Large-scale Density Gradient Research Articles

Numerical simulations of a continuously injected relativistic electron beam relaxation into a plasma with large-scale density gradients

In this paper the influence of large-scale decreasing and increasing gradients of the density of magnetized plasma on the relaxation process of a continuously injected relativistic electron beam with an energy of 660 keV (vb=0.9c) and a pitch-angle distribution is studied using particle-in-cell numerical simulations. It is found that for the selected parameters in the case of a smoothly decreasing gradient and in a homogeneous plasma the formation of spatially limited plasma oscillations of large amplitude occurs. In such cases, modulation instability develops and a long-wave longitudinal modulation of the ion density is formed. In addition, the large amplitude of plasma waves accelerates plasma electrons to energies on the order of the beam energy. In the case of increasing and sharply decreasing gradients, a significant decrease in the amplitude of plasma oscillations and the formation of a turbulent ion density spectrum are observed. The possibility of acceleration of beam electrons to energies more than 2 times higher than the initial energy of the beam particles is also demonstrated. This process takes place not only during beam propagation in growing plasma density, but also in homogeneous plasma due to interaction of beam particles with plasma oscillations of large amplitude.

The Cosmic Mach Number as an environment measure for the underlying dark matter density field

ABSTRACT Using cosmological dark matter (DM) only simulations of a (1.6 Gpc/h)3 volume from the Legacy simulation project, we calculate Cosmic Mach Numbers (CMN) and perform a theoretical investigation of their relation with halo properties and features of the density field to gauge their use as an measure of the environment. CMNs calculated on individual spheres show correlations with both the overdensity in a region and the density gradient in the direction of the bulk flow around that region. To reduce the scatter around the median of these correlations, we introduce a new measure, the rank ordered Cosmic Mach number ($\hat{\mathcal {M}}_g$), which shows a tight correlations with the overdensity $\delta =\frac{\rho -\bar{\rho }}{\bar{\rho }}$. Measures of the large-scale density gradient as well as other average properties of the halo population in a region show tight correlations with $\hat{\mathcal {M}}_g$ as well. Our results in this first empirical study suggest that $\hat{\mathcal {M}}_g$ is an excellent proxy for the underlying density field and hence environment that can circumvent reliance on number density counts in a region. For scales between 10 and $100 \, \mathrm{Mpc}\, h^{-1}$, Mach numbers calculated using DM haloes (> 1012 M⊙) that would typically host massive galaxies are consistent with theoretical predictions of the linear matter power spectrum at a level of $10{{\ \rm per\, cent}}$ due to non-linear effects of gravity. At redshifts z ≥ 3, these deviations disappear. We also quantify errors due to missing large-scale modes in simulations. Simulations of box size ≤1 Gpc h−1 typically predict CMNs 10–30 per cent too small on scales of ∼100 Mpc h−1.

Isolation of Autophagy Competent Lysosomes from Cancer Cells by Differential Large-Scale Multilayered Density Gradient Centrifugations.

Accurate isolation of functional and intact lysosomes enables the quantification and analyses of abundances, dynamic changes and enrichment levels of lysosomal content, allowing specific lysosomal investigations induced by autophagy. In this protocol chapter, we describe detailed practical instructions and advices for an efficacious lysosomal enrichment and isolation procedure by differential multilayered density gradient centrifugations using human cancer cell lines. By this method, intact and autophagy competent lysosomes can be isolated from cancer cells based on their distinct density and obtained fractions can further be analyzed for functional lysosomal assays, as well as for protein or metabolic loads to identify select spatiotemporal changes by comparative quantitative measurement. This method has been used to enrich lysosomes from a variety of cancer cells with activated chaperone-mediated autophagy, but can be optimized for other cell lines and tissues for multiple autophagy-induced conditions.

Turbulence transport in the solar corona: Theory, modeling, and Parker Solar Probe

A primary goal of the Parker Solar Probe (PSP) Mission is to answer the outstanding question of how the solar corona plasma is heated to the high temperatures needed for the acceleration of the solar wind. Various heating mechanisms have been suggested, but one that is gaining increasing credence is associated with the dissipation of low frequency magnetohyrodynamic (MHD) turbulence. However, the MHD turbulence models come in several flavors: one in which outwardly propagating Alfvén waves experience reflection from the large-scale flow and density gradients associated with the solar corona, and the resulting counterpropagating Alfvén waves couple nonlinearly to produce quasi-2D turbulence that dissipates and heats the corona, thereby driving the solar wind. The second approach eschews a dominant outward flux of Alfvén waves but argues instead that quasi-2D turbulence dominates the lower coronal plasma and is generated in the constantly upwelling magnetic carpet, experiencing dissipation as it is advected through the corona and into the solar wind, yielding temperatures in the corona that exceed a million degrees. We review the two turbulence models, describe the modeling that has been done, and relate PSP observations to the basic predictions of both models. Although PSP measurements are made in the super-Alfvénic solar wind, the observations are close to the coronal region, thus providing a glimpse into the likely properties of coronal turbulence. Observations of low-frequency MHD turbulence by PSP in the super-Alfvénic solar wind allow us to place constraints on models of the turbulently heated solar corona that drive the supersonic solar wind.

Simulating TeV gamma-ray morphologies of shell-type supernova remnants

ABSTRACT Supernova remnant (SNR) shocks provide favourable sites of cosmic ray (CR) proton acceleration if the local magnetic field direction is quasi-parallel to the shock normal. Using the moving-mesh magnetohydrodynamical (MHD) code arepo we present a suite of SNR simulations with CR acceleration in the Sedov–Taylor phase that combine different magnetic field topologies, density distributions with gradients and large-scale fluctuations, and – for our core-collapse SNRs – a multiphase interstellar medium with dense clumps with a contrast of 104. Assuming the hadronic gamma-ray emission model for the TeV gamma-ray emission, we find that large-amplitude density fluctuations of δρ/ρ0 ≳ 75 per cent are required to strongly modulate the gamma-ray emissivity in a straw man’s model in which the acceleration efficiency is independent of magnetic obliquity. However, this causes strong corrugations of the shock surface that are ruled out by gamma-ray observations. By contrast, magnetic obliquity-dependent acceleration can easily explain the observed variance in gamma-ray morphologies ranging from SN1006 (with a homogeneous magnetic field) to Vela Junior and RX J1713 (with a turbulent field) in a single model that derives from plasma particle-in-cell simulations. Our best-fitting model for SN1006 has a large-scale density gradient of ∇n ≃ 0.0034 cm−3 pc−1 pointing from south-west to north-east and a magnetic inclination with the plane of the sky of ≲10°. Our best-fitting model for Vela Junior and RX J1713 adopts a combination of turbulent magnetic field and dense clumps to explain their TeV gamma-ray morphologies and moderate shock corrugations.

Fingerprints for Early Detection of Changes in the AMOC

AbstractDifferent strategies have been proposed in previous studies for monitoring the Atlantic meridional overturning circulation (AMOC). As well as arrays to directly monitor the AMOC strength, various fingerprints have been suggested to represent an aspect of the AMOC based on properties such as temperature and density. The additional value of fingerprints potentially includes the ability to detect a change earlier than a change in the AMOC itself, the ability to extend a time series back into the past, and the ability to detect crossing a threshold. In this study we select metrics that have been proposed as fingerprints in previous studies and evaluate their ability to detect AMOC changes in a number of scenarios (internal variability, weakening from increased greenhouse gases, weakening from hosing and hysteresis) in the eddy-permitting coupled climate model HadGEM3-GC2. We find that the metrics that perform best are the temperature metrics based on large-scale differences, the large-scale meridional density gradient, and the vertical density difference in the Labrador Sea. The best metric for monitoring the AMOC depends somewhat on the processes driving the change. Hence the best strategy would be to consider multiple fingerprints to provide early detection of all likely AMOC changes.

A Technique for Removing Large-scale Variations in Regularly and Irregularly Spaced Data

Abstract In many astrophysical systems, smooth large-scale variations coexist with small-scale fluctuations. For example, a large-scale velocity or density gradient can exist in molecular clouds that have small-scale fluctuations by turbulence. In redshifted 21 cm observations, we also have two types of signals—the Galactic foreground emissions that change smoothly and the redshifted 21 cm signals that fluctuate fast in frequency space. In many cases, the large-scale variations make it difficult to extract information on small-scale fluctuations. We propose a simple technique to remove smooth large-scale variations. Our technique relies on multi-point structure functions and can obtain the magnitudes of small-scale fluctuations. It can also be used to design filters that can remove large-scale variations and retrieve small-scale data. We discuss how to apply our technique to irregularly spaced data, such as rotation measure observations toward extragalactic radio point sources.

Diagnostics of gaseous halo of the Milky Way by a shock wave from the Galactic Centre

A method to diagnose the gas distribution in the Galactic halo using a shock wave from the Galactic Centre (GC) is presented. Propagation of a shock wave caused by central explosion with released energy $\sim 10^{55}$ erg is calculated for various models of the gaseous halo, and the shock front morphology is compared with the observed North- (NPS) and South-Polar Spurs (SPS). The observed bipolar hyper-shell (BHS) structure of the spurs is reproduced, when a semi-exponential halo model with radially-variable scale height is adopted. A spherical halo model ($\beta$ model) is shown to be not appropriate to explain the observed BHS shape. Asymmetry of the spurs with respect to the rotation axis and to the galactic plane is explained by large-scale density gradient in the halo across the Galaxy. Such gradient may be produced by ram pressure of the IGM during motion of the Galaxy in the Local Group. A halo model having sinusoidal density fluctuations (clouds) can also explain the asymmetry, given appropriate cloud parameters are chosen. Further irregular features such as filaments and multiple curvatures superposed on the spurs are also understood as due to density fluctuations in the halo.

Goldstone models of modified gravity

We investigate scalar-tensor theories where matter couples to the scalar field via a kinetically dependent conformal coupling. These models can be seen as the low-energy description of invariant field theories under a global Abelian symmetry. The scalar field is then identified with the Goldstone mode of the broken symmetry. It turns out that the properties of these models are very similar to the ones of ultralocal theories where the scalar-field value is directly determined by the local matter density. This leads to a complete screening of the fifth force in the Solar System and between compact objects, through the ultralocal screening mechanism. On the other hand, the fifth force can have large effects in extended structures with large-scale density gradients, such as galactic halos. Interestingly, it can either amplify or damp Newtonian gravity, depending on the model parameters. We also study the background cosmology and the linear cosmological perturbations. The background cosmology is hardly different from its $\Lambda$-CDM counterpart whilst cosmological perturbations crucially depend on whether the coupling function is convex or concave. For concave functions, growth is hindered by the repulsiveness of the fifth force whilst it is enhanced in the convex case. In both cases, the departures from the $\Lambda$-CDM cosmology increase on smaller scales and peak for galactic structures. For concave functions, the formation of structure is largely altered below some characteristic mass, as smaller structures are delayed and would form later through fragmentation, as in some warm dark matter scenarios. For convex models, small structures form more easily than in the $\Lambda$-CDM scenario.

Wavelet-based cross-correlation analysis of structure scaling in turbulent clouds

We propose a statistical tool to compare the scaling behaviour of turbulence in pairs of molecular cloud maps. Using artificial maps with well defined spatial properties, we calibrate the method and test its limitations to ultimately apply it to a set of observed maps. We develop the wavelet-based weighted cross-correlation (WWCC) method to study the relative contribution of structures of different sizes and their degree of correlation in two maps as a function of spatial scale, and the mutual displacement of structures in the molecular cloud maps. We test the WWCC for circular structures having a single prominent scale and fractal structures showing a self-similar behavior without prominent scales. Observational noise and a finite map size limit the scales where the cross-correlation coefficients and displacement vectors can be reliably measured. For fractal maps containing many structures on all scales, the limitation from the observational noise is negligible for signal-to-noise ratios >5. (abbrev). Application of the WWCC to the observed line maps of the giant molecular cloud G333 allows to add specific scale information to the results obtained earlier using the principle component analysis. It confirms the chemical and excitation similarity of $^{13}$CO and C$^{18}$O on all scales, but shows a deviation of HCN at scales of up to 7' (~7 pc). This can be interpreted as a chemical transition scale. The largest structures also show a systematic offset along the filament, probably due to a large-scale density gradient. The WWCC can compare correlated structures in different maps of molecular clouds identifying scales that represent structural changes such as chemical and phase transitions and prominent or enhanced dimensions.

Characterisation of the subaquatic groundwater discharge that maintains the permanent stratification within Lake Kivu; East Africa.

Warm and cold subaquatic groundwater discharge into Lake Kivu forms the large-scale density gradients presently observed in the lake. This structure is pertinent to maintaining the stratification that locks the high volume of gases in the deepwater. Our research presents the first characterisation of these inflows. Temperature and conductivity profiling was conducted from January 2010 to March 2013 to map the locations of groundwater discharge. Water samples were obtained within the lake at the locations of the greatest temperature anomalies observed from the background lake-profile. The isotopic and chemical signatures of the groundwater were applied to assess how these inflows contribute to the overall stratification. It is inferred that Lake Kivu’s deepwater has not been completely recharged by the groundwater inflows since its turnover that is speculated to have occurred within the last ~1000 yrs. Given a recent salinity increase in the lake constrained to within months of seismic activity measured beneath the basin, it is plausible that increased hydrothermal-groundwater inflows into the deep basin are correlated with episodic geologic events. These results invalidate the simple two-component end-member mixing regime that has been postulated up to now, and indicate the importance of monitoring this potentially explosive lake.

Impact of uncertainties in the horizontal density gradient upon low resolution global ocean modelling

In this study, it is shown (i) that, as a result of the nonlinearity of the seawater equation of state, unresolved scales represent a major source of uncertainties in the computation of the large-scale horizontal density gradient from the large-scale temperature and salinity fields, and (ii) that the effect of these uncertainties can be simulated using random processes to represent unresolved temperature and salinity fluctuations. The results of experiments performed with a low resolution global ocean model show that this parameterization has a considerable effect on the average large-scale circulation of the ocean, especially in the regions of intense mesoscale activity. The large-scale flow is less geostrophic, with more intense associated vertical velocities, and the average geographical position of the main temperature and salinity fronts is more consistent with observations. In particular, the simulations suggest that the stochastic effect of the unresolved temperature and salinity fluctuations on the large-scale density field may be sufficient to explain why the Gulf Stream pathway systematically overshoots in non-stochastic low resolution ocean models.

THE MASS-SIZE RELATION FROM CLOUDS TO CORES. I. A NEW PROBE OF STRUCTURE IN MOLECULAR CLOUDS

We use a new contour-based map analysis technique to measure the mass and size of molecular cloud fragments continuously over a wide range of spatial scales (0.05 < r / pc < 10), i.e., from the scale of dense cores to those of entire clouds. The present paper presents the method via a detailed exploration of the Perseus Molecular Cloud. Dust extinction and emission data are combined to yield reliable scale-dependent measurements of mass. This scale-independent analysis approach is useful for several reasons. First, it provides a more comprehensive characterization of a map (i.e., not biased towards a particular spatial scale). Such a lack of bias is extremely useful for the joint analysis of many data sets taken with different spatial resolution. This includes comparisons between different cloud complexes. Second, the multi-scale mass-size data constitutes a unique resource to derive slopes of mass-size laws (via power-law fits). Such slopes provide singular constraints on large-scale density gradients in clouds.

On the Dynamics of the Slope Current System along the West European Margin. Part I: Analytical Calculations and Numerical Simulations with Steady-State Forcing

Abstract The dynamics of the baroclinic slope current system along the western European margin in the Bay of Biscay and along the northern Iberian Peninsula are investigated in two different models, one analytical and one numerical. Investigated here is the hypothesis that the steady-state slope current system is driven by the large-scale meridional density gradients. An analysis of the observed density fields evidences a four-layer structure with meridional gradients of alternate signs, which is also found in the numerical model. The linear analytical model of the continental margin shows that such a density structure is enough to obtain a steady-state four-layer slope current system comparable to the observed annual mean circulation. The slope currents result from a balance between bottom friction and meridional density gradients. The numerical simulation with an ocean general circulation model forced only by the large-scale density gradients at the lateral boundaries presents a four-layer slope current system similar to the circulation obtained in the analytical model. The study confirms that the large-scale meridional density gradients are the main driving mechanism for the steady-state slope current system; the large seasonality of these currents, however, requires a more extended model, which is discussed in a companion paper (Part II).

Analysing the structure of elongated star clusters

We have previously reported a measure which both quantifies and distinguishes between a (relatively smooth) large-scale radial density gradient and multiscale (fractal) subclustering. Here, we extend the applicability of to clusters which deviate significantly from an overall circular shape. varies systematically as clusters assume a more elongated shape, and it is therefore possible to correct for the effect, if the elongation of the cluster is also quantified. therefore remains a useful and robust analytical technique for classifying and quantifying the internal structure of star clusters, even when their overall shape is far from circular. The corrections required are small for individual clusters which are not extremely elongated (not more than three times longer than they are wide) of the same order as the uncertainty in the value of for a particular cluster type. We therefore recommend that no correction be applied to the calculation of for individual clusters, unless they are more than three times longer than their width, but that correction for elongation be applied when is used for statistical analyses of large numbers of observed or simulated clusters.

Nonspherical radiative supernova remnants

The inhomogeneity of the interstellar medium and the anisotropic energy release in the course of the explosion lead to the nonsphericity of supernova remnants, so that different parts of a particular remnant are found at different evolutionary stages. We propose an analytical description of the 3-D evolution of nonspherical supernova remnants in inhomogeneous media. The adiabatic, transition, and radiative stages are considered. In the case of anisotropic supernovae in the interstellar medium with a large-scale density gradient, the transitions of supernova remnants to the radiative stage with the formation of a cool dense shell occur at different moments in different parts of the shock front. So, different parts of such remnants can be at different evolutionary stages at a certain moment. Some observational manifestations of nonspherical remnants are discussed.

Constraints upon density gradients in evolved HII regions

We investigate the variation of radio flux ratios and brightness temperatures in Galactic HII regions. These are compared to the results of a radiative transfer analysis, and shown to imply that large-scale density gradients must be relatively modest. Where the radial variation in density gradients is approximated by n ∝ r − β , then we determine that exponents β are ⩽1 (and probably ≪1). Similarly, it is probable that proportionate variations in density are relatively modest, and no greater (in the mean) than ∼10 2 or so. Such results are important for studies of the formation of such regions, and appear consistent with various hydrodynamical models.

Sources of variability in Gulf of Maine circulation, and the observations needed to model it

Abstract Variability in the circulation of coastal oceans must ultimately be driven by changes in the meteorological conditions that force currents in the coastal ocean, and by variability in the waters entering the coastal ocean from elsewhere. If a coastal ocean is to be understood and modeled accurately, the external forcing that drives the largest portions of the circulation variability must be observed adequately. Thus, some method of comparing the relative importance of various sources of circulation variability must be developed, so that there is confidence that the most important sources of variability are included in any analysis or modeling. This is done for variability in the time-integrated transport across a section in a coastal ocean. The relative importance of various sources of circulation variability in the Gulf of Maine (GoM) are then quantified, with an emphasis on variability on timescales longer than tidal or weather-band timescales. In order to concentrate on interannual changes, the seasonal cycle is not included in estimates of variability. It is found that the variability forced by fluctuations in the winds and the volume of water entering from the Scotian Shelf to the GoM produce roughly comparable amounts of circulation variability. However, changes in the density structure of the GoM produce changes in time-integrated transport that are an order of magnitude larger, at least in the central GoM. The changes in the large-scale density gradients are governed by mixing processes in the Gulf and by changes in the water masses entering the GoM from the Scotian Shelf and the Northeast Channel. Unless the heat, freshwater and volume transport of the waters entering the GoM are routinely observed, numerical models will fail to capture much of the variability in the circulation of the Gulf. An analysis is given of the minimal set of observations needed to allow numerical models of the GoM to resolve adequately the true variability in the circulation.

Photoevaporation Flows in Blister HiiRegions. I. Smooth Ionization Fronts and Application to the Orion Nebula

We present hydrodynamical simulations of the photoevaporation of a cloud with large-scale density gradients, giving rise to an ionized, photoevaporation flow. The is found to be approximately steady during the large part of its evolution, during which it can resemble a champagne flow or a globule flow depending on the curvature of the ionization front. The distance from source to ionization front and the front curvature uniquely determine the structure of the flow, with the curvature depending on the steepness of the lateral density gradient in the neutral cloud. We compare these simulations with both new and existing observations of the Orion Nebula and find that a model with a mildly convex ionization front can reproduce the profiles of emission measure, electron density, and mean line velocity for a variety of emitting ions on scales of 1017-1018 cm. The principal failure of our model is that we cannot explain the large observed widths of the [O I] λ6300 line that form at the ionization front.

ChandraDetection of Ejecta in the Small‐Diameter Supernova Remnant G349.7+0.2

We present high-resolution X-ray observations of the small-diameter supernova remnant (SNR) G349.7+0.2 with Chandra. The overall SNR spectrum can be described by two spectral components. The soft component is in ionization equilibrium and has a temperature of ?0.8?keV; the hard spectral component has a temperature of ?1.4?keV, an ionization timescale of ?5 ? 1011?cm-3 s, and enhanced abundances of Si. The spatially resolved spectral modeling shows that S may also be enhanced, at least in some regions. The enhanced abundances clearly point to the presence of an ejecta component in this remnant. Using the available H I and CO data toward G349.7+0.2 we derive a column density of ?7 ? 1022?cm-2 along the line of sight to the SNR, which is consistent with our X-ray data. The X-ray morphology of G349.7+0.2 is strikingly similar to that at radio wavelengths?an irregular shell with a brighter eastern side?which is consistent with expansion in a medium with a large-scale density gradient. The remnant is known to be interacting with a molecular cloud (from the presence of OH (1720?MHz) masers), but this interaction is probably limited to the central portion of the SNR, as seen in SNR IC?443. We found that H I clouds are present in the SNR region, which supports the notion that G349.7+0.2 belongs to a class of remnants evolving in the intercloud medium (such are IC?443 and W44), which is also responsible for the remnant's morphology. G349.7+0.2 does not have the mixed-morphology found for other maser-emitting SNRs studied to date in X-rays, but its morphology can be explained by a projection model for mixed-morphology SNRs. We have identified a point source close to the center of the SNR with a luminosity of LX(0.5-10.0 keV) ~ (0.2-2.3) ? 1034?d22?ergs s-1, which is consistent with that of the compact central objects found in a few other Galactic SNRs.