Observed Density Fluctuations Research Articles

Magnetic Topology and the Structure of the Slow Solar Wind

We present a 3D reduced magnetohydrodynamic (RMHD) model of reflection driven Alfvén wave turbulence in an open magnetic field positioned near the solar equator. The non-linear interactions between outward and inward propagating waves generate turbulence. The RMHD equations describing the turbulence include the effects of solar wind outflow velocity on the dissipation of waves. For Alfvén wave turbulence to be a viable mechanism for heating the corona and accelerating the slow solar wind, there must be sufficient counter-propagating waves to generate the turbulence. In our previous study of the fast wind, we showed that the Alfvén wave turbulence model creates the energy needed for accelerating the fast solar wind, when observed density fluctuations are included in the model. Here, we will explore whether the conditions of the equatorial corona, a source of slow solar wind, are capable of generating sufficient Alfvén wave turbulence to account for the acceleration of the slow wind. We show that for a specific sets of model parameters, the energy from the Alfvén wave turbulence model is smaller by an order of magnitude than the energy needed to heat and accelerate the slow solar wind.

Observation of non-local effects in ion transport channel in J-TEXT plasmas

In cold pulse experiments in J-TEXT, the ion transport shows similar non-local response as the electron transport channel. Very fast ion temperature decreases are observed in the edge, while the ion temperature in the core promptly begin to rise after the injection of cold pulse. Moreover, the cutoff density is also found for the ion non-local effect. The experimental observed density fluctuation in a high frequency ranging from 500 kHz to 2 MHz is obviously reduced in the whole plasma region during non-local transport phase.

Experimental observation of density fluctuations in liquid metals associated with liquid–liquid, liquid–gas and metal–nonmetal transitions

We have developed a special technique and succeeded to carry out small-angle x-ray scattering measurements for some liquid metal systems. The purpose is to investigate effects of transitions such as liquid–liquid (LLT), liquid–gas (LGT) and metal–nonmetal (MNMT) transitions on mesoscopic density fluctuations in liquids. In liquid Te systems (Se–Te and Ge–Te mixtures), which show continuous LLT accompanying MNMT, parameters of density fluctuations show maxima almost in the middle of the transition, both in strength and spatial size. This work (and Kajihara et al 2012 Phys. Rev. B86 214202) was the first direct observation that density fluctuations exhibit maximum corresponding to LLT. However in this study, we could not clearly separate the effects of LLT and MNMT on the observed density fluctuations. Thus, we also investigated fluid Hg under high pressure and high temperature conditions, which shows MNMT near a critical point of LGT, to investigate how MNMT affects them. We observed distinct density fluctuations; a strength and a correlation length of them show maxima at around a critical isochore of LGT, and the former is basically consistent with a phase diagram (compressibility) of LGT; they do not show any peaks at MNMT region. Precise analysis revealed that MNMT only affects a shift of another parameter, a short-range correlation length. These results in fluid Hg indicate that the density fluctuations are mainly derived from a critical phenomena of LGT and MNMT does not play any critical role on them. We believe that the latter conclusion also holds true for liquid Te systems; MNMT plays no important role on the density fluctuations in liquid Te systems and LLT is the main origin of them.

What fraction of the density fluctuations in the Perseus cluster core is due to gas sloshing rather than AGN feedback?

Deep Chandra observations of the core of the Perseus cluster show a plethora of complex structure. It has been found that when the observed density fluctuations in the intracluster medium are converted into constraints on AGN induced turbulence, the resulting turbulent heating rates are sufficient to balance cooling locally throughout the central 220kpc. However while the signatures of AGN feedback (inflated bubbles) dominate the central 60kpc in X-ray images, beyond this radius the intracluster medium is increasingly shaped by the effects of gas sloshing, which can also produce subtle variations in X-ray surface brightness. We use mock Chandra observations of gas sloshing simulations to investigate what fraction of the observed density fluctuations in the core of the Perseus galaxy cluster may originate from sloshing rather than AGN induced feedback. Outside 60kpc, we find that the observed level of the density fluctuations is broadly consistent with being produced by sloshing alone. If this is the case, AGN-generated turbulence is likely to be insufficient in combating cooling outside 60kpc.

Density Fluctuations in a Polar Coronal Hole

Abstract We have measured the root-mean-square (rms) amplitude of intensity fluctuations, ΔI, in plume and interplume regions of a polar coronal hole. These intensity fluctuations correspond to density fluctuations. Using data from the Sun Watcher using the Active Pixel System detector and Image Processing on the Project for Onboard Autonomy (Proba2), our results extend up to a height of about 1.35 R ⊙. One advantage of the rms analysis is that it does not rely on a detailed evaluation of the power spectrum, which is limited by noise levels to low heights in the corona. The rms approach can be performed up to larger heights where the noise level is greater, provided that the noise itself can be quantified. At low heights, both the absolute ΔI, and the amplitude relative to the mean intensity, ΔI/I, decrease with height. However, starting at about 1.2 R ⊙, ΔI/I increases, reaching 20%–40% by 1.35 R ⊙. This corresponds to density fluctuations of Δn e/n e ≈ 10%–20%. The increasing relative amplitude implies that the density fluctuations are generated in the corona itself. One possibility is that the density fluctuations are generated by an instability of Alfvén waves. This generation mechanism is consistent with some theoretical models and with observations of Alfvén wave amplitudes in coronal holes. Although we find that the energy of the observed density fluctuations is small, these fluctuations are likely to play an important indirect role in coronal heating by promoting the reflection of Alfvén waves and driving turbulence.

Low scale Higgs inflation with Gauss–Bonnet coupling

Recent LHC data provides precise values of coupling constants of the Higgs field, however, these measurements do not determine its coupling with gravity. We explore this freedom to see whether Higgs field non-minimally coupled to Gauss–Bonnet term in 4-dimensions can lead to inflation generating the observed density fluctuations. We obtain analytical solution for this model and that the exit of inflation (with a finite number of e-folding) demands that the energy scale of inflation is close to Electro-weak scale. We compare the scalar and tensor power spectrum of our model with PLANCK data and discuss its implications.

Mixed inflaton and spectator field models: CMB constraints and μ distortion

We discuss mixed inflaton and spectator field models where both the fields are responsible for the observed density fluctuations. We combine the angular power spectrum of the CMB temperature anisotropy from the Planck 2013 result and other ground-based CMB observations in order to constrain both the general mixed model as well as some specific representative scenarios. Based on the Markov Chain Monte Carlo method, in addition to constraints on model parameters, we obtain the predictive posterior distributions of the CMB spectral μ distortion for those models. We demonstrate that the standard single-field inflaton model typically predicts μ ∼ 10−8 with a relatively narrow distribution, whereas for the mixed models, the distribution turns out to be much broader, and μ could be larger by almost an order of magnitude. Hence future experiments of μ distortion could provide a tool for the critical testing of the mixed source models of the primordial perturbation.

The nature of lemming cycles on Wrangel: an island without small mustelids

Lemming cycles are a key process in the functioning of tundra ecosystems. Although it is agreed that trophic interactions are important in causing the cycles, the actual mechanism is disputed. Some researchers attribute a major role to predation by small mustelids such as stoats and least weasels. Here we present a 40-year time series of lemming dynamics from Wrangel Island and show statistically that lemmings do exhibit population cycles in the absence of small mustelids. The observed density fluctuations differed, however, from those observed elsewhere, with long cycles and possibly higher densities of lemmings during the low phase. These differences in the shape of the population cycles may be related to the unique species assemblage of Wrangel Island, where arctic foxes are the only year-round resident lemming predator, and to the high diversity of landscapes, microclimatic conditions, and plants on the island. Both spectral analysis and wavelet analysis show a change in period length from five years in the 1970s to nearly eight years in the 1990s and 2000s. This change in dynamics coincides with reports of dampening or fading out of lemming cycles that have been observed in several regions of the Arctic in recent decades. As in the other cases, the changed lemming dynamics on Wrangel Island may be related to ground icing in winter, which could delay peak years.

From nanoparticles to nanocrystalline bulk: percolation effects in field assisted sintering ofsilicon nanoparticles

Nanocrystalline bulk materials are desirable for many applications as they combinemechanical strength and specific electronic transport properties. Our bottom-up approachstarts with tailored nanoparticles. Compaction and thermal treatment are crucial, butusually the final stage sintering is accompanied by rapid grain growth which spoilsnanocrystallinity. For electrically conducting nanoparticles, field activated sinteringtechniques overcome this problem. Small grain sizes have been maintained in spite ofconsolidation. Nevertheless, the underlying principles, which are of high practicalimportance, have not been fully elucidated yet.In this combined experimental and theoretical work, we show how the developingmicrostructure during sintering correlates with the percolation paths of the current throughthe powder using highly doped silicon nanoparticles as a model system. It is possible toachieve a nanocrystalline bulk material and a homogeneous microstructure. Forthis, not only the generation of current paths due to compaction, but also thedisintegration due to Joule heating is required. The observed density fluctuations on themicrometer scale are attributed to the heat profile of the simulated powder networks.

Observations of density fluctuations in a quiescent prominence

AbstractDensity fluctuations in a torsional quiescent prominence were studied with Solar Optical Telescope on board of Hinode satellite. Continuous observations were made in Ca II H line from ~15:00 UT May 03, 2008; the observational duration was ~1 hr with a cadence time ~ 30s. The emission intensity along the prominence axis as functions of altitudes and time shows fluctuations in brightness with local peaks in Fourier power spectra, indicating the presence of periods and intervallic displacements to be 3 ~ 11 min and 2 ~ 7 Mm respectively and statistically significant peaks at 368 ± 63s & 620 ± 41 s, and 3.3 ± 0.6 Mm regime. The distance is the line of sight projection. These intensity disturbances may perhaps be caused by density fluctuations originated from compressional waves in which the possible origin could be the combinations of p-mode oscillations induced at the surface and twisting of the flux tube.

Improvement in the magnetic properties of Tb/FeCo films for perpendicular magnetic recording media

By incorporating high M s Fe 65Co 35 alloy in the Tb/FeCo multilayer structure on 20 nm thick W underlayer, we obtained K u=4×10 6 erg/cm 3, M s=350 emu/cm 3, and H c=7.5 kOe. The W underlayer was deposited under 13.5mTorr Ar gas pressure to induce micro-roughness at the surface with lateral feature size of a few nm to 20 nm. Both X-ray diffraction and transmission electron microscopy methods confirmed that the Tb/FeCo multilayer was amorphous. We observed density fluctuation in the amorphous Tb/FeCo layer, which is the most probable candidate of domain wall pinning sites created in the amorphous magnetic film.

Application of precise phase detector for density profile and fluctuation measurements using CO2 imaging heterodyne interferometer on LHD

The two-color CO2∕YAG laser imaging heterodyne interferometer was recently equipped with a precise digital phase counter (eight channels, 0.5mrad resolution, 1MHz carrier frequency and sampling rate, 500kHz bandwidth, ±250rad recording range, and a maximum data record of 8.4×106 samples/channel). This modification enables detailed measurements of plasma density dynamics during pellet ablation and, in addition, observations of density fluctuations. Examples of fast rising density peaks in pellet discharges as well as spatial and spectral characteristics of plasma density fluctuations obtained with the heterodyne interferometer are presented.

A new inflaton model beginning near the Planck epoch

The Starobinsky model predicts a primordial inflation period without the presence of an inflaton field. The modified version of this model predicts a simple time dependence for the Hubble parameter H(t), which decreases slowly between the Planck epoch and the end of the inflation, H(t) = M Pl -betaM²Pl t, where beta is a dimensionless constant to be adjusted from observations. We investigate an inflaton model which has the same time dependence for H(t). A reverse engineered inflaton potential for the time dependence of H is derived. Normalization of the derived inflaton potential is determined by the condition that the observed density fluctuations, <FONT FACE=Symbol>dr</FONT>/<FONT FACE=Symbol>r »</FONT> 10-5, are created at ~ 60 e-folds before the end of inflation. The derived potential indicates an energy (mass) scale, Mend ~ 10(13) GeV, at the end of inflation. Using the slow roll parameters, which are obtained from this potential, we calculate the spectral index for the scalar modes nS and the relative amplitude of the tensor to scalar modes r. A tensor contribution, r ~ 0.13, and an approximately Harrison-Zeldovich density perturbation spectrum, nS ~ 0.95, are predicted.

Observations of Density Fluctuations in an Elongated Bose Gas: Ideal Gas and Quasicondensate Regimes

We report in situ measurements of density fluctuations in a quasi-one-dimensional 87Rb Bose gas at thermal equilibrium in an elongated harmonic trap. We observe an excess of fluctuations compared to the shot-noise level expected for uncorrelated atoms. At low atomic density, the measured excess is in good agreement with the expected "bunching" for an ideal Bose gas. At high density, the measured fluctuations are strongly reduced compared to the ideal gas case. We attribute this reduction to repulsive interatomic interactions. The data are compared with a calculation for an interacting Bose gas in the quasicondensate regime.

Studying the decay of the vacuum energy with the observed density fluctuation spectrum

We investigate here models that suggest that the vacuum energy decays into cold dark matter (CDM) and show that the density fluctuation spectrum obtained from the cosmic microwave background (CMB) data together with large galaxy surveys (e.g., the Sloan Digital Sky Survey), puts strong limits on the rate of decay of the vacuum energy. CDM produced by a decaying vacuum energy would dilute the density fluctuation spectrum, created in the primordial universe and observed with large galaxy surveys at low redshifts. Our results indicate that the decay rate of the vacuum energy into CDM is extremely small.

WMAP and supergravity inflationary models

We study a class of $N=1$ supergravity inflationary models in which the evolution of the inflaton dynamics is controlled by a single power in the inflaton field at the point where the observed density fluctuations are produced, in the context of the braneworld scenario, in light of Wilkinson Microwave Anisotropy Probe results. In particular, we find that the bounds on the spectral index and its running constrain the parameter space both for models where the inflationary potential is dominated by a quadratic term and by a cubic term in the inflaton field. We also find that ${\ensuremath{\alpha}}_{s}g0$ is required for the quadratic model whereas ${\ensuremath{\alpha}}_{s}l0$ for the cubic model. Moreover, we have determined an upper bound on the five-dimensional Planck scale, ${M}_{5}\ensuremath{\lesssim}0.019\mathrm{M},$ for the quadratic model. On the other hand, a running spectral index with ${n}_{s}g1$ on large scales and ${n}_{s}l1$ on small scales is not possible in either case.

CO 2 laser imaging interferometer on LHD

A CO2 laser (λ=10.6 μm) imaging interferometer is designed for electron density profile and electron density fluctuation measurements on large helical device (LHD). The purposes of this diagnostic are reliable electron density monitor in high density operation (especially in pellet injection discharge), precise measurement of electron density profiles and observations of density fluctuations. By using a CO2 laser, refraction effect becomes negligible, and the interferometer promises to be free from fringe jumps at high density. We plan to get 5 mm spatial resolution, (56 ch for each of two 280 mm slab beams, 50 ch for one 250 mm and 16 ch for 80 mm crossed beam), 3×10−3 rad phase resolution and 200 kHz frequency response for the above purpose. And fluctuation will be studied within the wave number range of 7×10−3⩽k⩽0.6 mm−1.

Investigation of the density fluctuation in sol-gel derived materials by light scattering techniques

Summary The experimental methods for studying of small-angle light scattering in porous media and a theoretical model for understanding the observed density fluctuations are presented in this paper. Parameters describing the structure of silica sol-gel materials were calculated basing on the random density fluctuation theory. The correlation length a1 and correlation length a2 were found that represent the average pore diameter and average size of Si—O—Si string aggregates, respectively. For the studies described here, the sol-gel matrices were prepared on the way of acid hydrolysis. Two types of materials in form of sol-gel thin layers were examined: type I made with the water solvent and type II made with alcohol solvent. It was stated that the size of the macropores depends on the sol-gel preparation recipe. The type of solvent influences the porosity of sol-gel glasses. The analysis of the experimental data indicated the strong inhomogeneities of sol-gel material type I what results from the different degree of Si—O—Si polymerization.

Observations of density fluctuations in Earth's magnetosheath with Geotail and Wind spacecraft

Large fluctuations of plasma densities in the earth's flank‐side magnetosheath (local times 1500–1700) are examined. The characteristics of the fluctuations are large amplitudes (ΔN/N ∼ 1), long oscillation periods (T ∼ 30 m) and anti‐ correlation between the plasma densities and magnetic field strengths. The variations of the solar wind plasma densities are negligible, whereas considerable fluctuations are found for the solar wind magnetic fields. Two possibilities are considered for the density fluctuations: 1) repeated traversals of the plasma depletion layer and the standing slow‐mode waves near the magnetopause and 2) convected slow‐mode waves generated at the earth's bow shock. Temperature anisotropy and density profile as expected from the repeated traversals are not confirmed with the observations. Based on the consideration that the present observations occur in the flanks where plasma flows are largely tangential to the magnetopause, we propose that the convected slow‐mode waves play a role in the generation of the density fluctuations.

Soho Observations of Density Fluctuations in Coronal Holes

In recent UVCS/SOHO White Light Channel (WLC) observations we found quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes at heliocentric distances of 1.9 to 2.45 solar radii. The motivation for the observation is the 2.5D MHD model of solar wind acceleration by nonlinear waves, that predicts compressive fluctuations in coronal holes. In February 1998 we performed new observations using the UVCS/WLC in the coronal hole and obtained additional data. The new data corroborate our earlier findings with higher statistical significance. The new longer observations show that the power spectrum peaks in the 10–12 minute range. These timescales agree with EIT observations of brightness fluctuations in polar plumes. We performed preliminary LASCO/C2 observations in an effort to further establish the coronal origin of the fluctuations.