate-scale structure in the melts. The decoupling phenomenon is associated with the appearance of migration channels in the network which leads to cooperative effects in the Li+ migration.

Probing the dark ages with the Square Kilometer Array

The epoch of reionization (EoR) sets a fundamental benchmark in cosmic structure formation, corresponding to the formation of the first luminous objects that act to ionize the neutral intergalactic medium (IGM). Recent observations at near-IR and radio wavelengths imply that we are finally probing into this key epoch of galaxy formation at z ⩾ 6. The Square Kilometer Array (SKA) will provide critical insight into the EoR, in a number of ways. First, the ability of the SKA to image the neutral IGM in 21-cm emission is a truly unique probe of the process of reionization, and is recognized as the next necessary and fundamental step in our study of the evolution of large scale structure and cosmic reionization. Second, study of HI 21-cm absorption toward the first radio loud objects probes small to intermediate scale structure in the neutral ‘cosmic web’, as well as HI in the first collapsed structures (proto-disks and mini-halos). And third, the incomparable sensitivity of the SKA allows for the study of the molecular gas, dust, and star formation activity in the first galaxies, as well as the radio continuum emission from the first accreting massive black holes. Such objects will be obscured at optical wavelengths due to absorption by the neutral IGM.

Intermediate scale structure of the interstellar medium towards NGC 6231 in Sco OB1 with FUSE

The FUSE far-ultraviolet interstellar spectra toward seven targets in NGC 6231 show that the molecules H_2, HD, and CO as well as various atomic species are distributed in several clouds. The main absorption component found on the sight lines lies in the Lupus cloud region at a distance of about 150 pc, and there is a weaker second one, presumably in the Sco OB1 shell surrounding NGC 6231 (d~1.8 kpc). H_2 excitation modelling is used to constrain the radiation field and the density in the gas; HD is used to estimate the abundance of H+. The small angular separation of the target stars allows column density variations to be probed over the field of view, on scales of 0.05 pc in the case of the Lupus cloud distance. They are 40% for H_2 and 60% for H I. A rather strong radiation field inside the molecular clouds suggests a separation of the gas into smaller cloudlets also along the line of sight.

Smart visualization of multi-scale turbulent structures

The vector wavelet multi-resolution technique based on the orthogonal wavelet transform was used to analyze the velocity vector data that were simultaneously obtained by sixteen X-wires in the turbulent near-wake of a circular cylinder at different streamwise positions. The instantaneous velocity vector field is decomposed into the multi-scale structures, and its sectional streamline of various scales is displayed. The large-scale structures in the plane of mean shear are consistent with the well-known K´ arm´ an vortices. The structures that correspond to the saddle region between K´´ vortices are identified with the rib-like structures. Even smaller scale structures are also observed. As increasing x/d, the decay of vortical structure and rib-like structure are observed. The topologies of Reynolds stress and RMS vorticity associated with the multi-scale structures are discussed at different downstream positions. It is found that the most significant contribution to Reynolds stress comes from the frequency components of f 219 Hz at x/d =2 0. However, the contribution to Reynolds stress coming from the intermediate-scale structure becomes weak at x/d =4 0. The RMS vorticity components of f = 875 Hz is the largest among all components.

On the presence of intermediate-scale heterogeneity in the upper mantle

SUMMARY Power spectra of tomographic models of the entire earth and of models of Europe and the Mediterranean area are compared. This comparison reveals a discrepancy (a factor 15–30) between the power of both kinds of models at intermediate spherical-harmonic degrees (around l = 30). Two possible explanations for this discrepancy are given: first, the fact that Europe is more complex than the global average; and second, the lack of resolution in the inversions. Another approach to the problem of the power spectrum of heterogeneities of intermediate length scale is the study of multipathing of long-period surface waves. The multipathing observed in the recordings of these waves, which manifests itself as an increasing complexity of their amplitude spectra due to increasing interference of spectral components with increasing epicentral distance, cannot be explained by present-day global earth models. The gradients in the models are simply not strong enough. However, random models with more power at intermediate scales, and consequently stronger gradients, are able to explain the observed multipathing. It is concluded that the global models underestimate the power in the heterogeneities of intermediate-scale structure in the earth. Finally, an explanation is given why current global models, although they do not contain sufficient power at shorter scales, do explain the traveltime and dispersion data used for the construction of the models.

Structure-property relationships in thin films and membranes

The molecular-scale species distributions and intermediate-scale structure of silicate sols influence the microstructures of the corresponding thin films prepared by dip-coating. Using multi-step hydrolysis procedures, we find that, depending on the sequence and timing of the successive steps, the species distributions (determined by 29Si NMR) and intermediate scale structure (determined by SAXS) can change remarkably for sols prepared with the same nominal composition. During film formation, these kinetic effects cause differences in the efficiency of packing of the silicate species, leading to thin film structures with different porosities.

Cosmic strings and intermediate scale structure of the universe

The paper suggests two possible explanations of the observed decrease in the average number density of galaxies with increasing distancer from the observer. Both are based on the fact that the invariant volume of space around and within a straight cosmic-string bundle increases more slowly withr than in Minkowski space.

Intermediate Scale Structure

What does intermediate (arcsecond) scale structure tell us about the activity deep in the VLBI cores of galaxies and quasars? There are certain key areas where a knowledge of this structure is particularly relevant. One is in defining the relationships between the various classes of objects leading to unified schemes. A second is in the understanding of jet physics. For the latter it is very important to know if the material in the jets supplied from the nucleus is heavy or light, slow or fast, ejected one side at a time or symmetrically. The optimum resolution to study jet material is on the arcsecond (≡ kiloparsec) scale where very high dynamic range maps are possible and where it becomes feasible to find out something about the physical conditions in the surrounding medium from optical and X-ray observations.